Conductor Ampacity Calculation
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Conductor Ampacity Calculation – Part One In part one of this subject, I will list all the terms definitions that we can subjected to. These terms will include the following sections: 1. Conductor/Cable Terms, 2. Conductor/Cable Insulation Materials Terms, 3. Conductor/Cable Tests Terms, 4. Conductor/Cable Manufacturing Process Terms. Definitions for All above terms are included in the below table, please review it very well before we proceed to explain the steps of conductor Ampacity Calculations.
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Conductor Ampacity Calculation
Transcript of Conductor Ampacity Calculation
Conductor Ampacity Calculation – Part One
In part one of this subject, I will list all the terms definitions that we can subjected to.
These terms will include the following sections:
1. Conductor/Cable Terms,
2. Conductor/Cable Insulation Materials Terms,
3. Conductor/Cable Tests Terms,
4. Conductor/Cable Manufacturing Process Terms.
Definitions for All above terms are included in the below table, please review it very well before we proceed to explain the steps of conductor Ampacity Calculations.
Term Definition
1- Conductor/Cable Terms
Ambient TemperatureAny all-encompassing temperature within a given area.
Ampacity
The maximum current an insulated wire or cable can safely carry without exceeding either the insulation or jacket material limitations. (Same as Current Carrying Ampacity)
Appliance Wire and Cable
Appliance wiring material is a classification of Underwriters Laboratories, Inc., covering insulated wire and cable intended for internal wiring of appliances and equipment.
Area of ConductorThe size of a conductor cross-section, measured in circular mils, square inches, etc.
AWG
Abbreviation for American Wire Gauge. A standard system used in the United States for designing the size of an electrical conductor based on geometric progression between two conductor sizes. Based on a circular mil. System 1 mil. Equals .001 inch.
Bare ConductorA conductor having no covering. A conductor with no coating or cladding on the copper.
Building WireWire used for light and power, 600 volts or less, usually exposed to outdoor environment.
Bunch StrandingA group of wire of the same diameter twisted together without a predetermined pattern.
Buried CableA cable installed directly into the earth without use of underground conduit. Also called “direct burial cable”.
BusWire used to connect two terminals inside of an electrical unit.
ButtJoining of two conductors end to end, with no overlap and with the axes in line.
Butt-SpliceA splice where in two wires from opposite ends butt against each other, or against a stop, in the center of a splice.
CableA group of individually insulated conductors in twisted or parallel configuration, with or without an overall coating.
Cable AssemblyA completed cable and its associated hardware ready to install.
Cable Filler
The material used in multiple conductor cables to occupy the spaces formed by the assembly of components, thus forming a core of the desired shape (normally cylindrical).
CablingThe twisting together of two or more insulated conductors to form a cable.
Circular MilThe area of a circle one mil. (.001”) is diameter, 7,845 x 10-7 sq. inches. Used in expressing wire cross sectional area.
Coating A material applied to the surface of a conductor to prevent environmental deterioration, facilitate soldering, or improve electrical
performance.
Coaxial CableA cable consisting of two cylindrical conductors with a common axis, separated by a dielectric.
Color CodeA system for a circuit identification through use of solid colors and contrasting tracers.
Combination UnilayA stranding configuration that uses two strand sizes to achieve a 3% reduction in the conductor diameter without compression.
Compact Stranded Conductor
A unidirectional or conventional conductor manufactured to a specified diameter, approximately 8 to 10% below the nominal diameter of a non-compact conductor of the same sectional area.
Compressed Stranding
A stranding configuration with concentric strands, in which either all layers or the outer layer only is passed through a die to reduce the conductor diameter by 3%.
CompoundAn insulating or jacketing material made by mixing two or more ingredients.
Concentric StrandingA central wire surrounded by one or more layers of helically wound strands in a fixed, round, geometric arrangement.
Concentricity
In a wire or cable, the measurement of the location of the center of the conductor with respect to the geometric center of the surrounding insulation.
ConductivityThe capability of a material to carry electrical current, usually expressed as a percentage of copper conductivity (copper being 100%).
Conductor An uninsulated wire suitable for carrying
electrical current.
Control CableA multi-conductor cable made for operation in control or signal circuits.
Cord A small, flexible insulated cable.
CoreIn cables, a component or assembly of components over which additional components (shields, sheath, etc.) are applied.
Cross-Sectional Area
The area of a conductor exposed by cutting the conductor perpendicular to its longitudinal plane, expressed in circular mils, square inches, or square millimeters.
Current-Carrying Capacity
The maximum current an insulated conductor or cable can continuously carry without exceeding its temperature rating. It is also called ampacity.
Derating Factor
A factor used to reduce the current carrying capacity of a wire when used in environments other than that for which the value was established.
DielectricAny insulating material between two conductors that permits electrostatic attraction and repulsion to take place across it.
Direct Burial Cable A cable installed directly in the earth.
DuctAn underground or overhead tube for carrying electrical conductors.
FeederThe circuit conductor between the service equipment and the final branch circuit over current device.
Fixture Wire
A conductor used in lighting or similar equipment or used to connect a lighting fixture to branch circuit conductors. Common types include TF, TFN, and TFFN.
Flat CableA cable with two smooth or corrugated, but essentially flat surfaces.
Flat ConductorA wire having a rectangular cross section, as opposed to round or square conductors.
Flat Conductor Cable A cable with a plurality of flat conductors.
Flex LifeThe measurement of the ability of conductor or cable to withstand repeated bending.
Flexible
The quality of a cable or cable component that allows for bending under the influence of outside force, as opposed to limpness which is bending due to the cable’s own weight.
Flexibility The ease with which a cable may be bent.
GaugeA term used to denote the physical size of a wire.
Hard Drawn Copper Wire
Copper wire that has not been annealed after drawing.
Hook-Up Wire
A single insulated conductor used for low current, low voltage (usually under 600 volts) applications within enclosed electronic equipment.
Insulation A material having high resistance to the flow of
electric current. Often called a dielectric in radio frequency cable.
Insulation Level-100%
Cable for use on grounded systems or where the system is provided with relay protection such that ground faults will be cleared as rapidly as possible but in any case within one minute.
Insulation Level-133%Cable for use on grounded systems or where the faulted section will be de-energized in a time not exceeding one hour.
Insulation Resistance (I.R)
The resistance offered by insulation to an impressed DC voltage, tending to produce a leakage current through the insulation.
Insulation Thickness The wall thickness of the applied insulation.
JacketAn outer covering, usually nonmetallic, mainly used for protection against the environment.
Kcmil 1,000 circular mils.
Leakage CurrentThe undesirable flow of current through or over the surface of insulation.
ListedConductors or other equipment included in a list published by a nationally recognized testing laboratory.
MC Metal-Clad CableMEC type designation for power and control cables enclosed in a smooth metallic sheath, or interlocking tape armor.
MCM One thousand circular mils.
MemberA group of insulated wires to be cabled with other stranded groups into multiple membered cable.
Messenger
The linear supporting member, usually a high strength steel wire, used as the supporting element of a suspended aerial cable. The messenger may be an integral part of the cable or exterior to it.
Metal-Clad Cable
Type MC; a multi-conductor cable, similar to type AC, in which the conductors are twisted together under aluminum or steel armor. With or without an overall PVC covering.
MilA unit used in measured diameter of a wire or thickness of insulation over a conductor. One one-thousandth of an inch (.001”).
Multi-ConductorMore than one conductor within a single cable complex.
Rated TemperatureThe maximum temperature at which an electric component can operate for extended periods without undue degradation or safety hazard.
Rated VoltageThe maximum voltage at which an electric component can operate for extended periods without loss of its basic properties.
Rope StrandA conductor whose cross-section is substantially circular.
Semi-Conducting Tape
A tape of such resistance that when applied between two elements of a cable, the adjacent surfaces of the two elements will maintain substantially the same potential.
Separator
A layer of insulating material such as textile, paper, polyester, etc. Used to improve stripping qualities, flexibility, mechanical or electrical protection to the components.
SheathThe outer covering or jacket of a multi-conductor cable.
Shield
A metallic layer placed around a conductor or group of conductors to prevent electrostatic interference between the enclosed wires and external fields.
Skin EffectThe tendency of alternating current, as its frequency increases, to travel only on the surface of a conductor.
Solid Conductor A single unit not divided into parts.Strand A single uninsulated wire.
Stranded ConductorA conductor composed of individual groups of wires twisted together to form an entire unit.
Temperature RatingThe maximum temperature at which an insulating material may be used in continuous operation without loss of its basic properties.
Tensile StrengthThe pull stress required to break a given specimen.
Test Lead
A flexible insulated lead wire used for making tests, connecting instruments to a circuit temporarily or for making temporary electrical connections.
Thermal RatingThe maximum and/or minimum temperature at which a material will perform its function without undue degradation.
Tinned CopperTin coating added to copper to aid in soldering and inhibit corrosion.
Tray Cable
A factory assembled multi-conductor or multi-pair control, signal or power cable specifically approved under the National Electrical Code for installation trays.
UFThermoplastic underground feeder and branch circuit cable.
USEUnderground Service Entrance cable, rubber-insulated, neoprene or XLP jacketed.
Voltage DropThe amount of voltage loss from original input in a conductor of given size and length or over a connection such as a termination.
Voltage LevelsPower-limited 0-300 volts. Low voltage 600-2000 volts. Medium voltage 5000-69000 volts.
High VoltageGenerally, a wire or cable with an operating voltage of over 35,000 volts.
Voltage RatingThe highest voltage that may be continuously applied to a wire in conformance with standards or specifications.
VW-1
A flammability rating established by Underwriters Laboratories for wires and cables that pass a specially designated vertical flame test, formerly designated FR-1. Multi-conductor flat or round portable power cables without grounding conductor.
Water AbsorptionWater by percent weight absorbed by a material after a given immersion period.
WireA single conductor, typically with a covering of insulation.
Wire GaugeA measure of the diameter or size of wires. The sizes are expressed by numbers.
Yield StrengthThe minimum stress at which a material will start to physically deform without further increase in load.
2- Insulation Materials Terms
CPEJacketing compound based on chlorinated polyethylene.
Ethylene Propylene Rubber (EPR)
An ozone-resistant rubber consisting primarily of ethylene propylene copolymer (EPM) or ethylene diene terpolymer (EPDM).
Flame RetardantA chemical added in insulation materials to make them less combustible, such as antimony oxide (to PVC) or alumna trihydrate.
Flame ResistanceThe ability of a material to restrict the spread of combustion to a low rate of travel, so that the flame will not be conveyed.
MTWThermoplastic-insulated machine tool wire. 90°C to 105°C, 600V.
NylonA group of polyimide polymers that are used for wire and cable jacket.
Non-Contaminating PVCA polyvinyl chloride formulation that does not produce electrical contamination through plasticizer migration.
PlasticizerA chemical agent added to plastics to make them softer and more pliable.
Polyester
Polyethylene terephtalate that is used extensively in the production of a high strength, moisture resistant film used as a cable core wrap.
PolyethyleneA thermoplastic material having the chemical identity of polymerized ethylene.
PolymerA substance made of many repeating chemical units or molecules. The term polymer is often used in place or plastic, rubber or elastomer.
Polypropylene: A thermoplastic polymer of propylene.
Polyvinyl Chloride (PVC)A thermoplastic material composed of polymers of vinyl chloride, which may be rigid or elastomeric, depending on specific formulation.
PPE Portable Power Elastomer. Same as Type W, except that it is a thermoplastic elastomer insulation and jacket, whereas Type W is all
thermostat.
Primary InsulationThe first layer of non-conductive material applied over a conductor, whose prime function is to act as electrical insulation.
PVCPolyvinyl chloride, a common thermoplastic insulation and jacketing material for building wire and cable.
RHType RH, a rubber or XLP-insulated conductor for use at 75°C in dry locations.
RHHType RHH, a rubber or XLP- insulated conductor for use at 90°C in dry locations.
RHWType RHW, a rubber or XLP-insulated conductor for use at 75°C in dry and wet locations.
RHW-2Type RHW-2, a rubber or XLP-insulated conductor for use at 90°C in dry and wet locations.
Secondary InsulationA high-resistance dielectric material whose flame is placed over primary insulation to protect it from abrasion.
Self-ExtinguishingThe characteristic of a material whose flame is extinguished after the igniting flame is removed.
SIS
Indicates single conductor having synthetic thermosetting insulation of heat resistant, moisture resistant, flame retarding grade. Also made with chemically cross-linked polyethylene insulation. Used for switchboard wiring only, 90°C.
SJ-SJEJunior hard service, rubber insulated pendant or portable cord. Same construction as type S, but 300B. Jacket thickness different.
SJO-SJEOSame as SJ, but neoprene, oil resistant compound outer jacket, 300V, 60°C
SJOOW-SJEOOWSame as type SJO, except oil resistant insulation and oil and weather resistant jacket.
SJTJunior hard service, thermoplastic or rubber insulated conductors with overall thermoplastic jacket. 300V, 60C to 105C.
SJTOSame as SJT, but oil resistant thermoplastic outer jacket. 60C
SO-SEOHard service cord, same construction as type S, except oil resistant neoprene jacket, 600V, 60°C to 90C
SOOWService cord with oil resistant jacket, oil resistant and insulation and weather resistant. Also is water resistant. 600V.
STHard service, jacketed, same as type S, except all plastic construction. 600V, 60°C to 105°C.
StabilizerA metallic compound added to PVC to maintain the integrity of the insulation compound during processing and use.
STOSame as ST, but with oil resistant, thermoplastic outer jacket. 600V, 60°C.
STOW/STOW
Service cord with oil resistant, thermoplastic jacket and weather resistant. STOW meets CSA approval for outdoor use. Can be water resistant. UL 600V.
STW/STW
Service cord with thermoplastic and weather resistant jacket, but not oil resistant. Can be UL water resistant. STW meets CSA approval for outdoor use. 600V.
Sunlight ResistanceThe ability of a conductor or cable insulation to resist degradation caused by exposure to ultraviolet rays.
TEWCanadian Standards Association type appliance wires. Solid or stranded single conductor, plastic insulated. 600V, 105C.
TFFNFixture wire; thermoplastic covered, stranded with a nylon sheath. 90C.
THHN90C, 600V, nylon-jacketed building wire for dry and damp locations.
THHN-2Incorrect reference, commonly misapplied when THWN-2 is called out.
THWThermoplastic, vinyl insulated building wire. Flame retardant, moisture and heat resistant. 75°C. Dry and wet locations.
THWN75°C, 600V, nylon jacketed building wire for dry or wet locations.
THWN-290°C, 600V, nylon-jacketed building wire for dry or wet locations.
XHHW-2High temperature (90°C), chemically cross-linked, polyethylene jacketed, small diameter building wire.
XLP Cross-linked polyethylene.
3- Conductor/Cable Tests Terms
Continuity CheckA test to determine whether electrical current flows continuously throughout the length of a single wire or individual wires in a cable.
Dielectric Test
A test in which a voltage higher than the rated voltage is applied for a specified time to determine the adequacy of the insulation under normal conditions.
Flammability Test
A test to determine the ability of a cable to resist ignition when placed near a source of heat or flame and to self extinguish when removed from this source.
Heat ShockA test to determine stability of a material by sudden exposure to a high temperature for a short period of time.
Hi Pot
A test designed to determine the highest voltage that can be applied to a conductor without electrically breaking down the insulation.
Life CycleA test to determine the length of time before failure in a controlled, usually, accelerated environment.
Megohommeter
A testing device that applied a DC voltage to a conductor and measures the resistance (in millions of ohms) offered by the conductors insulation.
Spark Test
A test designed to locate imperfections (usually pin-holes) in the insulation of a wire or cable by application of a voltage for a very short period of time while the wire is being drawn through the electrode field.
Tank Test
A voltage dielectric test in which the test sample is submerged in water and voltage is applied between the conductor and water as ground.
4- Manufacturing Process Terms
Anneal
The process of controlled heating and cooling of a metal to achieve predetermined characteristics as to tensile strength and elongation. . Annealing copper renders it less brittle.
Drawing
In wire manufacturing, pulling the metal through a die or series of dies to reduce diameter to a specified size.
Extrusion
The process of continuously forcing both a plastic or elastomer and a conductor core through a die, thereby applying a continuous coating of insulation or jacket to the core or conductor.
Oxidation
The process of uniting a compound with oxygen, usually resulting in an unwanted surface degradation of the material or compound.
In the next Article, I will explain the following points:
1. Measurement Units of Conductors Cross Section Area,
2. Conversions between different Measurement Units of Conductors Cross Section Area,
3. Standard Sizes of Conductors,
4. Conductor Ampacity Tables.
Please, keep following.
Conductor Ampacity Calculation – Part Two
In Article " Conductor Ampacity Calculation – Part One ", I listed the definitions for related terms which were included under the following sections:
1. Conductor/Cable Terms,
2. Conductor/Cable Insulation Materials Terms,
3. Conductor/Cable Tests Terms,
4. Conductor/Cable Manufacturing Process Terms.
Today, I will explain the following points:
1. Measurement Units of Conductors Cross Section Area,
2. Conversions between different Measurement Units of Conductors Cross Section Area,
3. Standard Sizes of Conductors,
1- Measurement Units of Conductors Cross Section Area
1.1 Traditional Measurement Units of Conductors Cross Section Area
As we know that the traditional Measurement Units of Conductors Cross Section Area are:
Square millimeters (mm2),
Square inches.
1.2 The measurement unit “SQUARE MIL”:
Definition:The Mil: is a convenient unit of measurement of the diameter of a conductor Noting that Mil = 0.001 inch = 0.0254 mm
Or, Inch = 1000 mils
Definition:The square Mil: is a unit of measurement used to determine the cross-sectional area of a square or rectangular conductor (views A and B of Fig.1).
Also, A square mil : is defined as the area of a square, the sides of which are each 1 mil. , as shown in view A of Fig.2.
Rule#1: Cross-Sectional Area Of A Square Conductor
For Square Conductors, as in View A in fig.1, to obtain the cross-sectional area in circular mils of a square conductor, multiply the dimension of any side of the square by itself.
So, Cross-sectional area (A ) in mils = square of Side Length Noting that Side Length must be in mils.
Fig (1)
Fig (2)
Example#1:
Assume that you have a square conductor with a side dimension of 3 mils. What its cross-sectional area in circular mils?
Solution:
Cross-sectional area (A ) in mils = square of Side Length = 3 x 3 = 9 square mils.
Rule#2: Cross-Sectional Area Of A Rectangular Conductor
For Square Conductors, as in View B in fig.1, to obtain the cross-sectional area in circular mils of a rectangular conductor, multiply the length times the width of the end face of the conductor (side is expressed in mils).
So, Cross-sectional area (A ) in mils = Length x Width Noting that Side Length and Width must be in mils.
Example#2:
Assume that one side of the rectangular cross-sectional area is 6 mils and the other side is 3 mils. What its cross-sectional area in circular mils?
Solution:
Cross-sectional area (A) = Length x Width = 6 mils × 3 mils= 18 square mils.
Example#3:
Assume that a conductor is 3/8 inch thick and 4 inches wide. What its cross-sectional area in circular mils?
Solution:
The 3/8 inch can be expressed in decimal form as 0.375 inch. Since 1 mil equals 0.001 inch, the thickness of the conductor will be 0.001 × 0.375, or 375 mils.
Since the width is 4 inches and there are 1,000 mils per inch, the width will be 4 × 1,000, or 4,000 mils. Cross-sectional area (A) = Length x Width = 375 mils × 4,000 mils= 1,500,000 square mils.
1.3 The measurement unit “CIRCULAR MIL”
Definition:The circular mil: is the standard unit of measurement of a round wire cross-sectional area (view C of Fig.1). This unit of measurement is found in American and English wire tables. Also, A circular mil: is the area of a circle having a diameter of 1 mil, as shown in view B of Fig.2.
Important!!!The diameter of a round conductor (wire) used to conduct electricity may be only a fraction of an inch. Therefore, it is convenient to express this diameter in mils to avoid using decimals. For example, the diameter of a wire is expressed as 25 mils instead of 0.025 inch.
Rule#3: Cross-Sectional Area Of A Round Conductor
For Round Conductors, as in View C in fig.1, to obtain the cross-sectional area in circular mils of a round conductor is obtained by squaring the diameter, measured in mils. So, Cross-sectional area (A) in mils = D2 , D in milsNoting that Diameter D must be in mils.
Example#4:
Assume that a wire having a diameter of 25 mils. What its cross-sectional area in circular mils?
Solution:
Cross-sectional area (A) = D2 = 25 x 25 = 625 circular mils
Comparing Square Mils with Circular Mils for the same Conductor:
Example#5:
A wire having a diameter of 25 mils, compare between its area calculated in Square Mils and in Circular Mils?
Solution:
a- Calculation of conductor area in square mils:
To determine the number of square mils in the same conductor, apply the conventional formula for determining the area of a circle (A = 3.14r2). In this formula, A (area) is the unknown and is equal to the cross-sectional area in square mils, and r is the radius of the circle, or half the diameter (D).
Through substitution, A = 3.14 x (12.5) 2 = 490.625 square mils.
b- Calculation of conductor area in circular mils:
A wire having a diameter of 25 mils has an area of 252, or 625 circular mils.
The cross-sectional area of the wire has 625 circular mils but only 490.625 square mils.
Important!!!For one conductor (wire or cable), a circular mil represents a smaller unit of area than the square mil.
Definition:A wire in its usual form is a single slender rod or filament of drawn metal. In large sizes, wire becomes difficult to handle. To increase its flexibility, it is stranded. Strands are usually single wires twisted together in sufficient numbers to make up the necessary cross-sectional area of the cable.
So, Stranded Conductor: A conductor composed of individual groups of wires twisted together to form an entire unit.
Rule#5: Cross-Sectional Area Of A Stranded wire/cable
The total area of stranded wire in circular mils is determined by multiplying the area in circular mils of one strand by the number of strands in the cable.
So, Cross-sectional area (A) in mils = N x Cross-sectional area (A) in mils for one standWhere N = number of strands in the cable
1.4 The measurement unit “American Wire Gauge (AWG)”
Definition:American Wire Gauge (AWG): A standard system used in the United States for expressing wire diameter and designing the size of an electrical conductor based on geometric progression between two conductor sizes, Based on a circular mil system.
The American Wire Gage (AWG) is the same as the Brown and Sharpe (BS) Gage.
Important!!!As the AWG number gets smaller, the wire diameter (gauge) gets larger.
Rule#6: Expressing wire/cable Size in AWG
1- Wire sizes up to size 4/0 AWG:They are expressed as XX AWG, with XX being the size wire. For example, a wire size expressed as 12 AWG.
2- Conductors larger than 4/0 AWG:They are sized in circular mils, beginning with 250,000 circular mils.For example, a wire size expressed as 250,000-circular-mils.
Noting that:1000 circular-mils = MCM = Kcmil For example, 250,000-circular-mils conductor was labeled 250 MCM or 250 Kcmil
Example#6:
What is the circular mil area of an 8 AWG solid conductor that has a 0.1285-in. diameter?
Solution:
Diameter (D) in mils = 0.1285 in. x 1000 = 128.5 mils
Cross-sectional area (A) of Round conductor in mils = D2 = 128.5 x 128.5 = 16,512.25 circular mils
Or 16,510 circular mils (rounded off).
2- Conversions between different Measurement Units of Conductors Cross Section Area
Rule#4: Unit Conversions
Mil = 0.001 inch = 0.0254 mmInch = 1000 mil = 25.4 mmSquare inch = 645.16 mm2 = 1,000,000 square milsCircular mil = 0. 7854 square mil 1000 circular-mils = MCM = Kcmil
Example#7:
A 12-gauge wire has a diameter of 80.81 mils. What is (1) Its area in circular mils and (2) Its area in square mils?
Solution:
(1) A = D2 = 80.81 X 80.81 = 6,530 circular mils
(2) A = 0.7854 x 6,530 = 5,128.7 square mils
Example#8:
A rectangular conductor is 1.5 inches wide and 0.25 inch thick. What is (1) Its area in square mils and (2) In circular mils? What size of round conductor is necessary to carry the same current as the rectangular bar?
Solution:
(1) 1.5 inches = 1.4 x 1000 = 1,500 mils
0.25 inch = 0.25 x 1000 = 250 mils A = 1,500 x 250 = 375,000 square mils
(2) To carry the same current, the cross-sectional area of the round conductor must be equal. There are more circular mils than square mils in this area. Therefore:
A = 375,000 / 0.7854 = 477,000 circular mils.
3- Standard Sizes of Conductors
An AWG table for copper wire (solid only) is shown at table #1.
Table#1
Notes to table#1:
The largest wire size shown in the table is 0000 (read "4 naught"), and the smallest is number 40.
The following sizes can be expressed in other way as follows: 0000 =4/0, 000 = 3/0, 00 = 2/0, 0 = 1/0.
It shows the diameter in mils, circular mil area, and area in square inches of AWG wire sizes.
It shows the resistance (ohms) per thousand feet and per mile of wire sizes at specific temperatures.
It shows the weight of the wire per thousand feet in the last column.
Example#9: (Using table #1)
You are required to run 2,000 feet of AWG 20 solid copper wire for a new piece of equipment. The temperature where the wire is to be run is 25º C (77º F). How much resistance will the wire offer to current flow?
Solution:
In table#1, under the gauge number column, find size AWG 20. Now read across the columns until you reach the "ohms per 1,000 feet for 25º C (77º F)" column.
You will find that the wire will offer 10.4 ohms of resistance to current flow. Since we are using 2,000 feet of wire, multiply by 2. Resistance of 1,000 feet for 25º C (77º F) = 10.4 ohms
So, Resistance of 2,000 feet = 10.4 ohms x 2 = 20.8 ohms
Important!!!An American Standard Wire Gauge (fig.3) is used to measure wires ranging in size from number 0 to number 36. To use this gauge, insert the wire to be measured into the smallest slot that will just accommodate the bare wire. The gauge number on that slot indicates the wire size. The front part of the slot has parallel sides, and this is where the wire measurement is taken. It should not be confused with the larger semicircular opening at the rear of the slot. The rear opening simply permits the free movement of the wire all the way through the slot.
Fig (3)
Table#2 introduces copper wire AWG sizes (solid and stranded) as follows:
AWG MM2 AWG MM2
30 0.05 6 16
28 0.08 4 25
26 0.14 2 35
24 0.25 1 50
22 0.34 1/0 55
21 0.38 2/0 70
20 0.50 3/0 95
18 0.75 4/0 120
17 1.0 300MCM 150
16 1.5 350MCM 185
14 2.5 500MCM 240
12 4.0 600MCM 300
10 6.0 750MCM 400
8 10.0 1000MCM 500
Table#2
Or you can use the following excel sheet converter from AWG to MM2 or vice versa.
For downloading your copy of AWG to MM2 Converter, please click on the link.
In the next Article, I will continue explaining the Conductor Ampacity calculations. Please, keep following.
Conductor Ampacity Calculation – Part Three
Today, I will explain the methods for Conductor Ampacity Calculations as follows.
The need for Ampacity calculation
When the ampacity of conductors is not accurate, the conductors may carry more currents than their rating and become overloaded, so they will heat up and short out.
Note: As the conductor heats up the current carrying capacity goes down.
When do we need Ampacity calculation?
1. When modifications are done to existing circuits,
2. On all new designs, the conductors/feeders ampacity should be checked.
Rule#1: Methods for Conductors Ampacity Calculations as per NEC code
As per NEC Article 310, Methods for Conductors Ampacity Calculations will depend on the voltage rating of these conductors, it is divided to:
1. Methods for Ampacity Calculations of Conductors Rated 0–2000 Volts,
2. Methods for Ampacity Calculations of Conductors Rated 2001 to 35,000 Volts.
Part one: Methods for Ampacity Calculations of Conductors Rated 0–2000 Volts
Rule#2: Methods for Ampacity Calculations of Conductors Rated 0–2000 Volts
As per 310.15(A)(1), The allowable Ampacities for conductors rated 0-2000 Volts shall be permitted to be determined by two methods:
1. Tables as provided in 310.15(B) or
2. Under engineering supervision, as provided in 310.15(C).
First Method: Conductor Ampacity Calculations from Tables as provided in 310.15(B)
In this method, I will explain the following points:
General Overview of Allowable Ampacity Tables for conductors rated 0 to 2000 volts,
General rules controlling the conductor ampacity calculations,
Factors affecting conductor ampacity.
1- General Overview of Allowable Ampacity Tables for conductors rated 0 to 2000 volts
The Allowable Ampacities for conductors rated 0 to 2000 volts are specified in the following tables:
Table 310.15(B)(16)
Table 310.15(B)(17)
Table 310.15(B)(18)
Table 310.15(B)(19)
Table 310.15(B)(20)
Table 310.15(B)(21)
To download a PDF file that includes The Allowable Ampacities for conductors rated 0 to 2000 volts, click on the link.
The above tables need to be modified to meet existing installation conditions as per the following tables:
TABLE 310.15(B)(2)(A)
TABLE 310.15(B)(2)(b)
TABLE 310.15(B)(3)(a)
TABLE 310.15(B)(3)(c)
TABLE 310.15(B)(7)
These tables will be explained later.
1.1 Allowable Ampacities Tables Construction:
For example, Table 310.15(B)(16) which provides Allowable Ampacities of Insulated Conductors Rated Up to and Including 2000 Volts, 60°C Through 90°C (140°F Through 194°F), Not More Than Three Current-Carrying Conductors in Raceway, Cable, or Earth (Directly Buried), Based on Ambient Temperature of 30°C (86°F).
Table 310.15(B)(16) will consists of:
1.1.A Conditions for table application
These conditions are existing in the table description at the top of the table (see below image), and in table 310.15(B)(16), these conditions are:
1. Conductors Rated Up to and Including 2000 Volts
2. Actual conductor temperature ranges from 60°C Through 90°C (140°F Through 194°F)
3. Not more than three current-carrying conductors in a raceway, cable or earth (directly buried).
4. An ambient temperature of 30°C (86°F).
1.1.B Conductor Material Sections
Two main Sections for conductor material type are existing (see above image):
1. One for copper,
2. One for aluminum or copper-clad aluminum conductors.
1.1.C Temperature Categories
Each of The two main sections for conductor material types is divided into three temperature categories (see below image):
1. 60°C (140°F),
2. 75°C (167°F) and
3. 90°C (194°F).
Noting that each temperature category is applicable with certain insulation properties which are listed in the row under each temperature category.
1.1.D Conductor Sizes Columns
Two Columns for conductor sizes range from 18 AWG to 2,000 kcmil (see below image):
1. The right left one beside copper conductors column is for copper conductors sizes,
2. The right one beside aluminum or copper-clad aluminum conductors column is for aluminum or copper-clad aluminum conductors sizes.
1.2 Notes for Allowable Ampacities Tables:
Note#1
The asterisk (*) next to the ambient temperature of 30°C (86°F) included in table description at the top of the table, Refer to table 310.15(B)(2) for application of the ampacity correction factors where the ambient temperature is other than 30°C (86°F).
Note#2
The asterisk (**) next to wire sizes 10, 12, and 14 AWG refers to article 240.4(D) and is letting you know that conductor sizes #14, #12, and #10 copper and aluminum are limited to certain size overcurrent protection devices (OCPD) even if the ampacity of the #14, #12, and #10 conductor is higher. The limitations are:
A- For Copper Conductors:
#14 copper limited to 15 amp OCPD
#12 copper limited to 20 amp OCPD
#10 copper limited to 30 amp OCPD
B- For Aluminum Conductors:
#14 aluminum not allowed
#12 aluminum limited to 15 amp OCPD
#10 aluminum limited to 25 amp OCPD
Note#3
Ampacity tables, particularly Table 310.15(B)(16), do not take into account all the many factors affecting ampacity.
If loads are not calculated in accordance with the requirements of Article 220, the table ampacities, even when corrected in accordance with ambient correction factors and the notes to the tables, might be too high. Especially where many cables or raceways are routed close to one another underground.
Note#4
Copper conductors of the same size have three different allowable ampacities. The same is true for aluminum (and copper-clad aluminum) conductors. This is because The maximum allowable ampacities depend on the conductor’s temperature rating.
For example, a 3 AWG copper conductor with a temperature rating of 60°C has a maximum allowable ampacity of 85 amperes (A). The maximum allowable ampacity of the same 3 AWG copper conductor with a temperature rating of 75°C is 100A. If the temperature rating of the 3 AWG copper conductor is 90°C, the allowable ampacity is 115A .
Note#5
All The Allowable Ampacities Tables refer to table 310.104(A) (see below image) which contains information about conductors rated 600V. Conductor information in this table includes trade name, type letter, maximum operating temperature, application provisions,
insulation, thickness of insulation, and outer covering (if any).
To download a PDF file for Table 310.104(A), click on the link.
Note#6: Conductor Lettering
You can note in allowable ampacity tables that each temperature category is applicable with certain wiring properties which is listed in the row under each temperature category.
When looking at these wiring properties, you will notice that the wires contain some type of letter identifier such as THHN or THW. These letters serve to identify specific properties of the conductor and/or its insulation. Listed below are some of the letters commonly used in allowable ampacity tables:
Letter Identifier
Conductor and/or Insulation Property
T ThermoplasticW Wet or DampR RubberF Fixture WiresFF Fixture wire, flexible stranding
-2Conductor is permitted 90ºC operating
temperatureH 75ºC insulation rating
HH 90ºC insulation rating
N Nylon outer cover
The combination of letters will tell you most of what you need to know about the conductor.
NOTE: This is a general rule and there are exceptions such as THW (see Table 310.13). The one H indicates 75° C rating but it can be used at a 90° C conductor with ballasts in dry locations.
Examples for Conductor Lettering:
TW - Thermoplastic (T) insulation, (W) suitable for wet and dry locations (When suitable for wet generally means it can be used for dry also), and is rated 60° C (no H in group).
RHW - Rubber (R) insulation, (H) rated 75° C and (W) suitable for wet and dry locations.
TF - Thermoplastic (T) insulation, (F) fixture wire
RFH – Rubber (R) insulation, (F) fixture wire, (H) rated 75° C
1.3 How to use these Allowable Ampacities Tables:
Step#1: Determine the proper table to use based on the existing wiring method conditions
For example, if we have a 6 AWG Type THWN copper wire in free air based on an ambient air temperature of 30° C (86° F), which table we will use?
Yes, it is table 310.15(B)(17), because tables Table 310.15(B)(16) and table 310.15(B)(18) don’t be used with single conductor in free air. Don’t use table TABLE 310.15(B)(19) because the ambient air temperature is 30° C, and THWN insulation is not listed in it.
Step#2: Determine the proper section in the selected table based on the conductor material type (Copper – Aluminum or Copper Clad Aluminum)
In same example above, because this is a copper conductor, we will use the section in the left side of table 310.15(B)(17).
Step#3: Locating the Ampacity Value
In the table section selected above, move down to the given conductor size, which is 6 AWG in our example, the amapcity value are existing in the 6 AWG Row.
Now, move to the right in the 6AWG row until you reach the column for THWN insulation for a copper conductor. This will be the 75° C (167° F) column.
Step#4: Reading the Ampacity Value
Now read the amapcity of the conductor, you should read 95.
Another example:
What is the allowable ampacity of a non-metallic-sheathed cable containing three 12 AWG Type THW-2 Copper wires based on an ambient air temperature of 30° C(86° F)?
Solution:
Step#1: Determine the proper table to use
Because this cable containing three conductors, use table Table 310.15(B)(16). Don’t use table 310.15(B)(17) or table TABLE 310.15(B)(19) because there are three conductor in a cable. Don’t use table 310.15(B)(18) because the ambient air temperature is 30° C, and THW-2 insulation is not listed in it.
Step#2: Determine the proper section in the selected table based on the conductor material type (Copper – Aluminum or Copper Clad Aluminum)
Because these are copper conductors, we will use the section in the left side of table Table 310.15(B)(16).
Step#3: Locating the Ampacity Value
In the table section selected above, move down to the given conductor size, which is 12 AWG , the amapcity value are existing in the 12 AWG Row.
Now, move to the right in the 6AWG row until you reach the column for THW-2 insulation for a copper conductor. This will be the 90° C (194° F) column.
Step#4: Reading the Ampacity Value
Now read the amapcity of the conductor, you should read 30.
In the next Article, I will continue explaining the Conductor Ampacity calculations from Tables as provided in 310.15(B) . Please, keep following.
Conductor Ampacity Calculation – Part Four
In Article " Conductor Ampacity Calculation – Part Three ", I explained the following points:
1. The need for ampacity calculation,
2. When do we need ampacity calculation?,
3. Methods for Conductors Ampacity Calculations as per NEC code,
4. Methods for Ampacity Calculations of Conductors Rated 0–2000 Volts,
5. First Method: Conductor Ampacity Calculations from Tables as provided in 310.15(B),
6. General Overview of Allowable Ampacity Tables for conductors rated 0 to 2000 volts.
Today, I will continue explaining the First Method: Conductor Ampacity Calculations from Tables as provided in 310.15(B) as follows.
For more information and good following, please review the following articles:
Conductor Ampacity Calculation – Part One
Conductor Ampacity Calculation – Part Two
2- General rules controlling the conductor ampacity calculations
In article " Conductor Ampacity Calculation – Part Three ", I explained the first point: how to use the allowable ampacity tables, this was as a general method, but this general method is controlled and refined by the other rules as follows.
Rule#1: Temperature Limitation of Conductors.
As per 310.15(A)(3), No conductor shall be used in such a manner that its operating temperature will exceed that designated for the type of insulated conductor involved.
The above rule said that:
Conductor with 60°C temperature rating must have ampacity from the 60°C column.
Conductor with 75°C temperature rating must have ampacity from the 60°C column or the 75°C column and according to Rule#2 in below.
Conductor with 90°C temperature rating must have ampacity from the 60°C column or the 75°C column and according to Rule#2 in below or from the 90°C column after applying correction factors.
Using The 90°C ColumnBe careful when using the 90°C column because no equipment is listed and identified for use with 90°C conductors other than individual lugs, terminal bars and equipment listed for use on circuits over 600V. The 90 ° C ampacity can be used for corrections only for factors:Ambient Temperature Number of Conductors
Rule#2: Selection Of Conductor Ampacity Based On Its Terminations’ Temperature Ratings
As per 110.14(C), Conductors must be sized in accordance with the lowest temperature rating of any terminal, device, or conductor insulation of the circuit.
Example#1:
A THHN conductor will have a 60°C termination on one end and a 75°C termination on the other, which ampacity column will be used?
Solution:
The lowest temperature rating of conductor’s two terminals is 60°C.
So, the conductor’s ampacity must not exceed the rating listed in the 60°C column Conductors.
How to Select Conductor Ampacity Based On Its Terminations’ Temperature Ratings
Selection of the appropriate conductor amapcity column depends on the temperature rating of the termination (or connection) points as follows:
First: For Conductor/equipment Rated 100A or Less or marked for 14 AWG through 1 AWG
The selected ampacity column will be as follows:
1- Use the 60°C column.
Example#2:
What is the Maximum allowed ampacity for a single #2 THHN Conductor installed in a circuit.
Solution:
Step#1: 110-14(c) (#14 thru #1), so Use the 60°C column
Step#2: use Table 310.15(B)(16), (60°C Column) and Read Ampacity
The conductor ampacity is 95 Amps
2- If any termination is either 60°C or unknown, Use the 60°C column regardless of the insulation rating of the conductor.
Example#3:
A THHN conductor will have a 60°C termination on one end and a 75°C termination on the other, which ampacity column will be used?
Solution:
Because one of the connection points has a 60°C rating, the conductor’s ampacity must not exceed the rating listed in the 60°C column Conductors.
3- If the equipment is listed and identified for use with certain conductor temperature rating, Use the column with temperature rating = the lowest temperature rating of conductor terminations.
If Temp. rating of listed conductor is
Termination#1
Termination#2
Use column
75°C 75°C 75°C 75°C75°C 75°C 60°C 60°C90°C 75°C 75°C 75°C90°C 75°C 60°C 60°C
Example#4:
A THHN conductor will have 75°C termination on one end and a 60/75°C termination on the other, which ampacity column will be used?
Solution:
A temperature rating of 60/75°C means the equipment has been listed for both 60°C and 75°C conductors; therefore, it is permissible to use the 75°C rating if the installed conductor is rated at least 75°C. Because all of the connection points in this example have at least a 75°C rating, the conductor’s ampacity can be based on the 75°C column
4- Use the 75°C column for conductors supplying power to motor marked with a design letter B, C or D and the conductor temperature rating is not less than 75°C.
If a motor marked with a design letter B, C or D use conductor with temp.
rating
Use column
75°C 75°C90°C 75°C
Example#5:
THHN conductors will have 75°C terminations on one end and a motor marked with a design letter D on the other end. After complying with the applicable requirements in Article 430, the conductors supplying power to this motor must have an ampacity of at least 55 amperes (A). What is the minimum size of THHN conductors required to supply power to this motor?
Solution:
Motor with a design letter D and the THHN conductors will have 75°C terminations on the other hand.
So, we must use the 75°C column even if THHN is not listed in the 75°C column and listed in the 90°C column.
Using table 310.15(B)(16), for a 55 A motor, the smallest conductor with ampacity higher than 55 A is 6 AWG (with 65A ampacity).
So, Conductors supplying power to this motor must be at least 6 AWG.
Notes for example#5:
In table 310.15(B)(16), The allowable ampacity of an 8 AWG THHN conductor (in the 90°C column) is 55A. Although conductors with a temperature rating of 90°C can be installed, the ampacity must not exceed the 75°C ampacity.
The ampacity of an 8 AWG conductor in the 75°C column is 50A. Because this motor requires a minimum ampacity of 55A, installing 8 AWG conductors is not permitted.
Second: For Conductor/equipment Rated over 100A or marked for larger than 1 AWG
The selected ampacity column will be as follows:
1- Use the 75°C column.
Example#6:
THWN conductors (larger than 1 AWG) will supply power to a circuit rated greater than 100A, the conductors will have 75°C terminations on both ends. Which ampacity column will be used?
Solution:
The ampacity can be based on the 75°C column because the conductors are larger than 1 AWG, and the circuit it is supplying is rated greater than 100A.
2- If the equipment is listed and identified for use with certain conductor temperature rating, Use the column with temperature rating = the lowest temperature rating of conductor terminations.
If Temp. rating of listed conductor is
Termination#1
Termination#2
Use column
75°C 75°C 75°C 75°C75°C 75°C 60°C 60°C90°C 75°C 75°C 75°C90°C 75°C 60°C 60°C
Example#7:
What is the maximum ampacity for a 1/0 AWG THHN copper conductor fed from a 150A breaker? Assume an ambient temperature of 30°C and no more than three current-carrying conductors in the raceway. The conductors will have 75°C terminations on one end, but the temperature rating of the terminations on the other end is unknown.
Solution:
We must not exceed the 75°C ampacity for this conductor. Although the temperature rating on one end is unknown, the ampacity of this 90°C conductor (since it is THHN) can be based on the 75°C column
Using table 310.15(B)(16), and Because the conductors are larger than 1 AWG and the circuit it is supplying is rated greater than 100A. This 1/0 AWG THHN copper conductor has a maximum ampacity of 150A.
Rule#3: Selection Of Ampicity For Multi-Ampacity Conductors
As per 310.15(A)(2), Where more than one ampacity applies for a given circuit length, the lowest value shall be used.
For example, in below image, The ampacity for No. 3 THHN (90°C) is 110A, but the correction factors of Table 310 reduce the conductor ampacity to only 96A because the lowest ampacity value = 110A x 0.87 = 95.7A and You round up to get 96A.
Exception to Rule#2
A higher ampacity is permitted, if the length of the reduced ampacity doesn’t exceed 10 ft and is not longer than 10% of the total length of the circuit.
For example, in below image,, The ampacity for each No. 12 THHN is 30A, but the correction factors in Table 310-15(b)(2)(a) reduce the conductor ampacity by 50% to be 15A inside the panelboard but it will be 30 amps outside the panelboard.
Rule#4: Minimum Size of Conductors
As per 310.106(A), the minimum size of conductors shall be as shown in Table 310.106(A) see below image, except as permitted in other sections, such the following:
Small conductor sizes 18 and 16 AWG as permitted by 240.4(D)(1) and (2)
Flexible cords as permitted by Table 400.4
Fixture wire as permitted by 402.6
Motors rated 1 hp or less as permitted by 430.22(F)
Cranes and hoists as permitted by 610.14
Elevator control and signaling circuits as permitted by 620.12
Class 1, Class 2, and Class 3 circuits as permitted by 725.49(A) and 725.127, Exception
Fire alarm circuits as permitted by 760.49(A); 760.127, Exception; and 760.179(B)
Motor-control circuits as permitted by 430.72
Control and instrumentation circuits as permitted by 727.6
Electric signs and outline lighting as permitted in 600.31(B) and 600.32(B)
Notes for Rule#4:
The smallest size conductor permitted by the NEC for branch circuits, feeders, or services is 14 AWG copper or 12 AWG aluminum.
Some local codes require a minimum 12 AWG for commercial and industrial installations.
Conductors smaller than 14 AWG are permitted for:
1. Class 1 remote-control circuits,
2. Fixture wire,
3. Flexible cords,
4. Motor control circuits,
5. Non-power-limited fire alarm circuits,
6. Power-limited fire alarm circuits.
As a guide when sizing conductors, the following table must always be used to determine the minimum size conductor:
In the next Article, I will explain the correction Factors affecting conductor ampacity Tables. Please, keep following.
