0.2 NEC 2005 Hanbook Introduction

Article 90

Introduction

90.1 PurposeSummary of Changes (A) Practical Safeguarding The purpose of this Code is

the practical safeguarding of persons and property from haz-• 90.2(B): Added FPN providing information on utilitiesards arising from the use of electricity.• 90.2(C): Added paragraph describing bracketed

references.The National Electrical Code (NEC) is prepared by theNational Electrical Code Committee, which consists of aTechnical Correlating Committee and 19 code-making pan-

Contents els. The code-making panels have specific subject responsi-bility within the Code. The scope of the National Electrical90.1 PurposeCode Committee is as follows:(A) Practical Safeguarding

(B) Adequacy This committee shall have primary responsibility for(C) Intention documents on minimizing the risk of electricity as a(D) Relation to Other International Standards source of electric shock and as a potential ignition

90.2 Scope source of fires and explosions. It shall also be responsi-(A) Covered ble for text to minimize the propagation of fire and(B) Not Covered explosions due to electrical installations.(C) Special Permission

90.3 Code Arrangement In addition to its overall responsibility for the National90.4 Enforcement Electrical Code, the Technical Correlating Committee is90.5 Mandatory Rules, Permissive Rules, and Explanatory responsible for NFPA 70A, National Electrical Code Re-

Material quirements for One- and Two-Family Dwellings, and for(A) Mandatory Rules correlation of the following:(B) Permissive Rules

1. NFPA 70B, Recommended Practice for Electrical(C) Explanatory MaterialEquipment Maintenance90.6 Formal Interpretations

2. NFPA 70E, Standard for Electrical Safety in the Work-90.7 Examination of Equipment for Safetyplace90.8 Wiring Planning

3. NFPA 73, Electrical Inspection Code for Existing(A) Future Expansion and ConvenienceDwellings(B) Number of Circuits in Enclosures

4. NFPA 79, Electrical Standard for Industrial Machinery90.9 Units of Measurement5. NFPA 110, Standard for Emergency and Standby Power(A) Measurement System of PreferenceSystems(B) Dual System of Units

6. NFPA 111, Standard on Stored Electrical Energy Emer-(C) Permitted Uses of Soft Conversiongency and Standby Power Systems(D) Compliance

1

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90.2 Article 90 — Introduction

calls attention to the fact that installations meeting the re-(B) Adequacy This Code contains provisions that are con-sidered necessary for safety. Compliance therewith and quirements of the NEC are also in compliance with the

fundamental principles outlined in IEC 60364-1, Electricalproper maintenance results in an installation that is essen-tially free from hazard but not necessarily efficient, conve- Installations of Buildings, Section 131. That TIA allowed

countries that do not have formalized rules for electricalnient, or adequate for good service or future expansion ofelectrical use. installations to adopt the NEC and by so doing to be fully

compatible with the safety principles of IEC 60364-1, Sec-FPN: Hazards often occur because of overloading of

tion 131. The addition of 90.1(D) will promote acceptancewiring systems by methods or usage not in conformityand adoption of the NEC internationally.with this Code. This occurs because initial wiring did

not provide for increases in the use of electricity. An The NEC is an essential part of the safety system ofinitial adequate installation and reasonable provisions for the Americas, and its future will be enhanced by increasedsystem changes provide for future increases in the use international acceptance.of electricity.

90.2 ScopeConsideration should always be given to future expansionof the electrical system. Future expansion might be unlikely (A) Covered This Code covers the installation of electricalin some occupancies, but for others it is wise to plan an conductors, equipment, and raceways; signaling and com-initial installation comprised of service-entrance conductors munications conductors, equipment, and raceways; and opti-and equipment, feeder conductors, and panelboards that cal fiber cables and raceways for the following:allows for future additions, alterations, designs, and so on. (1) Public and private premises, including buildings, struc-

tures, mobile homes, recreational vehicles, and floatingbuildings(C) Intention ThisCode is not intended as a design specifi-

(2) Yards, lots, parking lots, carnivals, and industrial substa-cation or an instruction manual for untrained persons.tions

FPN to (2): For additional information concerning suchTheNEC is intended for use by capable engineers and electri-installations in an industrial or multibuilding complex,cal contractors in the design and/or installation of electricalsee ANSI C2-2002, National Electrical Safety Code.

equipment; by inspection authorities exercising legal juris-diction over electrical installations; by property insurance

Requirements for locations such as these are found through-inspectors; by qualified industrial, commercial, and residen-out the Code. Specific items such as outside feeders andtial electricians; and by instructors of electrical apprenticesbranch circuits can be found in Article 225, grounding inor students.Article 250, surge arresters in Article 280, switches in Article404, outside lighting in Article 410, transformers in Article450, and carnivals in Article 525.(D) Relation to Other International Standards The re-

quirements in this Code address the fundamental principlesof protection for safety contained in Section 131 of Interna-

(3) Installations of conductors and equipment that connecttional Electrotechnical Commission Standard 60364-1, Elec-

to the supply of electricitytrical Installations of Buildings.

FPN: IEC 60364-1, Section 131, contains fundamental Often, but not always, the source of supply of electricity isprinciples of protection for safety that encompass protec- the serving electric utility. The point of connection from ation against electric shock, protection against thermal

premises wiring system to a serving electric utility systemeffects, protection against overcurrent, protection againstis, by definition, referred to as the service point. The conduc-fault currents, and protection against overvoltage. All of

these potential hazards are addressed by the requirements tors on the premises side of the service point are, by defini-in this Code. tion, referred to as service conductors. (These definitions

are found in Article 100.) The requirements for service con-ductors as well as for service-related equipment are foundIn addition to being the most widely adopted code for the

built environment in the United States, the NEC is also in Article 230. Article 230 applies only where the source ofsupply of electricity is from a utility.adopted and used extensively in many foreign countries.

Section 90.1(D) makes it clear that the NEC is compatible Where the source of supply of electricity is not theserving electric utility, the source may be a generator, awith international safety principles. Added as a Tentative

Interim Amendment (TIA) to the 1999 Code, this section battery system, a solar photovoltaic system, a fuel cell, or

2005 National Electrical Code Handbook2



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90.2Article 90 — Introduction

a combination of those sources. Requirements for suchsources of supply are found in Article 445 and Articles 700through 702 for generators, Article 480 for storage batteries,Article 690 for solar photovoltaic systems, and Article 692for fuel cells. The associated delivery wiring requirementsare found in Chapters 2 and 3 (except Article 230) and inArticles 700 through 702 for emergency, legally required,and optional standby power system circuits.

(4) Installations used by the electric utility, such as officebuildings, warehouses, garages, machine shops, and rec-reational buildings, that are not an integral part of agenerating plant, substation, or control center.

Section 90.2(A), which was rewritten for the 2002 Code,provides order and clarity concerning the portions of electricutility facilities covered by the NEC. [See 90.2(B) and therelated commentary for information on facilities and specificlighting not covered by the NEC.] Exhibit 90.1 illustratesthe distinction between electric utility facilities to which theNEC does and does not apply.

Industrial and multibuilding complexes and campus-style wiring often include substations and other installationsthat employ construction and wiring similar to those of elec-tric utility installations. Although such nonutility installa-tions are within the scope of the NEC, the NEC requirementsmay not always be all-inclusive, for example, in clearancesof conductors or in clearances from buildings or structuresfor nominal voltages over 600 volts. In such cases, the usercan find additional information in the National Electrical

Generation controland transmission

Garage

Gym

Machine shop

Warehouse

Officebuilding

Substation

Distributionand metering

NEC does not apply.

NEC applies to these buildings.

Safety Code (NESC), published by the Institute of Electricaland Electronics Engineers, Inc., P.O. Box 1331, 445 Hoes Exhibit 90.1 Typical electric utility complexes showing examples

of facilities covered and not covered by the provisions of theLane, Piscataway, NJ 08855-1331.NEC.

(B) Not Covered This Code does not cover the following: ments for floating buildings are covered in the NEC and arefound in Article 553.(1) Installations in ships, watercraft other than floating

buildings, railway rolling stock, aircraft, or automotivevehicles other than mobile homes and recreational (2) Installations underground in mines and self-propelledvehicles mobile surface mining machinery and its attendant elec-

trical trailing cableFPN: Although the scope of this Code indicates that the

(3) Installations of railways for generation, transformation,Code does not cover installations in ships, portions oftransmission, or distribution of power used exclusivelythis Code are incorporated by reference into Title 46,

Code of Federal Regulations, Parts 110–113. for operation of rolling stock or installations used exclu-sively for signaling and communications purposes

(4) Installations of communications equipment under theThe NEC does not specifically cover shipboard wiring. Titleexclusive control of communications utilities located46 of the Code of Federal Regulations, Parts 110–113, how-outdoors or in building spaces used exclusively for suchever, does contain many specific NEC-referenced require-installationsments. These requirements, which originated in the NEC,

(5) Installations under the exclusive control of an electricare enforced by the U.S. Coast Guard. Installation require-utility where such installations

National Electrical Code Handbook 2005 3



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a. Consist of service drops or service laterals, and asso- generally; Chapters 5, 6, and 7 apply to special occupancies,special equipment, or other special conditions. These latterciated metering, or

b. Are located in legally established easements, rights- chapters supplement or modify the general rules. Chapters1 through 4 apply except as amended by Chapters 5, 6, andof-way, or by other agreements either designated by

or recognized by public service commissions, utility 7 for the particular conditions.Chapter 8 covers communications systems and is notcommissions, or other regulatory agencies having

jurisdiction for such installations, or subject to the requirements of Chapters 1 through 7 exceptwhere the requirements are specifically referenced in Chap-c. Are on property owned or leased by the electric

utility for the purpose of communications, metering, ter 8.Chapter 9 consists of tables.generation, control, transformation, transmission, or

distribution of electric energy. Annexes are not part of the requirements of this Codebut are included for informational purposes only.FPN to (4) and (5): Examples of utilities may include

those entities that are typically designated or recognizedby governmental law or regulation by public service/ The reference to ‘‘the introduction’’ is intended to includeutility commissions and that install, operate, and maintain Article 90 in the application of the Code. Chapters 1 throughelectric supply (such as generation, transmission, or dis-

4 apply generally, except as amended or specifically refer-tribution systems) or communication systems (such asenced in Chapters 5, 6, and 7 (Articles 500 through 780).telephone, CATV, Internet, satellite, or data services).

Utilities may be subject to compliance with codes and For example, 300.22 (Chapter 3) is modified by 725.3(C)standards covering their regulated activities as adopted and 760.3(B) and is specifically referenced in 800.133(D)under governmental law or regulation. Additional infor- and 830.3(B). A graphic explanation of the NEC arrange-mation can be found through consultation with the appro-

ment, Figure 90.3, was added to the 2002 Code.priate governmental bodies, such as state regulatorycommissions, Federal Energy Regulatory Commission,and Federal Communications Commission.

An FPN was added to the 2005 Code to clarify the use ofthe word utility as used in 90.2(B)(4) and 90.2(B)(5). Thisexplanatory information now provides the authority havingjurisdiction a basis for judgment concerning this issue.

It is not the intent of 90.2(B)(5) to exclude the NEC asan installation regulatory document. After all, the NEC isfully capable of being utilized for electrical installations inmost cases, and 90.2(B)(5) does not pertain to areas whereportions of the NEC could not be used. Rather, 90.2(B)(5)lists specific areas where the nature of the installation re-quires specialized rules or where other installation rules,standards, and guidelines have been developed for specificuses and industries. For example, the electrical utility indus-try uses the NESC as its primary requirement in the genera-tion, transmission, distribution, and metering of electricalenergy. See Exhibit 90.1 for examples of electric utilityfacilities that may or may not be covered by the Code.

Chapter 1 — General

Chapter 2 — Wiring and Protection

Chapter 3 — Wiring Methods and Materials

Chapter 4 — Equipment for General Use

Applies generallyto all electrical

installations

Chapter 5 — Special Occupancies

Chapter 6 — Special Equipment

Chapter 7 — Special Conditions

Supplements or modifiesChapters 1 through 4

Chapter 9 — Tables

Annex A through Annex G

Chapter 8 — Communications Systems

Informational only; not mandatory

Applicable as referenced

Chapter 8 is not subject to the requirements of Chapters 1 through 7 except where the requirements are specifically referenced in Chapter 8.

(C) Special Permission The authority having jurisdictionfor enforcing this Code may grant exception for the installa-

Figure 90.3 Code Arrangement.tion of conductors and equipment that are not under theexclusive control of the electric utilities and are used to

90.4 Enforcementconnect the electric utility supply system to the service-entrance conductors of the premises served, provided such This Code is intended to be suitable for mandatory applica-installations are outside a building or terminate immediately tion by governmental bodies that exercise legal jurisdictioninside a building wall. over electrical installations, including signaling and commu-

nications systems, and for use by insurance inspectors. The90.3 Code Arrangement authority having jurisdiction for enforcement of the Code

has the responsibility for making interpretations of the rules,This Code is divided into the introduction and nine chapters,as shown in Figure 90.3. Chapters 1, 2, 3, and 4 apply for deciding on the approval of equipment and materials,




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and for granting the special permission contemplated in a 90.5 Mandatory Rules, Permissive Rules, andnumber of the rules. Explanatory Material

(A) Mandatory Rules Mandatory rules of this Code areSome localities do not adopt the NEC, but even in those those that identify actions that are specifically required orlocalities, installations that comply with the current Code prohibited and are characterized by the use of the termsare prima facie evidence that the electrical installation is shall or shall not.safe.

Section 90.4 advises that all materials and equipment Section 90.5, which was revised and reorganized for theused under the requirements of the Code are subject to the 1999 Code, clarifies that two distinctive types of rules areapproval of the authority having jurisdiction. The text of stated in the Code. Mandatory rules, characterized by the90.7, 110.2, and 110.3, along with the definitions of the terms terms shall and shall not, are covered in 90.5(A).approved, identified (as applied to equipment), labeled, andlisted, is intended to provide a basis for the authority havingjurisdiction tomake the judgments that fall within that partic- (B) Permissive Rules Permissive rules of this Code areular area of responsibility. those that identify actions that are allowed but not required,

The phrase ‘‘including signaling and communication are normally used to describe options or alternative methods,systems’’ was added to the 2002 Code to emphasize that, and are characterized by the use of the terms shall be permit-indeed, these systems are also subject to enforcement. ted or shall not be required.

Permissive rules are simply options or alternative methodsBy special permission, the authority having jurisdictionof achieving equivalent safety — they are not requirements.may waive specific requirements in this Code or permitA close reading of permissive terms is important, becausealternative methods where it is assured that equivalent objec-permissive rules are often misinterpreted. For example, thetives can be achieved by establishing and maintaining effec-frequently used permissive term shall be permitted can betive safety.mistaken for a requirement. Substituting ‘‘the inspector mustallow [item A or method A]’’ for ‘‘[item A or method A]It is the responsibility of the authority having jurisdictionshall be permitted’’ generally clarifies the interpretation.to interpret the specific rules of the Code. This paragraph

empowers the authority having jurisdiction, using specialpermission (written consent), to permit alternative methods

(C) Explanatory Material Explanatory material, such aswhere specific rules are not established in the Code. Forreferences to other standards, references to related sections ofexample, the authority having jurisdiction may waive spe-thisCode, or information related to aCode rule, is included incific requirements in industrial occupancies, research andthis Code in the form of fine print notes (FPNs). Fine printtesting laboratories, and other occupancies where the specificnotes are informational only and are not enforceable as re-type of installation is not covered in the Code.quirements of this Code.

Brackets containing section references to another NFPAThis Code may require new products, constructions, or document are for informational purposes only and are pro-

vided as a guide to indicate the source of the extracted text.materials that may not yet be available at the time the Codeis adopted. In such event, the authority having jurisdiction These bracketed references immediately follow the extracted

text.may permit the use of the products, constructions, or materi-als that comply with the most recent previous edition of thisCode adopted by the jurisdiction. A number of requirements in the NEC have been extracted

from other NFPA codes and standards. Therefore, a secondparagraph was added for the 2005 Code to prevent anyThis paragraph of 90.4 permits the authority having jurisdic-

tion to waive a new Code requirement during the interim misunderstanding about the purpose of bracketed referencesto other NFPA codes and standards — they are providedperiod between acceptance of a new edition of the NEC and

the availability of a new product, construction, or material only to indicate the section of the NFPA document fromwhich the material in the NEC was extracted. Althoughredesigned to comply with the increased safety required by

the latest edition. It is difficult to establish a viable future NEC requirements based on extracted material are under thejurisdiction of the technical committee responsible for theeffective date in each section of the NEC because the time

needed to change existing products and standards, as well particular document in which the extracted material resides,this revision to 90.5(C) makes it clear that the NEC require-as to develop new materials and test methods, usually is not

known at the time the latest edition of the Code is adopted. ments stand on their own and that extracted material




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with bracketed references does not indicate that other NFPA tions Governing Committee Projects. These regulations aredocuments are adopted through reference. included in the NFPA Directory, which is published annually

Fine print notes (FPNs) do not contain statements of and can be obtained from the Secretary of the NFPA Stan-intent or recommendations. They present additional supple- dards Council. The Formal Interpretations procedure can bementary material that aids in the application of the require- found in Section 6 of the Regulations.ment. In addition to explanatory material being in fine print The National Electrical Code Committee cannot be re-(small type), the material is further identified in the Code sponsible for subsequent actions of authorities enforcing theby the abbreviation FPN preceding the paragraph. Fine print NEC that accept or reject its findings. The authority havingnotes are not requirements of the NEC and are not enforce- jurisdiction is responsible for interpreting Code rules andable. should attempt to resolve all disagreements at the local level.

Footnotes to tables, although also in fine print, are not Two general forms of Formal Interpretations are recognized:explanatory material unless they are identified by the abbre- (1) those that are interpretations of the literal text and (2)viation FPN. Table footnotes are part of the tables and are those that are interpretations of the intent of the Committeenecessary for proper use of the tables. For example, the at the time the particular text was issued.footnotes at the end of Table 310.13 are necessary for the Interpretations of the NEC not subject to processing areuse of the table and therefore are mandatory and enforceable those that involve (1) a determination of compliance of aCode text. design, installation, product, or equivalency of protection;

Additional explanatory material is also found in the (2) a review of plans or specifications or judgment or knowl-annexes at the back of this handbook. Annex A is a reference edge that can be acquired only as a result of on-site inspec-list of product safety standards used for product listing where tion; (3) text that clearly and decisively provides thethat listing is required by the Code. Annex B provides guid- requested information; or (4) subjects not previously consid-ance on the use of the general formula for ampacity found ered by the Technical Committee or not addressed in thein 310.15(C). Annex C consists of wire fill tables for conduit document. Formal Interpretations of Code rules are pub-and tubing. Annex D contains example calculations. Annex lished in several venues, including necdigest�, the NFPAE presents various tables showing fire resistance ratings for Electrical Section News segment found in theNFPA Journal,Types I-V construction to correlate with the uses of Type NM in NFPA News, and in the National Fire Codes subscriptioncable, Annex F contains cross-reference tables for Chapter 3 service and are sent to interested trade publications.realignment with the 2002 edition, and Annex G contains Most interpretations of the NEC are rendered as themodel administration and enforcement legislation. personal opinions of NFPA electrical engineering staff or of

an involved member of the National Electrical Code Com-mittee because the request for interpretation does not qualifyFPN: The format and language used in this Code followsfor processing as a Formal Interpretation in accordance withguidelines established by NFPA and published in the

NEC Style Manual. Copies of this manual can be obtained NFPA Regulations Governing Committee Projects. Suchfrom NFPA. opinions are rendered in writing only in response to written

requests. The correspondence contains a disclaimer indicat-ing that it is not a Formal Interpretation issued pursuant toThis fine print note informs the user that a style manual is

available for the NEC. A style manual is basically a ‘‘how- NFPA Regulations and that any opinion expressed is thepersonal opinion of the author and does not necessarilyto’’ pamphlet for editors. The NEC Style Manual contains

a list of rules and regulations used by the panels and editors represent the official position of NFPA or the National Elec-trical Code Committee.who prepare the NEC. The NEC Style Manual, which was

revised for the 2002 edition of the Code, is available fromNFPA.

90.7 Examination of Equipment for Safety90.6 Formal Interpretations For specific items of equipment and materials referred to in

this Code, examinations for safety made under standardTo promote uniformity of interpretation and application ofthe provisions of this Code, formal interpretation procedures conditions provide a basis for approval where the record is

made generally available through promulgation by organiza-have been established and are found in theNFPARegulationsGoverning Committee Projects. tions properly equipped and qualified for experimental test-

ing, inspections of the run of goods at factories, and service-value determination through field inspections. This avoidsThe procedures for implementing Formal Interpretations ofthe necessity for repetition of examinations by different ex-the provisions of the NEC are outlined in the NFPA Regula-aminers, frequently with inadequate facilities for such work,




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and the confusion that would result from conflicting reports ways, and additional spaces allow for future increases inelectric power and communication circuits. Distribution cen-on the suitability of devices and materials examined for a

given purpose. ters located in readily accessible locations provide conve-nience and safety of operation.It is the intent of this Code that factory-installed internal

wiring or the construction of equipment need not be in-spected at the time of installation of the equipment, except

The requirement for providing the exclusively dedicatedto detect alterations or damage, if the equipment has been

equipment space mandated by 110.26(F) supports the intentlisted by a qualified electrical testing laboratory that is recog-

of 90.8(A) regarding future increases in the use of electricity.nized as having the facilities described in the preceding

The phrase ‘‘and communications circuits’’ was added forparagraph and that requires suitability for installation in

the 2005 Code to point out the importance of consideringaccordance with this Code.

communications circuits when planning future needs. Elec-trical and communications distribution centers should con-

FPN No. 1: See requirements in 110.3. tain additional space and capacity for future additions andFPN No. 2: Listed is defined in Article 100. should be conveniently located for easy accessibility.FPN No. 3: Annex A contains an informative list of Where electrical and communications distributionproduct safety standards for electrical equipment. equipment is installed so that easy access cannot be achieved,

a spare raceway(s) or pull line(s) should be run at the initialinstallation, as illustrated in Exhibit 90.2.Testing laboratories, inspection agencies, and other organiza-

tions concerned with product evaluation publish lists ofequipment and materials that have been tested and meet

(B) Number of Circuits in Enclosures It is elsewherenationally recognized standards or that have been found

provided in this Code that the number of wires and circuitssuitable for use in a specified manner. The Code does not

confined in a single enclosure be varyingly restricted.contain detailed information on equipment or materials but

Limiting the number of circuits in a single enclosurerefers to products as ‘‘listed,’’ ‘‘labeled,’’ or ‘‘identified.’’

minimizes the effects from a short circuit or ground faultSee Article 100 for definitions of these terms.

in one circuit.NFPA does not approve, inspect, or certify any installa-

tions, procedures, equipment, or materials, nor does it ap-These limitations minimize the heating effects inherentlyprove or evaluate testing laboratories. In determining thepresent wherever current-carrying conductors are groupedacceptability of installations or procedures, equipment, ortogether. See 408.35 for restrictions on the number of over-materials, the authority having jurisdiction may base accep-current devices on one panelboard.tance on compliance with NFPA or other appropriate stan-

dards. In the absence of such standards, the authority mayrequire evidence of proper installation, procedures, or use.The authority having jurisdiction may also refer to the listing 90.9 Units of Measurementor labeling practices of an organization concerned with prod- (A) Measurement System of Preference For the purposeuct evaluations that is able to determine compliance with of this Code, metric units of measurement are in accordanceappropriate standards for the current production of listed with the modernized metric system known as the Interna-items. tional System of Units (SI).

Annex A contains a list of product safety standards usedfor product listing. The list includes only product safety

According to a recent report titled ‘‘A Metric for Success’’standards for which a listing is required by the Code. Forby the National Institute of Standards and Technologyexample, 344.6 requires that rigid metal conduit, Type RMC,(NIST), most U.S. industries that do business abroad arebe listed. By using Annex A, the user finds that the listingpredominantly metric already because of global sourcing ofstandard for rigid metal conduit is UL 6, Rigid Metal Con-parts, service, components, and production. However, quiteduit. Because associated conduit fittings are required to bea few domestic industries still use U.S. Customary units.listed, UL 514B, Fittings for Cable and Conduit, is foundThe NIST report warns that domestic industries that ignorein Annex A also.global realities and continue to design and manufacture withnonmetric measures will find that they risk increasing theircosts. Nonmetric modular products (the building construc-90.8 Wiring Planningtion industry uses great quantities ofmodular parts) and those

(A) Future Expansion and Convenience Plans and speci- that interface with outside industry products are especiallyfications that provide ample space in raceways, spare race-




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Exhibit 90.2 A residential dis-tribution system showing spareraceways or pull lines thatallow for future circuits andloads for both electrical andcommunications systems.

Spare raceways or pull linesfrom electrical service panel and communications center to accessible attic space

Junctionboxes

Unfinishedutility area

Finishedbasement area

Racewayor pulllines

Servicepanel

Communicationscenter

NFPA documents), and safety calculations (e.g., minimumvulnerable to the added costs of adapting to a metric environ-distances taken from Table 110.31).ment.

Metric standards are beginning to appear in the domestic Examplebuilding construction industry because our national stan-

Using the hard-conversion method, determine the equivalentdards are being harmonized with international standards.

metric conversion for 24 in., generally the minimum coverThe National Electrical Code is an important building con-

requirements for direct burial cables and conductors in non-struction standard and moves another step in the metric

specific locations taken from row 1 of Table 300.5.direction.

Solution

Step 1.(B) Dual System of Units SI units shall appear first, andinch-pound units shall immediately follow in parentheses. 24 in. �

25.4 mm1 in.

� 609.6 mmConversion from inch-pound units to SI units shall be basedon hard conversion except as provided in 90.9(C). Step 2. Because the calculation is being performed as a hard

conversion, the 609.6 mm dimension may be changed, andthe selected equivalent cover requirement is 600 mm.Hard conversion is explained in FPN No. 1 following

90.9(D). Calculations to convert measurements from inch- For the 2005 Code as well as the 2002 Code, the mea-surements of 600 mm and 24 in. appear in Table 300.5 forpound units to metric units must be made using hard conver-

sion. The hard-conversion method is mandatory except for the minimum cover requirements for direct burial cables andconductors in nonspecific locations. For the 1999 NEC, thetrade sizes [e.g., raceway sizes in Table 300.1(C)], extracted

material (e.g., class and zone measurements from other selected SI unit of measure was required to be 609.6 mm.




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Example 1The 2005 Code (as well as the 2002 Code) permits muchmore latitude for the final selected dimension, and so the Determine the equivalent metric conversion for 10 ft whereequivalent minimum cover requirement of 600 mm is a more the calculation could have a negative impact on safety, suchpractical solution. Basically, a hard conversion permits a as the minimum distance of 10 ft given in Table 110.31,change in a dimension or allows rounding up or down to and where the measurement is from a fence to a live partbetter fit the physical constraints of the installation. from 601 volts to 13,799 volts.

Solution

(C) Permitted Uses of Soft Conversion The cases given Step 1.10 ft �

0.3048 m1 ft

� 3.048 min 90.9(C)(1) through (C)(4) shall not be required to use hardconversion and shall be permitted to use soft conversion. Step 2. Round up the calculation to 3.05 m, because a dis-

tance less than 3.048 could have a negative impact on safety.(1) Trade Sizes Where the actual measured size of a prod-The answer, 3.05 m, matches the minimum distance in Tableuct is not the same as the nominal size, trade size designators110.31 from a fence to a live part from 601 volts to 13,799shall be used rather than dimensions. Trade practices shallvolts.be followed in all cases.

Because safety is a concern for this conversion calcula-tion, the original Code distance (the U.S. Customary units

Metric trade sizes (metric designators) of conduits were for this example) remains the shortest permitted distance.added in the 1996 Code as fine print notes in each raceway The final metric equivalent ends up slightly larger. The exactarticle. Since the 2002 Code, these metric designators appear difference is of no practical concern, however, because 0.2in the Code text, preceding the trade size equivalents, in the mm is less than 1/32 in. From a practical point of view, araceway articles. variance of 1/32 in. in a length of 10 ft is insignificant.

For example, in 350.20(A) of thisCode, the size require-Example 2ment is stated as follows: ‘‘LFMC smaller than metric desig-

nator 16 (trade size 1/2) shall not be used.’’ In 351-5(a) of Using the soft-conversion method, determine the equivalentthe 1999 NEC, the size requirement was stated as follows: metric conversion for 30 in. where the calculation could‘‘Liquidtight flexible metallic conduit smaller than 1/2-in. have a negative impact on safety, such as a 30 in. minimumelectrical trade size shall not be used.’’ horizontal working space requirement in the rear of equip-

This change does not reflect a technical change but ment that requires access to nonelectrical parts according torather provides acceptable language to both domestic and 110.26(A)(1)(a).international users of the NEC. For ease of use, in Table 4

Solutionof Chapter 9, metric designators are separate columns.Step 1.

30 in. �25.4 mm1 in.

� 762 mm

(2) Extracted Material Where material is extracted from Step 2. Do not round off the calculation, because even aanother standard, the context of the original material shall slight reduction in the original distance could have a negativenot be compromised or violated. Any editing of the extracted impact on safety. The answer is 762 mm, which matchestext shall be confined to making the style consistent with the minimum distance of 110.26(A)(1)(a) for a minimumthat of the NEC. horizontal working space.

(3) Industry Practice Where industry practice is to expressunits in inch-pound units, the inclusion of SI units shall not (4) Safety Where a negative impact on safety would result,be required. soft conversion shall be used.

(D) Compliance Conversion from inch-pound units to SIThe following examples illustrate conversions from U.S.units shall be permitted to be an approximate conversion.Customary units to SI units. Example 1 shows the processCompliance with the numbers shown in either the SI systemof converting a dimension from feet to meters, where safetyor the inch-pound system shall constitute compliance withis a concern. Table 110.31 contains minimum permitted dis-this Code.tances from a fence to a live part for voltages 601 and greater.

Example 1 calculates the equivalent metric conversion for FPN No. 1: Hard conversion is considered a change in10 ft using the minimum distance of 10 ft in Table 110.31 dimensions or properties of an item into new sizes thatwhere the measurement is from a fence to a live part from might or might not be interchangeable with the sizes

used in the original measurement. Soft conversion is601 volts to 13,799 volts.considered a direct mathematical conversion and involves




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a change in the description of an existing measurement Commentary Table 90.1 Conversions Using the Hard-but not in the actual dimension. Conversion MethodFPN No. 2: SI conversions are based on IEEE/ASTMSI 10-1997, Standard for the Use of the International

U.S. Soft HardSystem of Units (SI): The Modern Metric System.Customary Conversions, Conversions, Equivalent U. S.

Units SI Units SI Units Customary UnitsCommentary Table 90.1 offers some examples of the hard-

1⁄2 in. 12.7 mm 13 mm 0.51 in.conversion process. U.S. Customary units were used in the3⁄4 in. 19 mm 19 mm 0.75 in.1993, 1996, and 1999 Code and were still valid for the 20021 in. 25.4 mm 25 mm 0.98 in.Code. Soft-conversion SI units were used in the 1996 and4 in. 102 mm 100 mm 3.94 in.

1999 Code. The hard-conversion SI units, which were added 12 in. 305 mm 300 mm 11.81 ftto the 2002 Code, were listed with their equivalent U.S. 2 ft 610 mm 600 mm 1.97 ftCustomary units. The equivalent U.S. units are given only 3 ft 914 mm 900 mm 2.95 ft

6 ft 1.83 m 1.8 m 5.91 ftto show the small variance between customary units and the15 ft 4.57 m 4.5 m 14.76 fthard-conversion units.

Warning signs that state specific clearances, such asrequired in 513.10(B), permit distance measurements in ei-ther inch-pound units or metric units.




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0.2 NEC 2005 Hanbook Introduction

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