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Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’semployees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,or disclosed to third parties, or otherwise used in whole, or in part,without the written permission of the Vice President, EngineeringServices, Saudi Aramco.

Chapter : Electrical For additional information on this subject, contactFile Reference: EEX10301

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Designing Low VoltageNon-Motor Industrial Feeder Circuits

Engineering Encyclopedia Electrical


Saudi Aramco DeskTop Standards

CONTENTS PAGES

CALCULATING AMPACITY RATINGS OF FEEDERS CONDUCTORS................ 1Factors Affecting Ampacity Ratings ........................................................................ 1Factors Affecting Continuous Current Ratings ........................................................ 2Busway................................................................................................................... 14

CALCULATING SHORT CIRCUIT RATINGS OF FEEDERCONDUCTORS........................................................................................................... 16

Introduction ............................................................................................................ 16Factors Affecting Conductor Short Circuit Ratings ............................................... 16Conductor Temperature Rise.................................................................................. 20Protective Device Clearing Times.......................................................................... 24

CALCULATING VOLTAGE DROPS OF A FEEDER CONDUCTORCIRCUIT...................................................................................................................... 27

Introduction ............................................................................................................ 27Factors Affecting Voltage Drop Calculations ........................................................ 28Effects of Resistance Variables on Voltage Drop .................................................. 31Effects of Reactance Variables on Voltage Drop ................................................... 33Voltage Drop Limits............................................................................................... 34Approximation Formula......................................................................................... 35

SELECTING FEEDER PROTECTIVE DEVICES...................................................... 38Types of Protective Devices................................................................................... 38Low Voltage Power Circuit Breaker (LVPCB) Ratings......................................... 43Protective Device Time/Current (T/C) Characteristics........................................... 45Factors Affecting Selection .................................................................................... 60

SELECTING FEEDER CONDUCTOR RACEWAY SIZES ...................................... 63Types of Raceways................................................................................................. 63Factors Affecting Raceway Size ............................................................................ 65

WORK AID 1: RESOURCES USED TO DESIGN A LOW VOLTAGENON-MOTOR FEEDER CIRCUIT............................................................................. 68

Work Aid 1A: 1993 National Electric Code Handbook......................................... 68Work Aid 1B: ANSI/IEEE Standard 141-1986 (Red Book) .................................. 68Work Aid 1C: SAES-P-114 ................................................................................... 68



Saudi Aramco DeskTop Standards

Work Aid 1D: SAES-P-100 ................................................................................... 70Work Aid 1E: SAES-P-104.................................................................................... 71Work Aid 1F: Applicable Procedures for Calculating Ampacity Ratingsof Feeder Conductors ............................................................................................. 73Work Aid 1G: Applicable Procedures for Calculating the Short CircuitRating of a Feeder Conductor ................................................................................ 75Work Aid 1H: Applicable Procedures for Calculating the Voltage Dropof a Feeder Conductor ............................................................................................ 78Work Aid 1I: Applicable Procedures for Selecting a Feeder ConductorProtective Device ................................................................................................... 79Work Aid 1J: Applicable Procedures for Selecting a Feeder ConductorRaceway Size ......................................................................................................... 80Work Aid 1K: Feeder Circuit Design Flow Chart.................................................. 81

GLOSSARY................................................................................................................. 82



Saudi Aramco DeskTop Standards 1

CALCULATING AMPACITY RATINGS OF FEEDERS CONDUCTORS

Note: Work Aid 1F has been developed to teach the Participant procedures to calculatefeeder conductor ampacity ratings.

Factors Affecting Ampacity Ratings

Although there are many factors that affect the ampacity ratings of conductors, this Modulewill restrict the discussion to the following three factors:

• Continuous Current• Short Circuit Current• Voltage Drop

Continuous Current

The National Electric Code (NEC) defines the continuous current rating of a conductor asampacity, which means “the current in amperes that a conductor can carry continuously underthe conditions of use without exceeding its temperature rating”.

The NEC recognizes that the maximum continuous current carrying capability of a conductorvaries with the different conditions of use and the insulation temperature rating of theconductor itself. For example, ambient temperature is a condition of use. A 75°C conductorinstalled outdoors in Saudi Arabia would have very little ampacity capability compared to thesame temperature-rated conductor installed indoors in an air-conditioned space. Anotherexample of condition of use is the number of conductors that are installed in a raceway(conduit).

Short Circuit Current

The short circuit current rating of a conductor is the maximum current that a conductor cancarry, for a specific and very short time interval, without reaching temperatures that willpermanently damage the conductor insulation. Note: The next Information Sheet will discussin detail the calculation of the short circuit ratings of conductors.

Voltage Drop

Because most electrical equipment is voltage-sensitive, it is very important to have withinequipment and design standard tolerances, the proper voltage at the terminals that are servingthe equipment. Any excessive voltage drop could possibly damage the equipment and impairits proper operation. Note: Calculating the voltage drop of feeder conductors will bediscussed later in this Module.




Factors Affecting Continuous Current Ratings

Conductor Material Types

The resistivity of a conductor, represented by the Greek letter rho (ρ), is defined as theresistance, in ohms (Ω), of the material for a specific sample of given cross-sectional area (A)and with given length (L). The most commonly used dimension for ρ is circular-mil-foot.Resistance (R) is directly proportional to ρ, and it is represented by the following equation:

• R = ρL/A• where:

R = dc resistance of the conductor in ohms (Ω)ρ = resistivity of the conductor in Ω-cmil/ftL = length of the conductor in feet (ft)A = cross-sectional area of the conductor in circular mils

(cmil)

The relationship should be very obvious that, if the resistivity (ρ) of the material is greater(Figure 1), the resistance (R) of the material also increases.

Type ofMaterial

Resisitivity at 200CΩΩ-cmil/ft

silver

copper

aluminum

tungsten

nickel

iron

nichrome

9.8

10.4

17.0

33.0

50.0

60.0

650.0

Figure 1. Resistivity Table




Aluminum is widely used as a transmission-line conductor, especially on high-voltage lines.Compared to a copper conductor of the same physical size, the aluminum wire only has60 percent of the conductivity, only 45 percent of the tensile strength, and only 33 percent ofthe weight. An aluminum wire, to have the same conductivity as copper, would have to be1.64 (100/61) times larger than the copper conductor. For the same conductor currentcarrying capacity, an aluminum wire would have approximately 75 percent of the tensilestrength and 55 percent of the weight of a copper wire. This lighter weight makes aluminumconductors the preferred choice for longer spans; fewer towers, crossarms, insulators, etc., arerequired for the spans. Note: Although permitted by the NEC, SAES-P-104 does not permitthe use of aluminum conductors for wires and cables in buildings, direct burial cables,overhead cable feeders, etc. ACSR is permitted for overhead distribution lines only. Thissection on aluminum is presented for general information purposes only.

When an aluminum conductor is stranded, the center strand is most often made of steel, whichprovides greater strength. ACSR is especially suited for long spans. Figure 2 shows thecomparison (cross-sectional area) between a copper, an aluminum, and an ACSR conductor,each of which has equal current-carrying capacity.

Figure 2. Conductor Type Comparison




Copper (Annealed) is the most generally used building conductor in insulated wires andcables. When covered with rubber insulation, the copper is coated with tin or lead alloy forits own protection, as well as for protection of the insulation, because both copper andinsulation contain substances that attack each other.

Copper is also quite often used in overhead distribution line conductors. Three kinds ofcopper are used: hard-drawn copper, medium hard-drawn copper, and annealed copper,which is often called “soft-drawn” copper. Copper wire is hard-drawn as it comes from thedrawing die. To obtain soft or annealed copper wire, the hard-drawn wire is heated to a veryhigh temperature (red heat) to soften it. Most utilities use medium hard-drawn copper fordistribution lines, especially in wire sizes smaller than No. 2 AWG.

Stranded Versus Solid Conductors - Conductors are classified as either solid or strandedand by size, ranging from very small (AWG No. 18) to very large (2000 kcmil) sizes.As the name implies, a solid conductor is a single strand conductor with a solid circular crosssection. Size AWG No. 1 is approximately the largest solid conductor that is manufactured.Anything larger would not be flexible enough to run in ducts, conduits, etc. Large solidconductors also are easily damaged by bending.

Stranded conductors are a group of solid wires (single strands) made into a single conductor.The strands of the conductor are arranged in concentric layers around a single core. SizeAWG No. 1/0 and larger are the usual sizes for stranded wire, although, because of itsflexibility, it is available and readily used in smaller sizes as well. The smallest number ofwires in a stranded conductor is three, and this number increases to a very large number forspecial applications.Note: SAES-P-104 requires the use of Class B or Class C stranding for all power conductorsfor normal power requirements.

Classification of stranded cables for industrial applications are as follows:

• Class B - This class of stranding is used for industrial cables for 600, 5000, and15,000 volt power systems. The number of strands vary from 7 to 127,depending on the wire size.

• Class C - This class of stranding is used when there is a need for additionalflexibility. The number of strands varies from 19 to 169. Also, the diameter ofeach strand is smaller than it is in wires of the same size that have Class Bstranding. The combination of more strands and smaller diameter strandsallows for more flexibility.

• Classes G and H - These classes of stranding allow for extremely flexiblecables. Classes G and H also are called rope or bunch stranding. Class G uses133 strands, and Class H uses 259 strands. Welding cable is an example ofClass G and Class H cables.




Figure 3 shows various strand configuration for cables.

Figure 3. Stranded Cable Configuration




Conductor Sizes

American Wire Gage (AWG) System - Wire sizes are expressed in numbers. Because thereare different numbering methods, it is imperative to always state which numbering method isbeing used when stating wire sizes. In America, the most often used wire gage is Browne andSharpe, which is also called the American Wire Gage (AWG). Wire sizes range from No. 40AWG (very small) to 0000 AWG or 4/0 AWG (pronounced four-aught). Larger wires greaterthan No. 4/0 AWG are sized in circular mils (cmil) or kilo-circular mils (kcmil), where 1kcmil equals 1000 cmils. The previous term for kcmil was MCM, which is now obsolete.One cmil equals the diameter of the wire in mils (1/1000 of an inch) squared. As the wire sizediameter and cross-sectional area (A) increases, its resistance decreases, which is once againexpressed by the following formula:

• R = ρL/A

International System (SI) - Saudi Aramco specifies the nearest metric wire size to thestandard U.S. ICEA NEMA standard sizes (AWG or kcmil). Note: See Work Aid 1, Figure36. The following formula is presented for informational purposes only:

• 1 cmil = 5.067 x 10-4 mm2 = 7.854 x 10-7 in2

Temperatures

Operating Temperature - The maximum continuous current carrying capability of aconductor is determined by the temperature at which it is allowed to operate over its lifetime.The type of insulation surrounding the material ultimately determines the operatingtemperature. The NEC temperature rating classifications are 60°C, 75°C, and 90°C. Note:SAES-P-104 does not permit use of 60°C insulation. If the operating temperature is exceededfor any long periods of time, the insulation ages prematurely, becomes hard and brittle, andeventually leads to early failure.

Ambient Temperature - The ambient temperature is defined as the temperature of themedium (air or earth) surrounding the conductor. When the ambient temperature increases,there is less of a temperature differential surrounding the conductor, which means that theheat dissipating rate of the conductor is less. As the ambient temperature increases, thecurrent carrying capability of the conductor must be decreased to prevent the conductor’soperating temperature, which is based on the insulation material, from being exceeded. TheNEC ampacity ratings for building wire are based on an ambient temperature of 30°C. SAES-P-100 specifies an ambient temperature of 35°C in indoor air-conditioned spaces, whereasSAES-P-104 specifies 30°C in indoor air-conditioned spaces. See Work Aid 1 for otherSAES-P-100 and SAES-P-104 listed ambient temperatures.




NEC Derating Factors - If the ambient temperature exceeds 30°C, the NEC requires deratingof the conductor in accordance with the correction factors listed in NEC Table 310-16 (seeWork Aid 1A, Handout 1).

Example A: NEC Table 310-16 lists the ampacity rating of a 500 kcmil (240 mm2) 75°Ccopper conductor at 380 A. What is the ampacity rating of the conductor at35°C?

Answer A: The correction factor listed in the NEC is 0.94. The corrected ampacity is 357A (.94 x 380).

Conductor Insulations

Types - The five most common types of insulation are listed as follows:

• Thermosetting compounds, solid dielectric• Thermoplastic compounds, solid dielectric• Paper-laminated tapes• Varnished cloth, laminated tapes• Mineral insulation, solid dielectric - granular

Current industrial applications use synthetic materials for cable insulation. Their resistance tomoisture and ease of handling make synthetic materials popular. These synthetic materialsalso have excellent electrical and mechanical properties. Synthetic insulations are groupedinto either thermoplastic or thermosetting. Thermosetting type cables have little tendency tosoften upon reheating, whereas thermoplastic will soften when reheated. Saudi Aramco-approved insulations for power conductors are the following:

• Polyvinyl chloride (PVC) - Thermoplastic Compound

This cable is flexible and has excellent electrical properties. PVC cables areused for 600 volt systems. The maximum operating temperature is 60°C. Themaximum short-circuit temperature rating is 150°C. PVC cable is available inmany different colors, and it usually carries a designation of T or TW. Themaximum voltage of PVC cable is 600 volts. Note: Polyvinyl chloride cable isnot used on Saudi Aramco installations.

• Cross-Linked Synthetic Polymer

This cable insulation is also for 600 volt systems, and it is moisture and heatresistant. The maximum operating temperature is 90°C, and the insulation isdesignated as XHHW.




• Ethylene Propylene Rubber (EPR)

This type of insulation offers excellent resistance to ozone, heat, and weather.The maximum operating temperature is 90°C. The voltage range of EPR cableis 600 to 69,000 volts.

• Silicone Rubber - Thermosetting Compound

This insulation system is highly resistant to flame, corona, and ozone, but it hasvery poor mechanical strength. Silicone rubber insulation has a maximumoperating temperature of 125°C with a short circuit temperature rating of 250°C.Silicone rubber cable is used in high temperature areas or fire protectionapplications only. The voltage range of silicone rubber cable is from 600 to5000 volts.

• Crossed-Linked Polyethylene (XLPE) - Thermosetting Compound

This cable has excellent electrical, moisture-resistant, and environmental-resistant properties. Corona affects XLPE type insulation. The maximumoperating temperature is 90°C. The maximum short circuit temperature is250°C. XLPE cable voltages range from 600 to 69,000 volts.

Applicable Provisions - NEC Table 310-13 lists the applicable provisions for insulatedconductors. For example, certain types of conductors may be used only in dry locations,whereas another type may be used only in wet or damp locations. Some of the conductorsmay be used in both locations. The applicable provisions ultimately determine the conductorampacity ratings. For example, a type XHHW conductor is rated for 90°C in dry and damplocations, and for 75°C in wet locations.

Example B: What is the ampacity rating of a No. 4/0 AWG, type XHHW copper conductorthat is being used in a wet location?

Answer B: Because the conductor is being used in a wet location, NEC Table 310-13(applicable provisions) lists the conductor operating temperature as 75°C. PerNEC Table 310-16, the ampacity rating of a No. 4/0 AWG, type XHHWcopper conductor, is 230 A.

Number of Conductors

The ampacity ratings of conductors, as listed in NEC Table 310-16, are based on no morethan three conductors installed in a raceway (conduit) at a given ambient temperature of 30°C.Exceeding the number of conductors (3) requires derating.




Current Carrying Conductors - Only conductors that normally carry current are consideredas current carrying conductors for determining the number of conductors in a raceway. Forexample, the equipment grounding conductor (green wire) is not considered a currentcarrying conductor. On the other hand, in a 3-phase, 4-wire, wye-connected system, thecommon conductor (neutral) carries approximately the same current as the phase conductors,and is therefore considered to be a current carrying conductor. Note: See Note 10 to NECTable 310-16 for other examples.

Fill Rate and Derating Factors - When the number of current carrying conductors, asexplained above, exceeds 3 in a raceway, Note 8 to Article 310-16 requires derating of theconductors. For example, 4 to 6 current carrying conductors in a raceway require that eachconductor be derated by 20 percent (.80 reduction factor). Seven to 9 conductors require thateach conductor be derated by 30 percent (.70 reduction factor). Note: For a complete listingof the reduction factors, see Note 8 to NEC Table 310-16 (Work Aid 1J, Handout 2).

Example C: Determine the ampacity of a 3-phase, 4-wire, feeder conductor that is using 500kcmil copper conductors, type THHN insulation (90°C), installed in steelconduit, 40°C ambient temperature, and feeding a mercury vapor lighting load.

Answer C: Because the ambient temperature exceeds 30°C, the conductors must bederated using the correction factor (0.91) from NEC Table 310-16. Note 10 toNEC Table 310-16 also requires that the neutral conductor be considered as acurrent carrying conductor for arc discharge (i.e., mercury vapor) lighting.Therefore, the conductors must be derated using the adjustment factor (0.80)for 4 to 6 current carrying conductors in a raceway (Note 8A to NEC Table310-16). The conductor ampacity is listed in NEC Table 310-16 is 430 A.Therefore, the ampacity rating is 315 A (430 x 0.91 x 0.80).

Example D: What is the ampacity rating of 6, type THHW, 75°C, size No. 4/0 AWG copperconductors in a raceway? Assume that the ambient temperature is 30°C.

Answer D: Per NEC Table 310-16, the ampacity is 230 A for no more than threeconductors in a raceway. Per Note 8 to NEC Table 310-16, the ampacityshould be reduced to 80 percent for 6 conductors. Therefore, the ampacityrating is 184 A(.80 x 230).

Parallel Conductors - NEC Article 310-4 permits parallel conductors as long as all of thefollowing conductor conditions are met: same length, same conductor material, sameinsulation type, and the same terminations.

Because of the skin effect in ac circuits, the current carrying capability per cmil of conductorarea decreases with the size of the conductor. Additionally, it is harder to dissipate the heatwithin large conductors. It is also more difficult to “pull” a large conductor through raceway.




For these reasons, it is often preferable to parallel small conductors rather than to use largeconductors.




Conductor Terminations

Although the NEC specifies the ampacity ratings of conductors, the UnderwritersLaboratories, Inc. (UL) approves the use and conditions of the electrical equipment. Thetermination provisions of the UL-approved equipment are based on 60°C and 75°Cterminations (terminals, lugs, etc.).

100 A or Less - NEC Article 110-14 (c) (1) specifies that termination provisions of equipmentfor circuits rated 100 A or less or marked for sizes No. 14 through No. 1 AWG conductorsshall be used with conductors rated for an operating temperature of 60°C. The NEC permitsthe following two exceptions:

• Higher-temperature-rated conductors may be used, provided the ampacity ofthe conductors is based on the 60°C operating temperature ampacity ratings.

• The equipment is listed for the higher-temperature-rated conductors.

Greater Than 100 A - NEC Article 110-14 (c) (2) specifies that termination provisions ofequipment for circuits rated over 100 A or marked for conductors larger than No. 1 AWGshall be used with conductors rated for an operating temperature of 75°C. The same twoexceptions apply for circuits rated more than 100 A, except that the ampacity of theconductors is based on an operating temperature of 75°C.

Example E: What is the ampacity rating of a 75°C, type THHW, size No. 4 AWGconductor that is being terminated at a 70 A molded case circuit breaker(MCCB)?

Answer E: Because the MCCB is rated at less than 100 A, the ampacity rating of theconductor must be limited to its 60°C rating of 70 A versus its 75°C rating of85 A.

Types of Feeders and Sizing Guidelines

Bus Feeders - SAES-P-104 specifies that bus feeders be sized in accordance with the NEC,but that a 20 percent growth factor must be added. However, the size, including the 20percent growth factor, must not exceed the maximum rating of the bus. SAES-P-104 allowsdeletion of the growth factor where 2 or more feeders serve the same load bus.

Transformer Feeders - SAES-P-104 specifies that feeders shall have an ampacity rating ofnot less than the full load ratings (fan-cooled ratings if fans are installed) of all connectedtransformers and all other connected loads.




Other Feeders - Although not explicitly specified in SAES-P-104, other feeders should besized to carry the load in accordance with the NEC.




Busway

In large commercial and industrial complexes, where high density (ampacity) loads areconnected, it is usually not economical to use insulated conductors as feeders. An alternativemethod is to use a busway, which is a metal enclosure containing factory assembledconductors of copper bars. It is standard industry practice, as well as Saudi Aramco practice,to consider the use of a copper busway whenever the load is greater than or equal to 1000 A.

Ampacity Ratings

Busways are available in the following three design types: low impedance feeder busway,high impedance feeder busway, and simple plug-in busway (Figure 4).

Plug-in busways are available in ampacity ratings ranging from 100 to 3000 A and in shortcircuit current withstand ratings ranging from 10 to 85 kA. Feeder type busways are availablein ampacity ratings ranging from 600 to 5000 A and in short circuit current withstand ratingsranging from 42 to 200 kA.




Figure 4. Busway Types

Industrial Feeder Uses

In industrial applications, feeder busways are almost exclusively used for horizontal runs fromthe main switchboards to the major power centers located throughout the plant. Busway risersare also often used to distribute power to all floors of multistory buildings.




CALCULATING SHORT CIRCUIT RATINGS OF FEEDER CONDUCTORS

Note: Work Aid 1G has been developed to teach the Participant procedures to calculateshort circuit ratings of feeder conductors.

Introduction

A cable must be protected from overheating due to excessive short circuit (fault) currents.The fault point may be on a section of the protected cable or on any other part of the electricsystem. During a phase fault, the I2R losses in the phase conductor elevate first thetemperature of the conductor, followed by the insulation materials, protective jacket, raceway,and surroundings. Since the short circuit current should be interrupted either instantaneouslyor in a very short time by the protective device, the amount of heat transferred from themetallic conductors outward to the insulation and to other materials is very small; therefore,the heat from I2R losses is almost entirely in the conductors. For practical purposes, it isassumed that 100% of the I2R losses is consumed to elevate the conductor temperature.During the period that the short circuit current is flowing, the conductor temperature shouldnot be permitted to rise to the point where it may damage the insulation. Providing cableprotection during a short-circuit condition involves, as a minimum, the following factors:

• Maximum available short circuit currents.• Maximum conductor temperature that will not damage the insulation.• Cable conductor size that affects the I2R value and its capacity to contain the

heat.• Longest time that the fault will exist and the fault current will flow.

Factors Affecting Conductor Short Circuit Ratings

Operating Temperature Versus Short Circuit Temperature

The operating temperature is the initial temperature of the conductor prior to the occurrenceof the short circuit. This operating temperature is assumed to be the temperature of theconductor in a 30°C ambient temperature environment and at rated ampacity, which is 60°C,75°C, or 90°C, as discussed in the previous section of this Information Sheet.

The short circuit temperature is the final temperature of the conductor after the short circuitoccurs. This short circuit temperature is assumed to be the maximum temperature rise that theconductor can sustain for a specified period of time without damaging the insulation.




Figure 5 lists the initial and final temperature ratings for various types of insulation commonlyused in industrial applications. The temperature rise is related by the following formula,which will be discussed in detail later in this Information Sheet.

• (I/A)2t = 0.0297 log10 [(T2 + 234)/(T1 + 234)]

Type ofInsulation

Continuous (Initial)Temperature Rating

T10C

Short Circuit (Final)Temperature Rating

T20C

Rubber

Cross-Linked Polymer

Silicone Rubber

Thermoplastic

Paper

Varnished Cloth

75

90

125

60, 75, 90

85

85

200

250

250

150

200

200

Figure 5. Conductor Temperature Ratings

Operating Speed of Protective Devices

The extraordinary temperatures generated under short circuit conditions may damage cablesover their entire length if the fault current is not interrupted quickly enough. The upstreamprotective devices must clear the fault before the damaging temperatures occur. Manyengineers select cables and/or protective devices such that the backup protective device couldalso clear the fault before thermal damage occurs.

Cable Impedance

Although the cable impedance decreases as the cable size increases, the increased cross-sectional area of the cable increases the withstand capability of the cable. For example, a No.2/0 AWG copper conductor having an impedance of 0.1150 Ω/1000 ft can withstand 30 kA ofshort circuit current for approximately 4 cycles (≈ .067 sec). Conversely, a 500 kcmil copperconductor having roughly half the impedance (0.0551 Ω/1000 ft) can withstand the same 30kA of short circuit current for almost 48 cycles (≈ 0.8 sec). The increased diameter of thecable allows the cable to absorb more heat without damaging the insulation.




Conductor Insulation

Figure 5 listed the maximum conductor temperature ratings. Any prolonged load/ambienttemperature combinations that increase the conductor temperature above the continuous(initial) temperature ratings, for example 75°C for a type THWN conductor, will damage theinsulation. Short circuits that elevate the conductor temperature above the final temperature,for example 150°C for the type THWN conductor, will also damage the insulation.

Short Circuit Current Available (ISCA)

It is general industry practice to use the subtransient reactance (X”d) to calculate theRMS symmetrical short circuit current available where instantaneous overcurrent relays(ANSI Device 50) are used to protect cables. And where cables are protected by fuses, cablelimiters, or low voltage breakers, it is industry practice to use X”

d to calculate theasymmetrical current available. Note: Short circuit currents were discussed in detail in EEX102.

Type of Fault

Three-phase faults and line-to-ground faults in a solidly grounded system are typically of sucha large magnitude that the protective devices quickly detect and clear the faultsinstantaneously, and therefore, usually provide adequate protection for the cables.

On the other hand, high impedance arcing faults are often persistent and escalate into moreserious, high magnitude phase-to-phase faults, resulting in extensive cable damage.

Type of System Grounding

For solidly grounded systems, although the fault current magnitudes are high, extensive cabledamage is usually avoided because of the very quick detection and clearing of faults.

Cable damage in low resistance grounded systems is also typically not extensive because theground fault current magnitudes are small (100 - 1200 A) and are quickly detected andcleared.

Because of the very low current magnitudes in high-resistance-grounded systems, cabledamage is limited unless the fault is not cleared and another ground fault escalates to a phase-to-phase fault resulting in extensive damage.

Line-to-ground faults in ungrounded systems can result in extensive cable insulation damageby the sustained overvoltages rather than by the fault current magnitudes (typically 1 - 10 A).




In ungrounded systems, cables should have, as a minimum, 133% or 173% insulation levelratings. Note: Saudi Aramco design standards do not permit use of ungrounded systems.




Conductor Temperature Rise

Insulation, Material Type, Size

On the assumptions that all heat is absorbed by the conductor metal (copper or aluminum) andthat there is no heat transmitted from the conductor metal to the insulation, the temperaturerise is a function of the conductor size (area in cmils), the magnitude of the short circuitcurrent (ISCA in amperes), and the fault duration (time in sec). These variable are related bythe following formula:

• (I/A)2t = (k1) log10 [(T2 + k2)/T1 + k2)]

• where:I = short circuit current (asymmetrical) in amperes (A)A = conductor cross-sectional area in circular mils (cmils)t = time of short circuit in seconds (sec)T1 = initial conductor temperature in degrees Celsius (°C), as listed by

the manufacturerT2 = final conductor temperature in degrees Celsius (°C), after the

short circuit, as listed by the manufacturerk1 = 0.0297 for copper = 0.0125 for aluminumk2 = 234 for copper = 228 for aluminum

Variables in Formula for Allowable Short Circuit Interrupting Time

The initial (T1) and final (T2) conductor temperatures are predetermined on the basis of thecontinuous current rating and the insulation material. For low voltage thermoplasticconductors specified in Saudi Aramco design standards, T1 will equal 75°C or 90°C and T2

will always equal 150°C (see Figure 5). Because Saudi Aramco design standards do notpermit the use of low voltage aluminum conductors, R1 equals 0.0297 and k2 equals 234.

The short circuit current (I) used in the formula to plot the cable damage curves is notnecessarily the short circuit current available (ISCA) in the event of a fault. The operating timeof the protective device must, however, allow the device to clear the fault for all values offault current in less time than the time (t) calculated in the above equation.




Short Circuit Cable Damage Charts

Cable damage charts (curves) are often used to ensure that the protective device will clear thefault prior to damaging the conductor. Figure 6 shows an Insulated Cable EngineersAssociation (ICEA) format for representing cable damage curves, and Figure 7 shows cabledamage representative curves used by many coordination engineers.

Example F: How quickly must a protective device clear a 30 kA (asymmetrical current)fault to protect a size No. 4/0 AWG copper conductor?

Answer F: A No. 4/0 AWG copper conductor can withstand a 30 kA fault forapproximately 16 cycles (≈ 0.2667 sec).

Example G: A low voltage power circuit breaker (LVPCB), with an instantaneous operatingtime of .05 sec (≈ 3 cycles), is used to protect a 90°C rated thermoplasticconductor in a circuit where 35 kA of asymmetrical fault current is available.What is the minimum size conductor (cross-sectional area in cmil) that willsafely carry the 35 kA for 0.05 sec?

Answer G: Rewriting the formula described previously in this Information Sheet yields thefollowing:

• A =[(I2t)/(.0297 log10 ((T2 + 234)/(T1 + 234)))]1/2

=[(35000)2 (.05)/(.0297 log10 ((150 +234)/(90+234)))]1/2

=167, 165 cmil

• Per NEC Table 8, Chapter 9, a size 3/0 AWG conductor has a cross-sectional area of 167,800 cmil.




Figure 6. ICEA Cable Damage Curves




Figure 7. Cable Damage Curves For Thermoplastic Insulation (750C - 1500C)




Protective Device Clearing Times

The total clearing time of various types of protective devices depends on the types of circuitbreakers (molded case or low voltage power) or fuses (current or non-current limiting) thatare being used.

Circuit Breakers

Note: All of the following times are approximate; the time/current characteristics of thespecific device being used should be checked for exact times.

Molded Case Circuit Breakers (MCCBs) typically operate in 1.1 (0.018 sec) cycles for lessthan or equal to 100 ampere frame (AF) sizes and 2.25 cycles (0.038 sec) for 225 AF andlarger sizes in their instantaneous ranges (5 times ampere trip), and over 100 seconds for lowmagnitude overloads.

Low Voltage Power Circuit Breakers (LVPCBs) typically operate in 3 cycles (0.05 sec) intheir instantaneous ranges, 6 - 30 cycles (0.10 - 0.50 sec) in their short time ranges, over 6000cycles (100 sec) in their long time ranges, and 6 - 30 cycles (0.10 - 0.50 sec) in their groundfault ranges.

Low Voltage Fuses

Low voltage fuses clear faults in approximately 60,000 cycles (1000 sec) at 1.35 to 1.5 timestheir continuous current ratings and in 0.25-0.50 cycles (0.004-0.008 sec) in their current-limiting ranges.

Typical Time/Current (T/C) Plot

Circuit Breakers - Figure 8 is a time/current characteristic plot of a LVPCB (DS-206)protecting a 500 kcmil copper conductor.




Figure 8. LVPCB Protection of a Cable




Fuses - Figure 9 is a time/current characteristic plot of a 20 A fuse protecting a No. 12 AWGcopper wire and a 400 A fuse protecting a 500 kcmil copper wire.

Figure 9. Fuse Protection of Cables




CALCULATING VOLTAGE DROPS OF A FEEDER CONDUCTOR CIRCUIT

Note: Work Aid 1H has been developed to teach the Participant procedures to calculatevoltage drops of a feeder conductor circuit.

Introduction

Designers of power systems must have practical knowledge of voltage drop calculations, notonly to meet required codes and standards, but to ensure that the required voltage of aparticular piece of equipment, for example a motor, is kept within manufacturer’s specifiedtolerances in order to prevent damage to the equipment. Note: The effects of voltage dropson different types of electrical equipment was discussed in Modules EEX 102.02 and EEX102.04.




Factors Affecting Voltage Drop Calculations

The voltage drop on a feeder depends on the following factors (Figures 10 and 11).

• Load Data

• Feeder Lengths

• Feeder Impedance

Figure 10. One-Line Diagram

Figure 11. Feeder Diagrams




Load Data

Voltage - The voltage drop (VD) is the difference between the voltage at the source end (VS)of the feeder, which is assumed to be a fixed value, and the voltage at the load end (VL) of thefeeder, which varies as a function of the load or feeder current (IL). The voltage drop VD

calculated is a line-to-neutral drop (one-way), the line-to-line voltage drop calculated for asingle-phase system is 2VD, and the line-to-line voltage drop calculated for a three-phasesystem is 3VD.

Current - The current (IL) flowing in the circuit is assumed to be the load or feeder current.The voltage drop is the load current times both the resistance (ILR) and reactance (ILX) of thecircuit.

Power Factor - The power factor (p.f.) of the load directly affects the voltage drop. As thepower factor decreases, the voltage drop will increase. For purposes of this Module, assumethat all power factors are lagging power factors.

Feeder Lengths

The voltage drop (ILZ) is directly proportional to the feeder lengths because the lineimpedance Z depends on the length. As the length of the feeder increases, the impedance ofthe line increases.

Feeder Impedance

Resistance - The resistance (R) per unit of length depends on the following factors.

• Conductor Material

• Conductor Size/Length

• Conductor Operating Temperature

Reactance - The inductive reactance (X) per unit of length depends on the following factors:

• Conductor Diameter

• Conductor Spacing

• Power Supply Frequency

• Raceway Type




Effects of Resistance Variables on Voltage Drop

Conductor Material

The resistance (R) of a conductive material is directly proportional to the resistivity (ρ) of thematerial as expressed by the following formula:

• R = ρL/A

Because the resistivity of copper is 10.4 Ω-cmil/ft, versus 17.0 Ω-cmil for aluminum, theresistance of a copper wire, of a given length and cross-sectional area, is less than theresistance of an aluminum wire of the same length and cross-sectional area.

Conductor Size/Length

Resistance is inversely proportional to the size (cross-sectional area) and directly proportionalto the length, which is again expressed by the following formula:

• R = ρL/A




Operating Temperature

Resistance values of conductors are usually provided at a given temperature, for example,30°C (86°F). As the temperature of the conductor increases, either because of an increase inload or because of ambient temperature, resistance will also increase. For a copper conductor,the change in resistance as a function of temperature can be represented by the followingformula(Figure 12):

• R2 = R1 [(234.5 + T2)/(234.5 + T1)]

• where R1 and R2 are the resistances of the conductor in ohms (Ω) attemperatures T1 and T2 respectively, in degrees Celsius (°C).

Figure 12. Resistance Versus Temperature Relationships




Example H: The ac resistance of a 500 kcmil copper conductor in steel conduit in a 30°Cambient environment is .029 Ω/1000 ft. What is the ac resistance of 2500 ft ofa 500 kcmil copper conductor in a 50°C ambient environment?

Answer H:

• T1 = 30°C, T2 = 50°C

• R1= (.029 Ω/1000 ft)(2500 ft) = .0725 Ω

• R2= R1 [(T2 + 234.5)/(T1 + 234.5)]= (.0725)[(50 + 234.5)/(30 + 234.5)]≈ .078 ΩΩ

Effects of Reactance Variables on Voltage Drop

Conductor Diameter

As the conductor diameter increases (larger wire sizes), the reactance of the conductordecreases because of mutual self-inductance.

Conductor Spacing

The total reactance of a line (X) actually consists of two terms, Xa and Xd, where Xa is afunction of the conductor material and Xa is a function of conductor spacing. As the spacingincreases, the reactance (Xd) increases because of mutual inductance. This increase inreactance is typically not evident for low voltage conductors because the reactance values intables already account for spacing.

Frequency

As indicated by the following formula, conductor reactance is directly proportional tofrequency:

• X = ωL = 2πfL

• where: ω = angular frequency in radians/secf = frequency in cycles/sec or Hertz (Hz)

Example I: The reactance of a 2000 ft run of a 500 kcmil copper conductor in steel is .096Ω at 60 Hz. What is the conductor’s reactance at 50 Hz? at 400 Hz?




Answer: (1) X60/60 = X50/50X50 = (50/60) X60 = (5/6)(.096) = .08 ΩΩ

(2) X60/60 = X400/400X400 = (400/60) X60 = (20/3)(.096) = .64 ΩΩ

Raceway Type

If the material (raceway) surrounding the conductor is magnetic, such as steel conduit, thereactance is greater than if the raceway is non-magnetic, such as aluminum or plastic conduit.

Voltage Drop Limits

National Electric Code (NEC)

NEC Article 215-2(b), FPN No. 2, states that “conductors for feeders as defined in Article100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power,heating, and lighting loads, or combinations of such loads, and where the maximum totalvoltage drop on both feeder and branch circuits to the farthest outlet does not exceed 5percent, will provide reasonable efficiency of operation”.

Saudi Aramco Standard SAES-P-100

SAES-P-100 specifies the following criteria pertaining to feeder voltage drops for non-motorcircuits under 600 volts:

• Maximum steady state voltage drop for a main, feeder, and branch circuit shallnot exceed five (5) percent.

• Summation of voltage drops in circuit from main to distribution center, andfrom feeder breaker to panelboard shall be two (2) percent at full load.




Approximation Formula

Phasor Relationships

Due to the phasor relationships shown in Figure 13, exact methods of calculating line voltagedrops require extensive knowledge of complex phasor algebra as was previously discussed inModule EEX 102.03. However, in most industrial electrical distribution systems, such asfound on Saudi Aramco installations, the approximate method formula, as presented below, isadequate for most line voltage drop calculations.

• Approximate Method Formula:

VD = IR cos θ + IX sin θ = I(R cos θ + X sin θ) = IZ

• where: VD = line-to-neutral voltage drop, volts (V), (one way)I = line current, amperes (A)R = circuit line resistance, ohms (Ω)X = circuit line reactance, ohms (Ω)Z = circuit line impedance, ohms (Ω)Z = R cos θ + X sin θθ = load power factor angle, degreescos θ = load power factor, decimalssin θ = load reactance factor, decimalsVS = source voltage, line-to-neutral, volts (V)VL = load voltage, line-to-neutral, volts (V)VD(line-to-line, 1φ system) = 2VD

VD(line-to-line, 3φ system) = 3VD




Figure 13. Voltage Drop Phasor Diagrams




Variable Assumptions

The error caused by variation of load current and power factor because of the voltage appliedto the load is not considered in the above formula for calculating voltage drops. In otherwords, the load current (IL) is assumed to be constant regardless of the voltage. However, ifthe error is significant, an iterative method may be used. Once the voltage drop is calculated,subtract the voltage drop (VD) from the source voltage (VS) and use this value to calculate anew load current, and then, using the new load current calculated, repeat the voltage dropcalculation. Generally, if the total drop calculated is less than ten percent, the iterativemethod is not used.

By convention, in the above formula, sin θ is considered to be positive for lagging powerfactor loads. As the angle of the load voltage approaches the angle of the line voltage, theerror in the calculation approaches zero.




SELECTING FEEDER PROTECTIVE DEVICES

Note: Work Aid 1I has been developed to teach the Participant procedures to select feederprotective devices.

Types of Protective Devices

Note: SAES-P-114 restricts low voltage line protection to circuit breakers.

Circuit Breakers

Molded-Case Circuit Breakers (MCCB) are a class of breaker rated at 600 volts and below,and they consist of a switching device and an automatic protective device assembled in anintegral housing of insulated material. MCCBs are capable of clearing a fault more rapidlythan a low voltage power circuit breaker (LVPCB). Solid-state trip units incorporated intosome styles of MCCBs provide for their coordination with LVPCBs. MCCBs are generallysealed to prevent tampering, and this sealing, in turn, precludes any inspection of the MCCBcontacts. MCCBs are generally not designed to be maintained in the field, and manufacturersrecommend total replacement if a defect appears.

MCCBs are available in several different types. The thermal magnetic type, which is the mostwidely used, employs thermal tripping for overloads and magnetic tripping for short-circuits.The magnetic type MCCB employs only instantaneous magnetic tripping for cases where onlyshort circuit interruption is required. The integrally-fused type MCCB combines regularthermal magnetic protection, together with current limiting fuses, to respond to applicationswhere higher short circuit currents are available. In addition, the current limiting type MCCBoffers high interrupting capacity protection, while at the same time limiting the let-throughcurrent to a significantly lower value than is usual for conventional MCCBs.

Low-Voltage Power Circuit Breakers (LVPCB), like MCCBs, are rated 600 volts andbelow. They differ from MCCBs, however, because they are typically open-constructionassemblies on metal frames and all parts are designed for accessible maintenance, repair, andease of replacement. LVPCBs are intended for service in switchgear compartments or inother enclosures of dead- front construction. Tripping units are field-adjustable and includeelectromagnetic, direct-acting, and solid-state types. LVPCBs can be used with integralcurrent-limiting fuses to meet interrupting requirements up to 200 kA RMS symmetrical.




Saudi Aramco Applications of Fuses

Low voltage fuses (600 volts or less) come in two basic shapes: the plug fuse and thecartridge fuse. Fuses may be current limiting or non-current limiting. Cartridge fuses areeither renewable or nonrenewable. Nonrenewable cartridge fuses are assembled at thefactory, and they are replaced after they open (melt) in service. Renewable cartridge fusescan be disassembled, and the fusible element can be replaced. A special type of low voltagefuse that is sometimes used is the dual-element or time-delay fuse. The dual-element fuse hasone element that is fast-acting, and it responds to overcurrents in the short circuit range, whileits other element permits short-duration overloads, but melts if the overload is sustained.Note: Saudi Aramco has very limited application of low voltage fuses.

Molded Case Circuit Breaker (MCCB) Ratings

The MCCB gets its name from the material (plastic) and manufacturing process (molded)used to make the frame (case) of the breaker. Figure 14 describes a MCCB. The MCCBserves as the disconnect, overload, and fault protection for the feeder circuit.

Figure 14. Molded Case Circuit Breaker (MCCB)




Frame Size

The frame size of the MCCB describes the physical size and continuous current ratings of theMCCB. Each different type and size of MCCB is assigned a frame designation. MCCBshave many different trip and interrupting ratings for the numerous frame sizes. Figure 15 liststhe different frame sizes for typical MCCBs.

Trip Ratings

The function of the trip unit in a MCCB is to trip the operating mechanism in the event of aprolonged overload or short circuit current. The traditional MCCB uses electromechanical(thermal-magnetic) trip units. Protection is provided by combining of a temperature sensingdevice (bimetallic strip) with a current sensing electromagnetic device, both of which actmechanically on the trip mechanism. More modern breakers use sophisticated electronic tripunits, which can be more precisely modeled than the electromechanical trip units. NECArticle 240-6 lists the standard trip ratings of MCCBs. Figure 15 also lists the continuouscurrent or trip ratings of typical MCCBs.




Interrupting Ratings

The interrupting rating or short circuit rating at a 40°C ambient temperature is commonlyexpressed in root mean square (RMS) symmetrical amperes. The interrupting capability ofthe breaker may vary with the applied voltage. For example, a breaker applied at 480 voltscould have an interrupting rating of 25,000 A at 480 volts, but the same breaker applied at240 volts may have an increased interrupting rating of 65,000 A.




All MCCBs operate instantaneously at currents well below their interrupting rating. Non-adjustable MCCBs will usually operate instantaneously at current values approximately fivetimes (5x) their trip rating. Low voltage breaker contacts separate and interrupt the faultcurrent during the first cycle of short circuit current. Because of this fast operation, themomentary and interrupting duties are considered to be the same. Therefore, all faultcontribution from generators, motors, and the dc components of the fault waveform must beconsidered. Figure 15 also lists the symmetrical and asymmetrical interrupting ratings oftypical MCCBs.

LineNo.

FrameSize

RatedContinuous

Interrupting Current Rating (AIC)(amps)

(amps)(AF)

Current(amps)(AT)

240 VoltsSym Asym

480 VoltsSym Asym

600 VoltsSym Asym

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

100

100

100

225

225

225

225

225

400

400

400

600

800

800

800

1000

1200

10-100

10-100

10-100

125-200

70-225

70-225

70-225

70-225

200-400

200-400

200-400

300-600

300-800

300-800

600-800

600-1000

700-1200

18,000

65,000

100,000

22,000

25,000

65,000

100,000

35,000

65,000

100,000

42,000

100,000

42,000

65,000

100,000

42,000

42,000

20,000

75,000

--

25,000

30,000

75,000

--

40,000

75,000

--

50,000

--

50,000

75,000

--

50,000

50,000

14,000

25,000

100,000

18,000

22,000

35,000

100,000

25,000

35,000

100,000

30,000

100,000

30,000

35,000

100,000

30,000

30,000

15,000

30,000

--

20,000

25,000

40,000

--

30,000

40,000

--

35,000

--

35,000

40,000

--

35,000

35,000

14,000

18,000

100,000

14,000

22,000

25,000

100,000

22,000

25,000

100,000

22,000

100,000

22,000

25,000

100,000

22,000

22,000

15,000

20,000

--

15,000

25,000

30,000

--

25,000

30,000

--

25,000

--

25,000

30,000

--

25,000

25,000

Figure 15. Typical MCCB Ratings




Low Voltage Power Circuit Breaker (LVPCB) Ratings

Frame Size

The rated continuous current of a LVPCB is the designated limit of RMS current, at ratedfrequency, that it is required to carry continuously without exceeding the temperaturelimitations based on a 40°C ambient temperature. The temperature limit on which the ratingof circuit breakers are based are determined by the characteristics of the insulating materialsused and the metals that are used in the current carrying components and springs. Standardframe size ratings for low voltage power circuit breakers are 800, 1600, 2000, 3200, and 4000amperes. Some manufacturers may have additional frame sizes. LVPCPs have either anelectromechanical trip or a solid-state trip that is adjustable or interchangeable from aminimum rating up to the ampere rating of the frame. Figure 16 lists the frame sizes oftypical LVPCBs.

Frame Size (amperes) Available Sensor Ratings (amperes)

8001600200032004000

50, 100, 150, 200, 300, 400, 600, 800100, 150, 200, 300, 400, 600, 800, 1200, 1600100, 150, 200, 300, 400, 600, 800, 1200, 1600, 20002400, 32004000

Figure 16. LVPCB Frame and Sensor Ratings

Sensor Ratings

A sensor on an LVPCB is simply a current transformer that reduces (transforms) the highmagnitude line currents to a magnitude that the trip units (typically electronic) can safelycarry for short periods of time. Figure 16 also lists the sensor ratings for an LVPCB.




Short Time Ratings

LVPCBs are designed and marked with the maximum voltage at which they can be applied.LVPCBs can be used on any system where the voltage is lower than the breaker rating. Theapplied voltage will affect the interrupting rating of the breaker. Standard maximum voltageratings for LVPCBs are 635 volts, 508 volts, and 254 volts. LVPCBs are usually suitable forboth 50 and 60 Hz.

The short-time current rating of a LVPCB specifies the maximum capability of the circuitbreaker to withstand the effects of short circuit current flow for a stated period, typically 30cycles or less. The short-time delay on the breaker’s trip units corresponds to the short-timecurrent rating. This time delay provides time for downstream protective devices closer to thefault to operate and to isolate the circuit. The short-time current rating of a modern dayLVPCB without an instantaneous trip characteristic is usually equal to the breaker’s shortcircuit interrupting rating. By comparison, MCCBs usually do not have a short-time rating.Figure 17 lists the short-time interrupting rating of typical LVPCBs.

Frame Interrupting Ratings (RMS Symmetrical Amperes)Size

(amperes)With Instantaneous Trip Short-Time Ratings (30cycles)

(With Short-Delay)

8001600200032004000

208-240V42,00065,00065,00085,000

130,000

480 V30,00050,00065,00065,00085,000

600 V30,00042,00050,00065,00085,000

208-240V30,00050,00065,00065,00085,000

480 V30,00050,00065,00065,00085,000

600V30,00042,00050,00065,00085,000

Figure 17. LVPCB Short-Time and Interrupting Ratings

Interrupting Ratings

The interrupting rating of a LVPCB is the RMS symmetrical current rating of the circuitbreaker. The asymmetrical interrupting rating is implied, and it is based on an X/R ratio of6.6 for unfused breakers and on an X/R ratio of 4.9 for fused breakers. An X/R ratio of 6.6corresponds to an asymmetrical factor (Ma) of 1.17. Under short circuit conditions, most lowvoltage systems have an X/R ratio of less than 6.6. Therefore, if an asymmetrical interruptingrating is not listed by the manufacturer, assume that the asymmetrical rating is 1.17 times thesymmetrical rating. Figure 17 also lists the symmetrical interrupting ratings of typicalLVPCBs.




Protective Device Time/Current (T/C) Characteristics

The response curves of all protective devices are plotted on common graphs so that they maybe compared at all current and time points. The standard means used to plot device T/Ccharacteristics is to plot the devices on log-log graph paper (Figure 18).

Standard log-log graphs are 4.5 cycles on the horizontal scale that represents current. Thecurrent axis ranges from 0.5 to 10,000 amperes. The vertical axis, representing time, rangesfrom 0.01 to 1000 seconds and/or 0.6 to 60,000 cycles. Because current limiting fuses andmolded case circuit breakers may operate in less than 0.5 cycles (.00835 seconds),manufacturers of these devices may reproduce T/C characteristic curves with 6 cycle verticalscales that represent times ranging from 0.001 to 10,000 seconds (.06 to 600,000 cycles). Thehorizontal current scale is also often “shifted” for a particular plot by multiplying the currentscale by a factor of 10, 100, or 1000 (x10, x100, x1000).

Thermal-Magnetic MCCB

SAES-P-114 permits thermal-magnetic (inverse-time) MCCBs for the protection of feeders.The MCCB is sized to protect the low voltage feeder conductors in accordance with theirampacities after all derating factors have been applied. The NEC permits the next standardsize MCCB that is available to protect the conductor if the MCCB rating does not exceed 800A. If the breaker rating being selected exceeds 800 A, the NEC requires the next smaller sizebreaker to be selected. Figure 19 shows a 600 A MCCB that protects a parallel set of 500kcmil, copper, type THWN feeder conductors.




Figure 18. Typical Log-Log Paper




Figure 19. MCCB Protecting A Feeder Conductor




An MCCB is tested in open air (not in an enclosure) to verify its nameplate ampere rating.The nameplate specifies a value of current that the circuit breaker is rated to carrycontinuously, without tripping, within specific operating temperature guidelines. In mostcases, however, an MCCB is used in an enclosure, but not in open air. Therefore, theperformance of a breaker inside an enclosure, as with any other overcurrent device, could beadversely affected by heat dissipation and temperature rise. These factors must be consideredregarding the ability of the breaker to comply with its nameplate ampere rating.

Section 220-10(b) of the NEC, which covers continuous and noncontinuous loads, states that:“Where a feeder supplies continuous loads or any combination of continuous andnoncontinuous loads, the rating of the overcurrent device shall be less than the noncontinuousload plus 125% of the continuous load”. The same NEC section permits the followingexception: “Where the assembly including the overcurrent devices protecting the feeder(s)are listed for operation at 100% of their rating, neither the ampere rating of the overcurrentdevice nor the ampacity of the feeder conductors shall be less than the sum of the continuousload plus the noncontinuous load”. Figure 20a shows a typical application using a standarddesign approach and Figure 20b shows the same application using 100 percent rated MCCBs.




Figure 20. Typical MCCB Applications




Low Voltage Power Circuit Breaker (LVPCB)

Long Time Functions include the long time pickup and the long time delay functions. Longtime pickup (LTPU) functions or long delay pickup (LDPU) settings are adjustable from0.5 - 1.0 times the plug rating (In). The plug rating is also a function of the sensor rating(current transformer), which establishes the continuous current rating of the breaker. Typicaltolerances for a modern day solid-state trip (SST) (e.g., Westinghouse Digitrip RMS) are -0%, + 10% (Figure 21).

Figure 21. Long Time Pickup (LTPU) T/C Characteristics




Long time delay (LTD) or long delay time (LDT) settings are adjustable from 2 - 24 secondsat6 times the plug rating (6In). Typical tolerances are + 0%, - 33% (Figure 22).

Figure 22. Long Time Delay (LTD) T/C Characteristics

Short Time Functions include the short time pickup and short time delay functions. Shorttime pickup (STPU) or short delay pickup (SDPU) settings are adjustable from 2 to 6 timesthe plug rating (2 - 6In) plus two variable settings of S1 (8In) and S2 (10In). Typical tolerancesare + 10%, - 10% (Figure 23).




Figure 23. Short Time Pickup (STPU) T/C Characteristics




Short time delay (STD) or short delay time (SDT) settings are available with five flatresponses of 0.1, 0.2, 0.3, 0.4 and 0.5 seconds. Typical tolerances are variable depending onthe setting (Figure 24).

I2t Function settings are available in three responses of 0.1*, 0.3*, and 0.5* seconds, and thesettings revert back to a flat response at 8In (Figure 24). The asterisk (*) refers to I2t settings.

Figure 24. Short Time Delay (STD) With I2t T/C Characteristics




Instantaneous Trip (IT) settings are adjustable from 2 to 6 times the plug rating (2 - 6In)plug two variable settings of M1 (8In) and M2 (12In). Typical tolerances are + 10%, - 10%(Figure 25).

Figure 25. Instantaneous Trip (IT) T/C Characteristics

Ground Fault Functions - SAES-P-114 requires ground fault protection, where required bythe NEC, that uses window-type current transformers (CTs) similar to the BYZ CT, as shownin Figure 26. The tripping function, unlike its MCCB counterpart, is part of the same SSTunit.




Figure 26. GFP With Window-Type CT




Ground fault pickup (GFPU) settings have eight discrete adjustments (A, B, C, D, E, F, H, K),which are a function of the plug ratings (In). Figure 27 shows a sample listing of the settingsfor plug ratings of 100, 200, 250, and 300 amperes. Typical tolerances are + 10%, - 10%(Figure 28).

In A B C D E F H K100 25 30 35 40 50 60 75 100200 50 60 70 80 100 120 150 200250 63 75 88 100 125 150 188 250300 75 90 105 120 150 180 225 300

Figure 27. Sample GFPU Code Letters and Settings

Figure 28. Ground Fault Pickup (GFPU) T/C Characteristics




Ground fault time (GFT) or ground fault delay time (GFDT) settings, like the SDT settings,are available with 5 flat responses of 0.1, 0.2, 0.3, 0.4, and 0.5 seconds. Typical tolerancesare variable depending on the setting (Figure 29).

I2t functions settings are also available for ground fault functions in 3 responses of 0.1*, 0.2*,and 0.3* seconds, and the settings revert back to a flat response at 0.625 In (Figure 29).

Figure 29. Ground Fault Time (GFT) With I2t T/C Characteristics




Coordination Principles

Molded Case Circuit Breakers (MCCBs) are difficult to coordinate, especially in thesmaller frame sizes of 100 and 225 amperes. Even the larger frame breakers are difficult tocoordinate unless sufficient impedance (very long cable runs) exists between the feederbreakers and the other downstream breakers. Modern-day larger frame breakers with solid-state (electronic) trips are much easier to coordinate.

Two MCCBs in series will coordinate if their plotted T/C characteristic curves do not overlapor cross one another.

Low Voltage Power Circuit Breakers (LVPCBs) are fairly simple to coordinate because ofthe numerous settings and adjustments available for their long time, short-time, instantaneous,and ground fault functions.

As with MCCBs, two LVPCBs in series will coordinate if their plotted T/C characteristiccurves do not overlap or touch one another.

Figure 30 shows a main breaker (DS-632) coordinating with a feeder breaker (DS-206) that isprotecting a 500 kcmil copper conductor.




Figure 30. LVPCB Coordination




Factors Affecting Selection

The properly rated MCCB for a specific application can be selected by determining thefollowing listed parameters. There are other factors, for example, unusual operatingconditions, altitude, frequency, etc; however, for purposes of this Module, the factorsaffecting MCCB selection are limited to the following:

• Temperature• Continuous Loads• Voltage• Round-up Rule• Round-down Rule

Temperature

Thermal magnetic circuit breakers are temperature sensitive. At ambient temperatures below40°C, circuit breakers carry more current than they are rated to carry in continuous current.Nuisance tripping is not a problem under these lower temperature conditions, althoughconsideration should be given to closer protection coordination to compensate for theadditional current carrying capability. In addition, the actual mechanical operation of thebreaker could be affected if the ambient temperature is significantly below the 40°C standard.Note: The 40°C standards for MCCBs is based on the fact that 40°C is the averagetemperature of an enclosure, for example, a lighting panelboard.

For ambient temperatures above 40°C, breakers will carry less current than they are rated tocarry in continuous current. This condition promotes nuisance tripping, and it can createunacceptable temperature conditions at the terminals. Under this condition, the circuitbreaker should be recalibrated for the higher ambient temperature. Recalibrating MCCBs isnot a “user” option; recalibration must be accomplished by the manufacturer. The best optionwhen ordering new breakers is to order ambient-compensated breakers.

Continuous Loads

MCCBs are rated in amperes at a specific ambient temperature. The ampere rating is thecontinuous load current that the breaker will carry in the ambient temperature for which it iscalibrated. Most manufacturers calibrate their standard breakers for a 40°C (104°F) ambient.

The continuous current ampere rating typically signifies the amount of current that the MCCBis designed to carry continuously in open air. In accordance with Article 220-10(b) of theNEC, all overcurrent protection devices may be loaded to a maximum of 80% of theircontinuous ampere rating, unless specifically listed for 100% application. A number ofmanufacturers offer circuit breakers that can be applied at 100% of their continuous rating.These 100% rated breakers specifically outline on their nameplates the minimum size




enclosure, the minimum ventilation (if needed), and the minimum conductor size forapplication at 100% of their ratings.




Voltage

The voltage rating of an MCCB is determined by the maximum voltage that can be appliedacross its terminals, the type of distribution system, and how the breaker is applied in thesystem. For example, the most prevalent secondary distribution voltage in commercial andinstitutional buildings today is 480Y/277 volts, with a solidly grounded neutral. It is also avery common secondary voltage in industrial plants and even in some high rise commercialbuildings that are centrally air-conditioned. In panelboards, it is important that the MCCBhave the lowest possible voltage rating that will do the job and meet the specifications. Anoverly conservative selection can make a considerable difference in the cost.

It is critical that the proper application, testing, and governing standards be understood beforeany attempt is made to apply dual voltage-rated breakers. Article 240-83(e) of the NECdefines the allowed application of circuit breakers with straight or slash voltage markings.

International applications, where 415Y/240, 380Y/220, or 220Y/127 volt systems are used,are all grounded “Y” systems similar to the U.S. 480Y/277 volt system. Only IEC 947-2breakers tested per Appendix C and marked accordingly (C 480V) are suitable for applicationon delta systems. Single pole testing at maximum voltage is not a requirement of IEC 947-2except for this optional test under Appendix C. This test is a standard test under the UL 489requirements for all voltage ranges.

Round-up and Round-down Rules

Per NEC Article 240-3, conductors should be protected against overload in accordance withtheir ampacities as specified by NEC Article 310-15 and Tables 310-16 through 310-19 forconductors rated 0-2000 V. The next higher standard device is permitted if the protectivedevice does not exceed 800 A (round-up rule). See NEC Article 240-3(b). The conductorampacity must be greater than the protective device setting/rating if the protective deviceexceeds 800 A (round-down rule). See NEC Article 240-3(c).




SELECTING FEEDER CONDUCTOR RACEWAY SIZES

Note: Work Aid 1J has been developed to teach the Participant procedures to select feederconductor raceway sizes.

Types of Raceways

Note: For purposes of this Module, only wireways and conduits will be considered inselecting of feeder conductor raceway sizes.

Wireways

The NEC (Article 362-1) defines wireways as sheet metal troughs with hinged or removablecovers for housing and protecting electric wires and cable and in which conductors are laid inplace after the wireway has been installed. Figure 31 describes a typical length of wirewaywith a hinged cover. Note: The NEC does not permit concealment of wireway; it mustremain exposed.

Figure 31. Length of Wireway

Conduits

SAES-P-114 lists the following specifications pertaining to conduits:

• Conduit, above ground in outdoor, industrial facilities shall be hot-dipgalvanized rigid steel.

• Direct buried conduit shall be (a) threaded, hot-dip galvanized rigid steel andpolyvinyl chloride (PVC) coated or (b) type direct burial (DB) PVC conduit perNEMA TC 8.

• Other types of conduits shall be permitted where used and installed inaccordance with the NEC.




Rigid Metal Conduit (RMC) has almost unlimited restrictions for use in industrialinstallations. RMC is most often used where moisture is present and where cables aresubjected to mechanical damage. RMC uses threaded fittings.

Intermediate Metal Conduit (IMC) is very similar to RMC, and it also has very limitedrestrictions for use. The major difference between the two conduits is that RMC providesbetter mechanical damage for conductors than does IMC. Note: SAES-P-104 prohibits theuse of IMC in classified areas.

Electric Metallic Tubing (EMT) is a non-threaded metallic conduit used primarily for indoorprotection of conductors. EMT is not permitted in permanent moisture locations, although itis permitted for both exposed and concealed wet locations. EMT uses compression typecouplings and fittings.

Rigid Nonmetallic Conduit (PVC) is permitted for both concealed and exposed locations aslong as it is not subject to physical damage. PVC is not permitted in hazardous locations.

Flexible Metal Conduit is most often used for motor circuits or other types of circuitssubjected to vibrations. Flexible metal conduit is not permitted in underground installations,in most hazardous locations, in wet locations, or in any areas subject to physical damage.

Factors Affecting Raceway Size

The number of conductors permitted in a raceway is restricted by the NEC. The total cross-sectional area of the conductors, which includes the insulation, must not exceed a specifiedpercentage of the wireway or conduit cross-sectional area. The NEC refers to this restrictionas “percentage fill”. Exceeding the percentage fill can cause physical damage to theconductors as they are being installed (pulled) through the raceway. Additionally, the heatbuildup in the raceway could be excessive, resulting in damage to the insulation.

Conductor Insulation

Because each type of conductor has different insulation thicknesses, the percentage fill foreach type of conductor (insulation) is different. Chapter 9, Table 5, of the NEC lists thedimensions (diameter and area) for each size and type of rubber-covered and thermoplastic-covered conductors.




Number of Conductors

The number of the same type of conductors (insulation) in conduit can be determined fromChapter 9, Tables 3A and 3B of the NEC. The number of conductors in wireway, or mixedsizes and types of conductors (insulations), must be calculated based on the fill rates permittedby the NEC.

Tables - NEC Tables 3A and 3B are based on a 40 percent fill rate of conductors in a giventrade size of conduit (Figure 32). If conductor insulation types are mixed, the tables cannotbe used and the fill rate for a particular mix of conductors in a given trade size of conduitmust be calculated. The number of conductors, for fill rate purposes, includes all conductors,regardless of whether they are considered as current-carrying conductors. For example, the“green” equipment grounding conductor is considered for percentage fill restrictions eventhough it does not carry current except under line-to-ground fault conditions.

Figure 32. Conduit Fill Rate Example

Example J: Referring to Table 3B of the NEC, what is the maximum number of 500 kcmil,type THHN conductors in 3-inch conduit?

Answer J: Per Table 3B (page 914 of Work Aid 1A, Handout 1), the maximum number of500 kcmil type THHN conductors in 3-inch conduit is 4.




Fill Rate Calculations - NEC Article 362-5 restricts the fill rate of wireways to the following:no more than 30 current-carrying conductors, and/or the sum of all conductor areas shall notexceed a fill rate of 20 percent of the interior cross-sectional area of the wireway. Note: Thederating factors specified in Article 310 for more than 3 conductors in a raceway are notapplicable to the 30 current-carrying conductors and 20 percent fill rate specified above.

Table 1, Chapter 9, of the NEC specifies a fill rate of 40 percent for more than threeconductors in a raceway. Table 4 lists the diameter, total area, and 40 percent fill rate area ofconduit. Tables 5, 5A, and 5B lists the dimensions (diameter and area) of bare, rubber-covered, and thermoplastic-covered conductors. All of the tables must be used to calculatethe fill rate of a conduit, when the insulation types and wire sizes installed in conduit aremixed.

Example K: Given the following list (Figure 33) of conductors, what is the minimum sizeconduit permissible to enclose the conductors?

CircuitIdentification

InsulationType

Size of CopperConductors

Feeder AFeeder BFeeder C

THHWTHHNTHHN

3-No.4/0, 1-No.2/04-No.2/0, 1-No.1/0

4-No.1/0, 1-No.2

Figure 33. Example K Conductor Listing

Answer K:

Feeder A: 3 x 0.3904 + 1 x 0.2781 = 1.4493 in2 (Table 5)Feeder B: 4 x 0.2265 + 1 x 0.1893 = 1.0953 in2 (Table 5)Feeder C: 4 x 0.1893 + 1 x 0.1182 = 0.8754 in2 (Table 5)

Total = 3.4200 in2

Per Table 4, a 3.5-inch conduit (3.96 in2) is acceptable at a 40 percent fill rate.




WORK AID 1: RESOURCES USED TO DESIGN A LOW VOLTAGE NON-MOTORFEEDER CIRCUIT

Work Aid 1A: 1993 National Electric Code Handbook

For the content of Work Aid 1A, refer to Handout 1.

Work Aid 1B: ANSI/IEEE Standard 141-1986 (Red Book)

For the content of Work Aid 1B, refer to Handout 2.

Work Aid 1C: SAES-P-114

1. Section 4.2.1 specifies that all system and equipment protection shall conform toNFPA 70 (National Electric Code - NEC) as supplemented by this Standard.

2. Section 4.7.1 specifies that low voltage feeder circuits shall be protected by circuitbreakers and that all interrupting devices shall be fully rated.

3. Section 4.7.3 specifies that overcurrent protection for wires and cables connected tofull-size molded case circuit breakers shall be based on NEC Article 110-14.

4. Section 4.8.3 specifies that all low voltage power circuit breakers shall include eitheran integral protective device with adjustable ground unit or shall be tripped by aground overcurrent relay.

5. Section 4.8.4 specifies that low voltage 480 volt molded case circuit breakers rated 70amperes and greater shall be provided with one of the following:

a) An electronic trip device with adjustable ground unit, where available from themanufacturer for the required frame size, or

b) A thermal-magnetic trip unit with separate adjustable 50G ground sensor andshunt trip device.

6. Section 10.2.3 specifies that protection of low voltage radial feeders may be providedby integral solid-state breaker trip devices, which shall be supplied with long-time,short-time phase overcurrent trips, and time-delayed ground fault trips. Instantaneoustrips shall only be provided for dedicated feeders where there is no downstreamprotective device.




Work Aid 1D: SAES-P-100

1. Section 5.6 specifies that maximum steady state total voltage drop for a main, feeder,and branch circuit shall not exceed five (5) percent.

2. Section 5.6.1 (E) specifies that summation of voltage drops in circuit from main todistribution center and from feeder breaker to panelboard shall be two (2) percent atfull load.

3. Section 5.6.1 (F) specifies that other voltage drops shall average two (2) percent with amaximum of four (4) percent to the most distant outlet at full load.

4. Section 6.2 specifies that the following criteria shall be used to establish equipmentderating when specific requirements are not covered in an SAES or SAMSS.

Location Ambient TemperatureAverage MonthlyNormal Maximum

0C

MaximumDaily Peak

0C

Outdoors

Indoors -Well Ventilated Buildings

Indoors -Air-Conditioned BuildingsUnmanned Areas

Indoors -Air-Conditioned BuildingsManned Areas

45

40

35

30

50

50

35

30

Figure 35. SAES-P-100 Ambient Temperatures




Work Aid 1E: SAES-P-104

1. Section 4.1 specifies that design and installation of wiring and cable systems shall bein accordance with NFPA 70 (National Electric Code - NEC), as supplemented by thisStandard.

2. Section 4.2.1 specifies that wire and cable shall have copper conductors.

3. Section 4.2.2 specifies that low voltage wire and cable (600 V or 600/1000 V andbelow) shall have a minimum rating of 75°C.

4. Section 4.2.5 specifies that power conductors shall be of stranded copper, except thatsolid copper conductors 6 mm2 (No. 10 AWG) and smaller may be used in non-industrial locations and for specialty applications.

5. Section 4.2.10 specifies that for 600 V and below power conductors, the minimum sizepermitted is 2.5 mm2 (No. 14 AWG).

6. Section 4.3.1 specifies that direct buried conduit shall be threaded, rigid steel, hot-dipgalvanized, and PVC coated, or type DB PVC conduit.

7. Section 4.3.2 specifies that conduit above ground in outdoor industrial facilities shallbe threaded, rigid steel, and be hot-dip galvanized.

8. Section 4.3.4 specifies that the minimum conduit size shall be 3/4 inch, except oninstrument panels, inside buildings, and on prefabricated skids, where the minimumsize conduit shall be 1/2 inch.

9. Section 4.3.6 specifies that electric metallic tubing (EMT) is acceptable only innonhazardous indoor locations.

10. Section 5.1 specifies that the minimum burial depths for 600 V and belowunderground installations shall be the following:

• Direct Buried cables - 600 mm (24 in.)

• Direct buried PVC conduit - 460 mm (18 in.)

• Duct bank and direct buried rigid steel conduit - 460 mm (18 in.)

• PVC conduit, rigid steel conduit, or duct bank under roads, parking lots, andother areas that are subject to vehicular traffic - 600 mm (24 in.)




11. Section 8.1 specifies that the sizing of power cables in the Saudi Aramco system shallbe the following:

• load factor - 100 percent of maximum steady state load

• direct buried cable - 40°C ambient temperature

• cable in conduit (in air) - 60°C exposed to sun and 50°C shaded or indoor.

12. Section 8.2.1 specifies that feeders supplying transformers shall have an ampacity ofnot less than the sum of the full-load ratings (fan-cooled ratings, if fans are installed)of all connected transformers and all other connected loads.

13. Section 8.2.2 specifies that a feeder cable serving a load bus shall be sized inaccordance with the NEC plus a 20 percent growth factor, but the size is not to exceedthe maximum rating of the bus. Note: Section 8.2.3 permits deletion of the growthfactor where two or more feeders serve the same bus.

14. Section 8.2.4 specifies that the ampacity of a feeder directly connecting the secondaryof a transformer to a load bus shall not be less than the full-load rating (forced-airrating, if fans are installed) of the transformer.

15. Section 8.2.5 specifies that a derating factor of 15 percent shall be applied to powercable that requires fireproofing.




Work Aid 1F: Applicable Procedures for Calculating Ampacity Ratings of FeederConductors

Step 1. Determine the feeder load current in accordance with the following formulas: Note:1.2 factor is the 20 percent growth factor required by SAES-P-104.

a. Three-phase non-motor loads:

• IL = 1.2 x kVA/( 3 x kV) or

• IL = 1.2 x kW/( 3 x kV x p.f.)

b. Three-phase transformer feeder loads based on the self-cooled (OA) ratingor forced-air (FA) rating if fans are installed:

• IL = kVA/( 3 x kV)

Step 2. Initially select a 75°C or 90°C conductor form NEC Table 310-16 (Handout 2,page 248). Note: Consider use of parallel conductors for required conductor sizes500 kcmil and larger.

Step 3. Apply derating (correction) factors (if applicable) as follows:

a) Fireproofing (where required): 15 percent

b) Ambient temperature: Select the correction factor from NEC Table 310-16(Handout 2, page 248).

c) More than three current-carrying conductors in a raceway: Select thecorrection factor from Note 8 to NEC Table 310-16 (Handout 2, page 253).

Step 4. Specify the selected conductor (or parallel conductors) as follows:

• Size - AWG or kcmil Note: See Figure 36 for the nearest metric equivalentsize conductor.

• Material - Copper (Cu)

• Insulation Type - i.e., THWN, THHN, etc.

• Number of Conductors - i. e., 4/C, 8/C




CONDUCTOR SIZES

AWGor

kcmil* mm2

AWGor

kcmil* mm2

141210

8642

1/0

2.546

1016253550

2/04/0

250*350*500*750*

1000*

70120120185240400500

Figure 36. Standard Saudi Aramco Wire Sizes




Work Aid 1G: Applicable Procedures for Calculating the Short Circuit Rating of aFeeder Conductor

Step 1. Estimate the protective device clearing times as follows:

a) Molded Case Circuit Breakers (MCCBs): 1.1 cycles (18 msec) for less than100 ampere frame (AF) and 1.5 cycles (25 msec) for 225 AF and larger inthe MCCB’s instantaneous range (approximately five times the amperetrip), and over 100 sec at low magnitude overloads.

b) Low Voltage Power Circuit Breakers (LVPCBs): 3 cycles (0.05 sec) forinstantaneous ranges, 6 to 30 cycles (0.10 - 0.50 sec) in the short timeranges, over 100 sec in the long time ranges, and 6 to 30 cycles (0.10 - 0.50sec) for their ground fault ranges.

Step 2. Calculate the magnitude of the fault current (total asymmetrical) in accordance withthe following formula:

• Iasy = Isym x ko

• where: Isym = rms symmetrical fault current

Note: Isym is assumed as a “given quantity” for purposes of this Work Aid.

ko= correction factor accounting for the dc component of current.= 1.6 for MCCBs= 1.3 for LVPCBs

Step 3. Calculate the duration (t) of of the short circuit in accordance with the followingformula:

• t = .0297 log10 [(T2 + 234)/T1 + 234)] x (A/Iasy)2

• where: t = short circuit duration in secondsT1 = initial conductor temperature in degrees Celcius (°C)

= 75°C or 90°C for NEC thermoplastic (PVC) conductorsT2 = final conductor temperature in degrees Celsius (°C)

= 150°C for NEC thermoplastic (PVC) conductorsA = conductor cross-sectional area in circular mils (cmils)

Note: See Table 8 of the NEC Handbook (Handout 1,page 919).

Iasy= maximum asymmetrical short circuit current in amperescalculated in Step 2.




Step 4. Compare the short circuit duration (t) calculated in Step 3 to the estimatedprotective device clearing times from Step 1. If t is less than the protectivedevice’s clearing time, increase the conductor size to the next standard availablesize and repeat Steps 3 and 4 until t is greater than the protective device’s clearingtime.




Work Aid 1H: Applicable Procedures for Calculating the Voltage Drop of aFeeder Conductor

Step 1. Obtain the specified one-line diagram.

Step 2. Calculate load current (IL).

IL = (kVA)/[( 3)(kV)] = (kW)/[( 3)(kV)(p.f.)]

Step 3. Calculate the load power factor angle θ = cos-1 p.f.

Step 4. Calculate the load reactive factor (sin θ). sin θ = sin (cos-1 p.f.)

Step 5. Determine feeder impedance (ZΩ) per 1000 feet from NEC Table 9 (Handout 1,page 920).

Step 6. Calculate the feeder impedance.

ZΩ = ((R + jX) Ω per 1000 ft) (number of feet)

Step 7.Calculate VD line-to-neutral.

VD = I(R cos θ + X sin θ)

Step 8. Calculate VD line-to-line.

VD = 3VD (3f system)VD = 2VD (1φ system)

Step 9. Calculate the load voltage (VL).

VL = VS - VD

Step 10. Calculate VD as a percentage (VD%).

VD% = 100[(VS - VL)/VS]

Step 11. If VD% exceeds 2 percent, increase the conductor to the next standard size andrepeat Steps 6 through 11.




Work Aid 1I: Applicable Procedures for Selecting a Feeder Conductor ProtectiveDevice

Step 1. Select the ampacity rating of an MCCB or an LVPCB based on the ampacity of theconductors as calculated in Work Aid 1F. Note: NEC Article 240-6 (Handout 1,pages 142 and 143) lists the standard ampere ratings of circuit breakers. Note:NEC Article 240-3 (Handout 1, page 141) requires low voltage conductors to beprotected in accordance with their ampacities as listed in NEC Tables 310-16through 310-19 (Handout 1, pages 248 through 251) and their accompanying notes(Handout 1, pages 252 through 255).

Step 2. If the conductor ampacity does not correspond with a standard ampere rating of acircuit breaker, select the next standard ampere-rated device. (Note: See NECArticle 240-3(b) (Handout 1, page 141). If the next standard ampere-ratedprotective device exceeds 800 A, the conductor ampacity must be increased toequal or exceed the device rating or the protective device must be decreased to thenext standard lower ampere-rated protective device. Note: See NEC Article 240-3(c) (Handout 1,page 141).




Work Aid 1J: Applicable Procedures for Selecting a Feeder Conductor Raceway Size

Step 1. Select the size of the equipment grounding conductor from NEC Table 250-95(Handout 1, page 199) based on the size of the selected protective device fromWork Aid 1I.

Step 2. For type of conductor and number of conductors all of the same size, select theconduit (raceway) size from Tables 3A, 3B, or 3C of the NEC (Handout 1, pages913, 914, and 915). Note: The term “conductors” includes the phase conductors,the neutral conductor, and the equipment grounding conductor.

Step 3. For type of conductor and number of conductors of different sizes:

a) Using Table 5 of the NEC (Handout 1, page 916), compute the total cross-sectional area of the conductors.

b) Select a conduit from Table 4 of the NEC (Handout 1, page 915),where the40% fill rate area of the standard conduit size is greaterthan the conductorcross-sectional area computed in Step 3a.




Work Aid 1K: Feeder Circuit Design Flow Chart

Figure 37. Feeder Circuit Design Flow Chart




GLOSSARY

AWG American Wire Gauge

branch circuit The circuit conductors that are between the final overcurrent protectivedevice and the load.

bus feeder A type of feeder that supplies power to the main bus of switchgear, aswitchboard, or a panelboard.

busway Solid copper bars that are insulated and covered in a metal enclosure and thatare used to carry large currents for short distances.

cable A stranded conductor or a combination of conductors that are insulated from eachother.

circuit conductor The conductor that is the current carrying element of a branch or feedercircuit. This circuit conductor is usually cable or busway.

circular mils (cmil) A measure of the diameter of a wire that equals the diameter of the wirein mils (1/1000 of an inch) squared.

conduit A metallic or non-metallic tube that is used to mechanically protect electricalwires and cables.

correction factor Factors that are used to derate equipment due to high ambienttemperatures, type of installation, spacing, etc.

derating factor Factors that are used to derate equipment due to high ambienttemperatures, type of installation, spacing, etc. See correction factor.

direct burial A method of installing cable underground, where the cable is placed in a trenchor ditch and covered with soil.

duct bank A method of underground installation of cable, where the duct bank consists ofmetallic or non-metallic conduit encased in concrete. The complete assembly is buried in atrench or ditch.

feeder circuitAll circuit conductors that are between a source and the final branch circuitovercurrent protective device.

forced-cooled rating A kVA rating on a transformer that is the transformer (FA)rating with the fans operating.




growth factor A factor that is applied to equipment to allow for futureexpansion.

ICEA Insulated Cable Engineers Association

IEC International Electrotecnical Commission

kcmil A unit of measure of wire sizes that equals 1000 circular mils(cmil).

NEC National Electric Code

NEMA National Electrical Manufacturer’s Association

power cable A conductor or a group of conductors that supplies current toequipment.

raceway Any channel that holds wires, cables, or bus bars and it may bemetallic or non-metallic. Examples of raceway are conduit, cable duct, and cable tray.

self-cooled rating (OA) A rating on a transformer that is the kVA rating of a transformerwithout the use of any additional cooling methods, such as fans.

transformer feeder A type of feeder that supplies power to a transformer.

utilization voltage The operating voltage required by a specific piece of equipment.Utilization voltage is the equipment’s rated operating voltage.

voltage drop The amount of voltage that is measured (lost) on a conductorbetween the source and the load. The amount of voltage drop depends on the cableimpedance, amount of current, cable size, and length of cable.

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