National electrical codeb

NATIONAL ELECTRICALHANDBOOK Twenty-Sixth EditionBased on the Current 2008 National Electrical Code® National Fire Protection Association.. The 26th edition of McGraw-Hill’s National Elec

McGraw-Hill’s

About the Authors

Brian J McPartland is an electrical consultant and educator who teaches

the nuts and bolts of the National Electrical Code® He is coauthor of

McGraw-Hill’s National Electrical Code®Handbook, 25th Edition.

Joseph F McPartland is an electrical contracting consultant and

coau-thor of McGraw-Hill’s National Electrical Code®Handbook, 25th Edition.

Frederic P Hartwell is a working electrician, President of Hartwell

Electrical Services, Inc., and has been certified by the InternationalAssociation of Electrical Inspectors as a Certified Master ElectricalInspector He is the senior member of NEC®CMP 9 He is coauthor ofMcGraw-Hill’s American Electricians’ Handbook, 15th Edition

NATIONAL ELECTRICAL

HANDBOOK Twenty-Sixth EditionBased on the Current 2008

National Electrical Code®

National Fire Protection Association.

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Preface xi

Introduction to the National Electrical Code® xiii

Brief History of the National Electrical Code® xv

About the 2008 NE Code® xvii

Article Page

Chapter 3

314 Outlet, Device, Pull, and Junction Boxes; Conduit Bodies; Fittings;

354 Nonmetallic Underground Conduit with Conductors: Type NUCC 768

427 Fixed Electric Heating Equipment for Pipelines and Vessels 946

506 Zone 20, 21, 22 Locations for Combustible Dusts or Ignitible

520 Theaters, Audience Areas of Motion Picture and Television Studios,

530 Motion Picture and Television Studios and Similar Locations 1257

605 Office Furnishings (Consisting of Lighting Accessories

620 Elevators, Dumbwaiters, Escalators, Moving Walks, Platform Lifts,

640 Audio Signal Processing, Amplification, and Reproduction

Chapter 7

725 Class 1, Class 2, and Class 3 Remote-Control, Signaling, and

Article Page

Chapter 8

820 Community Antenna Television and Radio Distribution Systems 1496

Chapter 9

Annex C Conduit and Tubing Fill Tables for Conductors and Fixture

Annex F Availability and Reliability for Operations Power Systems;

and Development and Implementation of Functional

Performance Tests (FPTS) for Critical Operations

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The 26th edition of McGraw-Hill’s National Electrical Code®Handbook has

been thoroughly revised to reflect the changes given in the 2008 NationalElectrical Code This is a reference book of commentary, discussion, and anal-ysis on the most commonly encountered rules of the 2008 National ElectricalCode Designed to be used in conjunction with the 2008 NE Code book pub-lished by the National Fire Protection Association, this Handbook presentsthousands of illustrations—diagrams and photos—to supplement thedetailed text in explaining and clarifying NECregulations Description of thebackground and rationale for specific Code rules is aimed at affording abroader, deeper, and readily developed understanding of the meaning andapplication of those rules The style of presentation is conversational andintended to facilitate a quick, practical grasp of the ideas and concepts thatare couched in the necessarily terse, stiff, quasi-legal language of the NEC

document itself

This Handbook follows the order of “articles” as presented in the NE Codebook, starting with “Article 90” and proceeding through “Appendix.” TheCode rules are referenced by “section” numbers (e.g., “250.138 Cord- and Plug-Connected Equipment.”) This format ensures quick and easy correlationbetween NEC sections and the discussions and explanations of the rulesinvolved This companion reference to the NECbook expands on the rules andpresents common interpretations that have been put on the many difficult andcontroversial Code requirements A user of this Handbook should refer to the

xii PREFACE

NECbook for the precise wording of a rule and then refer to the correspondingsection number in this Handbook for a practical evaluation of the details.Because many NECrules do not present difficulty in understanding or inter-pretation, not all sections are referenced But the vast majority of sections arecovered, especially all sections that have proved troublesome or controversial.And particular emphasis is given to changes and additions that have been made

in Code rules over recent editions of the NEC Although this new edition,

McGraw-Hill’s National Electrical Code®2008 Handbook, does not contain the

complete wording of the NE Code book, it does contain much greater analysisand interpretation than any other so-called Handbook contains

Today, the universal importance of the NE Code has been established by thefederal government (OSHA and other safety-related departments), by state andlocal inspection agencies, and by all kinds of private companies and organiza-tions In addition, national, state, and local licensing or certification as an elec-trical contractor, master electrician, or electrical inspector will require a firmand confident knowledge of the NEC With requirements for certification orlicensing now mandated in nearly every jurisdiction across the country, theneed for Code competence is indispensable To meet the great need for infor-mation on the NEC, McGraw-Hill has been publishing a handbook on the

National Electrical Codesince 1932 Originally developed by Arthur L Abbott inthat year, the Handbook has been carried on in successive editions for eachrevision of the National Electrical Code

One final point—words such as “workmanlike” are taken directly from theCode and are intended in a purely generic sense Their use is in no way meant

to deny the role women already play in the electrical industries or their tance to the field

impor-Frederic P HartwellBrian J McPartlandJoseph F McPartland

Introduction to the

McGraw-Hill’s National Electrical Code®Handbook is based on the 2008

edi-tion of the National Electrical Code as developed by the National Electrical Code

Committee of the American National Standards Institute (ANSI), sponsored bythe National Fire Protection Association® (NFPA®) The National ElectricalCode is identified by the designation NFPANo 70-2008 The NFPAadopted the

2008 Code at the NFPATechnical Meeting held in June, 2007

TheNational Electrical Code, as its name implies, is a nationally accepted guide

to the safe installation of electrical wiring and equipment The committee soring its development includes all parties of interest having technical compe-tence in the field, working together with the sole objective of safeguarding thepublic in its utilization of electricity The procedures under which the Code isprepared provide for the orderly introduction of new developments and improve-ments in the art, with particular emphasis on safety from the standpoint of itsend use The rules of procedure under which the National Electrical CodeCom-mittee operates are published in each official edition of the Code and in sepa-rate pamphlet form so that all concerned may have full information and freeaccess to the operating procedures of the sponsoring committee The Code hasbeen a big factor in the growth and wide acceptance of the use of electricalenergy for light and power and for heat, radio, television, signaling, and otherpurposes from the date of its first appearance (1897) to the present

spon-TheNational Electrical Codeis primarily designed for use by trained electricalpeople and is necessarily terse in its wording.

The sponsoring National Electrical CodeCommittee is composed of a TechnicalCorrelating Committee and 20 Code-Making Panels, each responsible for one ormore Articles in the Code Each Panel is composed of experienced individualsrepresenting balanced interests of all segments of the industry and the publicconcerned with the subject matter In an effort to promote clarity and consis-tency of field interpretations of NECpassages, the National Electrical CodeStyleManual was completely rewritten in 1999, with the current version effective in

2003 All code-making panels have been asked to review their articles forusability and editorial conformity to this publication, and copies are availablefrom the NFPA, Batterymarch Park, Quincy, MA 02269

The National Fire Protection Association also has organized an ElectricalSection to provide the opportunity for NFPAmembers interested in electricalsafety to become better informed and to contribute to the development of NFPA

electrical standards This new Handbook reflects the fact that the National trical Codewas revised for the 2008 edition, requiring an updating of the previ-ous Handbook which was based on the 2005 edition of the Code Theestablished schedule of the National Electrical CodeCommittee contemplates anew edition of the National Electrical Codeevery 3 years Provision is made underthe rules of procedure for handling urgent emergency matters through a Tenta-tive Interim Amendment Procedure The Committee also has established rulesfor rendering Formal (sometimes called Official) Interpretations Two generalforms of findings for such Interpretations are recognized: (1) those making aninterpretation of literal text and (2) those making an interpretation of the intent

Elec-of the National Electrical Codewhen a particular rule was adopted All TentativeInterim Amendments and Formal Interpretations are published by the NFPAasthey are issued, and notices are sent to all interested trade papers in the elec-trical industry

TheNational Electrical Codeis purely advisory as far as the National Fire tection Association is concerned but is very widely used as the basis of law andfor legal regulatory purposes The Code is administered by various local inspec-tion agencies, whose decisions govern the actual application of the National Elec-trical Codeto individual installations Local inspectors are largely members of theInternational Association of Electrical Inspectors, 901 Waterfall Way, Suite 602,Richardson, TX 75080-7702 This organization, the National Electrical Manu-facturers Association, the National Electrical Contractors Association, the Edi-son Electric Institute, the Underwriters’ Laboratories, Inc., the InternationalBrotherhood of Electrical Workers, governmental groups, and independentexperts all contribute to the development and application of the National Electri-cal Code

Pro-xiv INTRODUCTION TO THE NATIONAL ELECTRICAL CODE

Brief History of the

TheNational Electrical Codewas originally drawn in 1897 as a result of the unitedefforts of various insurance, electrical, architectural, and allied interests Theoriginal Code was prepared by the National Conference on Standard ElectricalRules, composed of delegates from various interested national associations.Prior to this, acting on an 1881 resolution of the National Association of FireEngineers’ meeting in Richmond, Virginia, a basis for the first Code was sug-gested to cover such items as identification of the white wire, the use of singledisconnect devices, and the use of insulated conduit

In 1911, the National Conference of Standard Electrical Rules was disbanded,and since that year, the National Fire Protection Association (NFPA) has acted assponsor of the National Electrical Code Beginning with the 1920 edition, the

National Electrical Code has been under the further auspices of the AmericanNational Standards Institute (and its predecessor organizations, United States

of America Standards Institute, and the American Standards Association), withtheNFPAcontinuing in its role as Administrative Sponsor Since that date, theCommittee has been identified as “ANSI Standards Committee C1” (formerly

“USAS C1” or “ASA C1”)

Major milestones in the continued updating of successive issues of the

National Electrical Codesince 1911 appeared in 1923, when the Code was ranged and rewritten; in 1937, when it was editorially revised so that all thegeneral rules would appear in the first chapters followed by supplementary

rear-rules in the following chapters; and in 1959, when it was editorially revised toincorporate a new numbering system under which each Section of each Article

is identified by the Article Number preceding the Section Number

In addition to an extensive revision, the 1975 NECwas the first Code to bedated for the year following its actual release That is, although it was released

in September of 1974, instead of being called the 1974 Code—as was done forthe 1971 and all previous editions of the NEC—this Code was identified as the

1975 Code That’s the reason there appears to be 4 years, instead of the usual 3,between the 1971 and 1975 editions

The 2008 edition of the NECwas also extensively revised In addition to theusual number of additions, deletions, and other modifications, a number ofmajor Articles were added, including Art 708 covering critical operationspower systems This responded to the need for facilities that have an electricalbackbone capable of remaining operational not just during a period of evacua-tion (e.g., Art 700) but for protracted periods during disasters

For many years the National Electrical Code was published by the NationalBoard of Fire Underwriters (now American Insurance Association), and thispublic service of the National Board helped immensely in bringing about thewide public acceptance which the Code now enjoys It is recognized as themost widely adopted Code of standard practices in the U.S.A The NationalFire Protection Association first printed the document in pamphlet form in

1951 and has, since that year, supplied the Code for distribution to the publicthrough its own office and through the American National Standards Institute.The National Electrical Code also appears in the National Fire Codes, issuedannually by the National Fire Protection Association

xvi BRIEF HISTORY OF THE NATIONAL ELECTRICAL CODE

About the 2008

The trend for ever-increasing numbers of proposals for changes and adoptedchanges in successive editions of the NEChas not reversed itself The 2008 NEC

is based on 3688 public proposals and 2349 public comments that haveresulted in literally hundreds of additions, deletions, and other modifications—both minor and major There are completely new articles covering equipmentand applications not previously covered by the Code There are also new regu-lations and radical changes in old regulations that affect the widest possiblerange of everyday electrical design considerations and installation details.Much of the analysis and discussion about the specifics related to the variousadditions, deletions, and modifications in the 2008 NECare based on the infor-mation available in two familiar documents: the “Report on Proposals” (ROP)and the “Report on Comments” (ROC) These documents provide a wealth ofinformation Both the 2007 ROP and the 2007 ROC for the 2008 NECare avail-able from the National Fire Protection Association, Batterymarch Park, Quincy,

MA 02269; by phoning (800) 344-3555; or through the NFPA Web site atwww.nfpa.org Those two documents are highly recommended references thatwill facilitate completion of the Herculean task that looms immediately aheadfor every designer and installer

Everyone involved in the layout, selection, estimation, specification, tion, as well as installation, maintenance, replacement, etc., of electrical sys-tems and equipment must make every effort to become as thoroughly versed in

inspec-and completely familiar with the intimate details related to the individualchange as is possible And, this must be done as soon as possible.

Clearly, compliance with the NECis more important than ever, as evidenced

by the skyrocketing numbers of suits filed against electrical designers and

installers In addition, inspectors everywhere are more knowledgeable and petent and they are exercising more rigorous enforcement and generally tight-ening control over the performance of electrical work Another factor is the

com-Occupational Safety and Health Administration’s Design Safety Standard for Electrical Installations That standard, which borrowed heavily from the rules

and regulations given in the NEC, is federal law and applies to all places of

employment in general industry occupancies Although the OSHA Design Safety Standard is based heavily on the NEC, due to the relatively dynamicnature of the NEC, there will eventually be discrepancies But, for thoseinstances where a more recent edition of the Code permits something that isprohibited by the OSHA standard, OSHA officials have indicated that such aninfraction—although still an infraction—will be viewed as what OSHA refers to

as a “de minimus violation,” which essentially boils down to no fine Of coursethat is not always the case “Listing” and “labeling” of products by third party

testing facilities is always permitted but frequently not required by the NEC, but

it is made mandatory in most places of employment by the Occupational Safety and Health Administration (OSHA) Design Safety Standard for Electrical Sys- tems The OSHA requirement for certification may take precedence over the

less stringent position of the NECregarding listing of equipment To be certain

as to whether or not OSHA must be followed instead of a more recent edition ofthe NEC—which will be the minority of times—one can write to the OSHADirectorate of Enforcement Programs, Mr Richard Fairfax, Director, 200 Con-stitution Ave., NW, Washington, DC 20210 The impact of the NEC—even onOSHA regulations, which are federal law—is a great indicator of the Code’s farreaching effect

The fact that the application of electrical energy for light, power, control, naling, and voice/data communication, as well as for computer processing andcomputerized process-control continues to grow at a breakneck pace alsodemands greater attention to the Code As the electrical percentage of the con-struction dollar continues upward, the high-profile and very visible nature ofelectrical usage demand closer, more penetrating concern for safety in electri-cal design and installation In today’s sealed buildings, with the entire interiorenvironment dependent on the electrical supply, reliability and continuity ofoperation has become critical Those realties demand not only a concern foreliminating shock and fire hazards, but also a concern for continuity of supply,which is essential for the safety of people, and, in today’s business and indus-try, to protect data and processes, as well

sig-And, of course, one critical factor that, perhaps, emphasizes the importance

of Code-expertise more than anything else, is the extremely competitive nature

of construction and modernization projects, today The restricted market andthe overwhelming pressure to economize have caused some to employ extrememethods to achieve those ends without full attention to safety The Code rep-resents an effective, commendable, and, in many instances, legally binding

xviii ABOUT THE 2008 NATIONAL ELECTRICAL CODE

standard that must be satisfied, which acts as a barrier to any compromises

with basic electrical safety It is a democratically developed consensus dard that the electrical industry has determined to be the essential foundationfor safe electrical design and installation; and compliance with the NEC willdictate a minimum dollar value for any project

stan-In this Handbook, the discussion delves into the letter and intent of Coderules Read and study the material carefully Talk it over with your associates;engage in as much discussion as possible In particular, check out any ques-tions or problems with your local inspection authorities It is true that onlytime and discussion provide final answers on how some of the rules are to beinterpreted But now is the time to start Do not delay Use this Handbook tobegin a regular, continuous, and enthusiastic program of updating yourself onthis big new Code

This Handbook’s illustrated analysis of the 2008 NECis most effectively used

by having your copy of the new Code book at hand and referring to each section

as it is discussed The commentary given here is intended to supplement andclarify the actual wording of the Code rules as given in the Code book itself

ABOUT THE 2008 NATIONAL ELECTRICAL CODE xix

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as well as the issues addressed by Tentative Interim Amendments (TIA) that areissued, periodically, by the NE Code committees They should know the intent

of Code requirements (i.e., the spirit as well as the letter of each provision) and

be familiar with the safety issue at the heart of the matter And, most important,they should keep a copy of the NECand this Code handbook close by for readyreference and repeated study

(B) Adequacy It’s worth noting that compliance with the provisions of the

National Electrical Codecan effectively minimize fire and accident hazards in anyelectrical system Although the Code assures minimum safety provisions,actual design work must constantly consider safety as required by special types

or conditions of electrical application For example, effective provision of matic protective devices and selection of control equipment for particularapplications involve engineering knowledge above routine adherence to Coderequirements Then, too, designers and installers must know the physicalcharacteristics—application advantages and limitations—of the many materialsthey use for enclosing, supporting, insulating, isolating, and, in general, pro-tecting electrical equipment The task of safe application based on skill andexperience is particularly important in hazardous locations Safety is not auto-matically made a characteristic of a system by simply observing codes Safetymust be designed into a system

auto-In addition to safety considerations, the need for future expansion and othercommon sense aspects—such as voltage drop—must be considered and fac-tored into the overall system design The Code in this section makes it clear thatmore than Code compliance will be necessary to ensure a system that is notonly safe but also functional and capable of providing for future needs, withoutcompromising system-operating continuity or integrity It is up to the designerand installer, in consultation with the owner, to provide adequate capacity,selectivity, isolation, and protection beyond its minimum requirements inorder to achieve the desired system characteristics Remember, it is always per-

missible to do more than the Code requires, but never permissible to do less

than the Code-prescribed minimum

Addressing voltage drop illustrates these principles No definite standardshave been adopted for the maximum allowable voltage drop in mostinstances There is a good reason for this In most cases voltage drop is aninefficiency or inconvenience, but it does not rise to the level of a safety hazard.For example, a motor run at 10 percent voltage drop, but with appropriaterunning overload protection, will have a greatly reduced life span, but notcreate a shock, fire, or electrocution hazard The National Electrical Codedoesnote, however, in a nonmandatory explanatory note, that if the voltage dropfrom the point of service entrance to the final outlet does not exceed 5 percent,there will be “reasonable efficiency of operation.” The note also explains thatnot more than 3 percent voltage drop should occur in the feeder system ahead

of the branch-circuit supply points, which leaves the other 2 percent for thebranch circuit In the end, the extent to which voltage drop in an electricalsystem is to be tolerated is the owner’s decision, because the NEC does notmandate design flexibility

There are some instances, however, where voltage drop does directly bear onsafety and the NECcontains mandatory rules accordingly For example, if theconductors to a fire pump are not sized to prevent the voltage drop while start-ing (i.e., while under locked-rotor conditions) from exceeding 15 percent, mea-sured at the controller terminals, the control contactor for the motor maychatter and not reliably hold in, resulting in a failure to start with disastrousconsequences

(C) Relation to Other International Standards. This section simply states thatthe National Electrical Code addresses the same safety issues addressed by theInternational Electrotechnical Commission (IEC) Standard for “ElectricalInstallations of Buildings.” Because the NECcovers the same consideration forsafety as related to protection against electrical shock, protection against ther-mal effects, protection against overcurrent, protection against fault current,protection against overvoltage, faults between circuits, and so forth that arecovered by the IEC Standard, it was considered necessary to establish that fact.This statement in this section facilitates the adoption of the Code by foreigncountries and is consistent with the ongoing process of harmonizing the NEC

and other accepted standards from around the world

90.2 Scope

(A). Simply stated, the Code applies to all electrical work—indoors and

outdoors—other than that work excluded by the rules of part (B) in this section.

Installation of conductors and equipment, anywhere on the load-side of thepoint of connection to the serving utility, must comply with the provisionsgiven in the NEC

The scope of the NECincludes the installation of optical fiber cable, part (A).

As part of the high-technology revolution in industrial and commercial ing operations, the use of light pulses transmitted along optical fiber cables hasbecome an alternative method to electric pulses on metal conductors for data,voice, and video networks, as well as for control and signaling Although thetechnology of fiber optics has grown dramatically over recent years, it is stillprimarily used as a “trunk line” or “backbone” for high-speed networks, whilehorizontal distribution is generally accomplished via a twisted-pair or coaxialcopper medium Although coaxial cable can handle high rates of data trans-mission involved in data processing and computer control of machines andprocesses, optical fiber cables far outperform metallic conductors—even coaxialcable—when it comes to bandwidth as well as cost of materials (See Fig 90-1.)

build-NECArt 770, “Optical Fiber Cables,” covers the installation and use of optic cables

fiber-Part (A)(1) provides a laundry-list of specific indoor installations that

must be in compliance with the applicable requirements given in the Code.Note that this section makes clear that the NE Code also applies to “floatingbuildings” because the safety of Code compliance is required for all placeswhere people are present Coverage of floating buildings is contained in NEC

gener-word “equipment” in parts (A)(2) and (A)(3) makes clear that the NE Code

applies to electrical circuits, systems, and components in their manner ofinstallation as well as use

The following discussion and the discussion in 90.2(B)(5) are very closely

related and often hotly debated Information has been provided from both sides

of the discussions as well as the commentary from the Code-making panels(CMPs) where available The purpose is to allow each designer and installer tomake their own judgment with regard to how these matters will be resolvedbased on a full understanding of both sides of the arguments.

Although generally exempt from compliance with the NEC, according to

90.2(A)(4) certain utility-owned or -operated occupancies must be wired per

the NEC The wording in this section along with the companion rule of

90.2(B)(5) is intended to identify those utility electrical installations that are

subject to the rules of the NECand those that are not Basically stated, any utilityoccupancy that is not an “integral part” of a “generating plant, substation, orcontrol center” must comply with the NECin all respects Clearly, any officespace, storage area, garage, warehouse, or other nonpower-generating area of abuilding or structure is not an “integral part” of the generation, transmission, ordistribution of electrical energy and therefore is covered by the NEC

There has been discussion and disagreement over the meaning of the phrase,

“not an integral part of a generating plant etc.” Some feel that the phrase

“not an integral part of” applies to the process of generation, and so forth

Fig 90-1.The NEC covers the technology of fiber optics for communication and data transmission.

Others believe that it applies to the building That is, if an occupancy identified

in 90.2(B)(4) is part of a generating plant, it is exempt from compliance with the

NEC Although that doesn’t seem to make sense, past comments made by theCMP indicate that it is the intent of this rule to exempt, say, office spaces within

a generating plant However this is not completely clear from the wording used

To prevent any problem with this section, one could choose to interpret thisrule to require NECcompliance for any occupancy that is “not an integral part

of the process” and wire such spaces in accordance with the NEC Such pretation cannot be disputed That is, satisfying the more rigorous NECrequire-ments cannot be construed as a violation But, if one does not comply, thepotential for legal liability exists

inter-Some may feel that the term integral part should be interpreted to mean

“integral part of the process” (i.e., generation, transformation, or distribution ofelectric energy), according to commentary in the NEC Committee Reports forthe 1987 NEC Others feel that it should be taken to mean an “integral part” ofthe building or structure Be aware that the first contention seems more reason-

able That is, just because an office is in a generating plant, it shouldn’t be

exempt from the NEC, especially since these areas will be occupied by the eral public And it seems logical that the same should apply to the cafeteria,bathrooms, and other areas within the plant that are not directly related to thetask of generating and delivering electrical energy and will be occupied byother than qualified plant electrical personnel

gen-With that said, it should be noted that the wording here could be read bothways and it will be up to the local AHJ to interpret what is and what is notrequired to comply with the NEC

It should be noted that equipment installed by the utility to perform ated functions, such as outdoor lighting at an outdoor substation, is intended to

associ-be considered as an “integral” part to the process and is therefore exempt fromcompliance with the NEC(Figs 90-2 and 90-3)

(B) Not Covered. The rules of the Code do not apply to the electrical work

described in (1) through (5) The most common controversy that arises

con-cerns exclusion of electrical work done by electric utilities (power companies),especially outdoor lighting

This rule emphatically explains that not all electrical systems and equipmentbelonging to utilities are exempt from Code compliance Electrical circuits andequipment in buildings or on premises that are used exclusively for the “gener-ation, control, transformation, transmission, and distribution of electricenergy” are considered as being safe because of the competence of the utilityengineers and electricians who design and install such work Code rules do notapply to such circuits and equipment—nor to any “communication” or “meter-ing” installations of an electric utility But, any conventional electrical systemsfor power, lighting, heating, and other applications within buildings or on

structures belonging to utilities must comply with Code rules where such

places are not “used exclusively by utilities” for the supply of electric power tothe utilities’ customers

An example of the kind of utility-owned electrical circuits and equipment

covered by Code rules would be the electrical installations in, say, an office

the particular installation is part of the utility’s system for transmitting and distributing power to the

utility’s customers—provided that such an installation is accessible only to the utility’s personnel and

access is denied to others Outdoor, fenced-in utility-controlled substations, transformer mat tions, utility pad-mount enclosures, and equipment isolated by elevation are typical utility areas to which the NEC does not apply The same is also true of indoor, locked transformer vaults, or electric rooms (Sec 90.2) But electrical equipment, circuits, and systems that are involved in supplying light- ing, heating, motors, signals, communications, and other load devices that serve the needs of person- nel in buildings or on premises owned (or leased) and operated by a utility are subject to NE Code rules, just like any other commercial or industrial building, provided that the buildings or areas are not integral parts of a generating plant or substation.

installa-building of the utility But, in the Technical Committee Report for the 1987 NEC,the Code panel for Art 90 stated that it is not the intent of this rule to have NEC

regulations apply to “office buildings, warehouses, and so forth that are an gral part of a utility-generating plant, substation, or control center.” According

inte-to comments from the CMP, NECrules would not apply to any wiring or ment in a utility-generating plant, substation, or control center and would notapply to conventional lighting and power circuits in office areas, warehouses,maintenance shops, or any other areas of utility facilities used for the genera-tion, transmission, or distribution of electric energy for the utility’s customers.ButNECrules would apply to all electrical work in other buildings occupied by

equip-utilities—office buildings, warehouses, truck garages, repair shops, etc., thatare in separate buildings or structures on the generating facility’s premises.And that opinion was reinforced by the statements of the CMP that sat for the

1996NEC With that said, it should be noted that the actual wording used here

in the Code could be read both ways and it will be up to the local AHJ to pret what areas are and what areas are not required to comply with the NEC

inter-The wording used in 90.2(B)(5)(b) recognizes non-NEC-complying utilityinstallations “in legally established easements or rights-of-way designated by orrecognized by public service commissions,” etc This clearly exempts utilityactivities on public streets, alleys, and similar areas, even for street and arealighting for adjacent parking lots However, the 2008 NECdeleted the phrase “or

Fig 90-3.Those buildings and structures that are directly related to the generation, transmission, or distribution of energy are intended to be excluded from compliance with the NEC However, the rules covering this matter also indicate that functionally associated electrical equipment—such as the out- door lighting for the utility-owned and -operated outdoor substation—are also exempt from the NEC

by other agreements” from this list The concern was that this provision openedthe door to utility noncompliance throughout a facility, provided an agreementcould be struck with the owner and ratified by the governmental authorityhaving jurisdiction over utility practice Since utilities are governed by theNational Electrical Safety Code (NESC) whose provisions are entirely inappro-priate for premises wiring, this concern is not inconsequential.

However, the change is extremely controversial because it has the potential

to unravel over a century of established precedent regarding site lighting byutilities, where all of the work is on the line side of any service point, or wherethere is no service point whatsoever, as illustrated in Fig 90-4 Virtually everyelectric utility has permission to supply outdoor lighting according to ratesestablished by the governing authority, and that lighting need not be in a pub-lic way or in an easement, provided it is not premises wiring The key to under-standing the problem is the concept of a service point, defined in Art 100 TheNESC applies on the supply side of service points, where they exist The NEC

applies on the load side of service points, where they exist It is instructive toreview the premises underlying the 2005 NEClanguage

The entire premise behind allowing the NESC, substantially different fromtheNEC, to apply to utility work is a simple one: The organizational perma-nence, engineering supervision, and workforce training in the utility environ-ment are fundamentally different than for premises wiring Therefore, different

Fig 90-4. This drawing shows an actual example of a practice that is widespread throughout the United States and many other countries There is no service point, the parking lot luminaire is not premises wiring, and the maintenance will be performed by a utility line crew in the same bucket truck as services the street lighting, at the same time The drug store is, in effect, buying the 27,500-lm output from each of the two 250-W high-pressure sodium (HPS) lamps The 2008 NEC purports to

standards can be applied to installations under their exclusive control.Whether this also applies to an Energy Service Company (ESCo) doing mainte-nance under contract with the utility is a regulatory matter that will depend onthe degree of command and control exercised by the regulated utility.

Area lighting wired to the NESC will lack local disconnects, specific current protection, and separate equipment grounding conductors, for just afew examples Is this a safety issue if a utility line crew does the maintenance?Apparently not, given the ubiquitous presence of street lighting wired this way.Would it be a safety issue if it were premises wiring, maintained by others? Cer-tainly, given that the NEChas never allowed such practices over its long history.The fact that these two statements are self-evidently both true leads to this con-clusion: You cannot write and apply installation rules without taking the oper-ational context into account The NECdoes exactly the same thing over and overagain when it creates special exceptions and allowances for work that will beperformed under qualified maintenance and supervision

over-Part (C) gives the AHJ the discretion to permit other than “utilities” the

option to install conductors between the utility supply and the service entranceconductors for individual buildings without complying with the NEC Essen-tially it allows the inspector to permit the use of another standard such as in theutilities code, the NESC Such permission is typically limited to campus-typeenvironments where the utility supply to the premises is medium-voltage anddistribution to, and between, buildings is installed and maintained by on-sitepersonnel It’s worth noting that any such permission granted by the AHJ must

be written permission to satisfy the definition of “special permission,” as given

in Art 100 Today, however, such occupancies frequently take service at an vated voltage at a central point, and all the medium voltage feeders to serve thebuildings are just that, feeders As soon as the service point becomes a centralmedium voltage switch, this provision can no longer be applied to the individ-ual buildings

ele-There are far more mundane uses for this permission Many CATV (seeArticle 820) companies rely on powered amplifiers mounted near the top of util-ity poles to keep their signal strength where it needs to be Those amplifiers willhave a small disconnect and overcurrent protective device located adjacent tothe amplifiers There are no provisions within the body of the NECthat allow for

a service disconnect to be located at such a location, which is certainly not ily accessible However, the entire installation is confined to the pole top, and

read-special permission under 90.2(C) is routinely granted in such cases.

90.3 Code Arrangement. This section provides guidance on which rule takesprecedence where two rules covering a particular installation are at odds Basi-

cally, the rules in Chaps 1 through 4 apply at all times, except for installations

covered by Chap 8, which stands alone Installations covered by Chaps 5, 6,and 7 must always comply with the requirements given in Chaps 1 through 4,

unless a specific rule in Chaps 5 through 7 requires or permits an alternate

method One implication of this principle is that exceptions in Chaps 1through 4 that allow for different procedures in Chaps 5 through 7 are unnec-essary The NECStyle Manual has been rewritten to take this into account, andsuch exceptions are disappearing from the NECfor this reason Provisions in

Chaps 1 through 7 of the NEConly apply in Chap 8 when a Chap 8 articlespecifically cites them, and the numbers of such citations in Chap 8 articles aresteadily increasing for this reason Chapter 9 consists of tables that are manda-tory, but only applicable as referenced in earlier articles The graphic provided

in this section facilitates understanding of the relationship between variousCode chapters

90.4 Enforcement. This is one of the most basic and most important of Coderules because it establishes the necessary conditions for use of the Code.The NE Code stipulates that when questions arise about the meaning orintent of any Code rule as it applies to a particular electrical installation,including signaling and communication systems covered by Chap 8, the elec-trical inspector having jurisdiction over the installation is the only one autho-rized by the NE Code to make interpretations of the rules The wording ofSec 90.4 reserves that power for the local inspection authority along with theauthority to approve equipment and material and to grant the special permis-sion for methods and techniques that might be considered alternatives to thoseCode rules that specifically mention such “special permission.”

It should be noted that any deviation from standard Code enforcement must

be done in accordance with the provisions given in Art 100 by the definition of

“Special Permission.” The most salient requirement is the need for tation That is, in order to comply with the definition of “special permission,”such permission must be in writing This will serve to provide a written record

documen-of the circumstances surrounding the granting documen-of a waiver

The NE Code permits the electrical inspector to “waive specific ments” or “permit alternate methods” in any type of electrical installation Inresidential, commercial, and institutional electrical systems—as well as inindustrial—inspectors may accept design and/or installation methods that donot conform to a specific Code rule, provided they are satisfied that the safetyobjectives of the Code rule are achieved In other words, there must be a find-ing of equivalent safety before the permission is granted, and the permission todeviate from them must be provided in writing as required by the first sentence

require-of the second paragraph in this section and stated by the definition require-of “specialpermission” given in Art 100 (Fig 90-5)

This recognition of practices at variance with the Code is provided only forspecial conditions and must not be interpreted as a general permission toengage in non-Code methods, techniques, or design procedures In fact, it islikely that inspectors will exercise this authority only with reluctance and thenwith great care, because of the great responsibility this places on the inspector

Fig 90-5.Inspector’s authority may

be exercised either by enforcement of

that individual’s interpretation of a

Code rule or by waiver of the Code

rule when the inspector is satisfied

that a specific non-Code-conforming

method or technique satisfies the

This is especially true because such permission may only be granted in writing.Clearly, this requirement for documentation will give many inspectors pausefor reflection and reconsideration It seems almost certain for the exercise ofthis prerogative.

90.5 Mandatory Rules and Explanatory Material. This section provides guidanceregarding proper application of the NEC Although the NE Code consists essen-tially of specific regulations on details of electrical design and installation,there is much explanatory material in the form of notes to rules

Part (A) of this section addresses “mandatory” rules, which typically employ

the phrases “shall” or “shall not.” Compliance with the Code consists in fying all requirements and conditions that are stated by use of the word “shall”

satis-or “shall not” where used in the body of a Code rule satis-or Exception Those wsatis-ords,anywhere in any rule or exception, designate a mandatory rule Failure to com-ply with any mandatory Code rule constitutes a “Code violation.”

Part (B) of this section indicates the wording that is used in “permissive

rules.” These rules are typified by the use of phrases such as “shall be permitted”

or “shall not be required.” Such rules typically provide or accept alternate sures or suspend requirements under certain conditions It is not necessary to

mea-do what these rules permit; it is essentially an optional approach

Note that under the provisions of the NECStyle Manual, the word “may” isnot to be used to set forth a permissive rule When “may” is being used to indi-cate permission, it can only be used in the context of a discretionary action ofthe authority having jurisdiction For example, NEC430.26 authorizes, but doesnot require, an AHJ to permit the application of a demand factor to the loads on

a motor feeder being sized under 430.24 This is an excellent example of theappropriate use of the word, as in “ the authority having jurisdiction maygrant permission for feeder conductors ”

Part (C) explains that fine-print notes (FPNs) are included, following certain

Code rules, to provide additional information regarding related rules or dards This information is strictly advisory or “explanatory” in nature and pre-sents no rule or additional requirement The same is true for bracketedinformation that references other NFPA®standards The inclusion of the refer-enced standard is to inform the reader of the origin of “extracted text,” wherethat text is taken from an NFPAstandard However, the reference to another NFPA

stan-standard in no way makes the referenced stan-standard part of the Code; nor doessuch reference oblige compliance with other rules in the referenced standard.FPNs explain NECrules, they do not change NECrules If, in reading a FPN, itappears to allow or require something different from the rule that precedes it,then you are misreading the rule and you should read the rule again

90.6 Formal Interpretations. Official interpretations of the National Electrical Code

are based on specific sections of specific editions of the Code In most cases,such official interpretations apply to the stated conditions on given installa-tions Accordingly, they would not necessarily apply to other situations thatvary slightly from the statement on which the official interpretation was issued

As official interpretations of each edition of the Code are issued, they are

published in the NFPA Fire News, and press releases are sent to interested trade

papers

All official interpretations issued on a specific Code edition are reviewed bythe appropriate CMP In reviewing a request for a formal interpretation, a Codepanel may agree or disagree They will render a simple “yes” or “no” to thequestion, which places the burden on the questioner to provide a question thatcan be answered in the affirmative or negative At some point in future Codes,the CMP might clarify the Code text to avoid further misunderstanding ofintent On the other hand, the Code panel may not recommend any change inthe Code text because of the special conditions described in the request for anofficial interpretation For these reasons, the NFPAdoes not catalog official inter-pretations issued on previous editions of the Code within the Code itself SuchFormal Interpretations can be obtained through the National Fire ProtectionAssociation®.

UnderNFPArules, Formal Interpretations require a four-fifths vote, which caneasily result in sufficient dissent to preclude their issuance They are issued on

a specific edition of a standard, and are retained until the wording to whichthey applied changes In addition, when a formal interpretation is issued, thetechnical committee (in this case a CMP) is encouraged (but not required) toreview the disputed text that provoked the request for interpretation when theyprocess the next edition A classic example of a Formal Interpretation, on thetext of the 1978 NEC, asked whether reinforcing steel in a concrete foundationwas “available” for connection after the concrete had hardened It was commonfor inspection authorities in Florida at the time to insist that footings be jack-hammered and connections be made so as to bring these concrete-encased elec-trodes into the grounding electrode system The panel’s answer was “No” andthat interpretation retained its validity until the 2005 NEC changed the word

“available” to “present” in what is now 250.50

It should be remembered that, according to 90.4, the authority having diction has the prime responsibility of interpreting Code rules in its area anddisagreements on the intent of particular Code rules in its area; and disagree-ments on the intent of particular Code rules should be resolved at the locallevel if at all possible A Formal Interpretation is not really a viable avenue for

juris-a couple of rejuris-asons One is the juris-amount of time it will tjuris-ake for the CMP to der its decision, which is generally months The other is that even if yourequest a Formal Interpretation and the CMP agrees with your application,there is no guarantee that the authority having jurisdiction will accept the find-ings of an official interpretation, nor are they required to do so

ren-Although this section deals with Formal Interpretations, it should be notedthat changes in the Code are promulgated in a very similar manner That is,changes to Code rules are generally precipitated by a request for change fromthe field Guidance for submittal of a Code change is provided immediately fol-lowing the Index in the back of the Code

90.7 Examination of Equipment for Safety. It is not the intent of the National trical Codeto include the detailed requirements for internal wiring of electricalequipment Such information is usually contained in individual standards forthe equipment concerned Note that Annex A at the end of the Code bookincludes the recognized product standards that the testing laboratories use toevaluate the products for which NECrules require listings

The last sentence does not intend to take away the authority of the localinspector to examine and approve equipment, but rather to indicate that therequirements of the National Electrical Codedo not generally apply to the internalconstruction of devices which have been listed by a nationally recognized elec-trical testing laboratory.

Although the specifics of Code rules on examination of equipment for safetyare presented in 110.2 and 110.3, the general Code statement on this matter ismade here in 90.7 Although the Code does place emphasis on the need forthird-party certification of equipment by independent testing laboratories, itdoes not make a flat rule to that effect However, the rules of the U.S Occupa-tional Safety and Health Administration (OSHA) are very rigid in insisting onproduct certification

Codes and standards must be carefully interrelated and followed with careand precision Modern work that fulfills these demands should be the objective

of all electrical construction people

90.8 Wiring Planning. These two sections address concepts that are essentially

design-oriented Part (A) alerts the reader to the fact that simply designing to

Code-mandated minimums will not provide for any future expansion tional capacity in raceways, boxes, enclosures, and so forth, should be consid-

Addi-ered, but spare capacity is not required Part (B) points out the fact that

minimizing the number of conductors within a given raceway will minimizethe number of circuits affected during a fault Additionally, extra room in yourraceways (i.e., fewer conductors than the maximum permitted) will also facili-tate pulling of the conductors into the raceway Again, providing extra room inraceways or limiting the number of circuits is only required as indicated else-where in the Code (e.g., 314.16 on box fill)

90.9 Metric Units of Measurement Part (A) identifies metric measurements as

the preferred measurement, although English units (i.e., inch-pounds-feet) are

also provided as indicated by part (B).

In part (C), the Code discusses when one is required to use a “soft”

conver-sion and where a “hard” converconver-sion is permitted A “soft converconver-sion” is directmathematical conversion, for example, 1 m = 39.3 in.; a “hard conversion” ismore practical, e.g., 1 m = 3 ft It may seem counterintuitive to have a “hard”conversion as the inexact conversion Another way to express the concept isthat a hard conversion is the conversion a hard-core metric user would do, that

is, use a round number for his or her metric measurement The various nations that follow in the NECat this point regarding “hard” and “soft” con-version are primarily aimed at CMPs They must make the decisions aroundwhich metric unit would unacceptably degrade safety, or cause wholesalechanges in industry specifications For example, CMP 9 used soft conversions

expla-in Table 314.16 because hard conversions would result expla-in every steel boxbeing at variance from NECprovisions, not by much, but enough to force exten-sive redesign of manufacturing facilities with not real safety benefit CMP 1made the decision that reducing the minimum workspace width in front of apanel from 762 mm (the soft conversion from 30 in.) to 750 mm (the hard con-version) would unacceptably degrade safety, and so that dimension has beenretained as a soft conversion

The rule of part (D), Compliance, addresses the coexistence of the two

sys-tems of measurement There the Code states that use of either the SI or theEnglish units “shall constitute compliance with this Code.” Clearly, designersand installers may use either of the designated values However, it should benoted that only one, or the other units of measure should be used throughout agiven project Inspectors have raised objection to mixing and matching units ofmeasure

Chapter One

ARTICLE 100 DEFINITIONS

TheNECreserves Art 100 to cover the essential definitions required to properlyapply its provisions Not included are general terms that are commonlydefined, or technical terms that are used in the same way as in related codesand standards In addition, if a term is only used in one article, it will bedefined within that article and not in Art 100 Part I of the article appliesthroughout the NEC; Part II covers definitions that only apply to installationsoperating over 600 V, nominal Consult Art 100 if you are unclear as to how aspecialized electrical term is defined that appears in the NEC

Accessible (as Applied to Wiring Methods):

Accessible (as Applied to Equipment):

Accessible, Readily (Readily Accessible):

The best way to look at these definitions is to consider all three at the same timebecause although they are necessarily related, there are important differences.Each of the three terms involves the concept of unimpeded approach That is,accessible items, whether wiring methods, equipment, or either of these, ifreadily accessible, must be capable of unimpeded approach as required, butthat is about the extent of what these terms have in common

Wiring methods are accessible if they can be removed or exposed without

damaging the building finish or structure Wiring methods are any of the NECode-recognized techniques for running circuits between equipment, as cov-ered in the articles in Chap 3 of the Code Wiring methods are also accessible

if they are not permanently closed in by the building structure or finish Anysurface wiring method would obviously qualify if in plain view, but what about

above a suspended ceiling? The definition uses the word “exposed” which isalso defined in Art 100 as being on or attached to the surface, or behind panelsdesigned to allow access Since suspended ceiling panels are clearly designedfor that purpose, wiring such as that shown in Fig 100-1 above a suspendedceiling is exposed, and since it is exposed, it is also accessible.

The same word used to describe equipment does not mean quite the same

thing Equipment covers all the products that are connected or hooked up by a

recognized wiring method, together with the other components of the wiring

system Equipment is accessible if it allows close approach It is not accessible

if it is guarded by a locked door or by height or other barrier that effectively

pre-cludes approach by personnel The word guarded is also defined in Art 100,

and it means protected by any of various means to remove the likelihood of

“approach or contact by persons or objects to a point of danger.”

Consider the busbars in a panelboard located chest high in a corridor, andthen think about the panelboard itself, including its enclosing cabinet Are thebusbars themselves “accessible”? No, because they are guarded by the dead-front Is the panel accessible? Yes, the deadfront makes it safe to approach, andnothing about its location precludes approach What if the panelboard is for atenancy, and is located in another tenancy for which access to the supplied ten-ancy is forbidden? Such a panel would still be accessible, but not to those forwhom access is required by the NEC

This brings us to the final concept, readily accessible This term also applies

to equipment, and requires access without climbing over or removing obstacles,

or arranging for a ladder or lift to reach the equipment, as covered in Fig 100-2.Equipment in the open and reachable only by ladder is probably accessible, butcould never be considered “readily accessible.” Overcurrent (OC) devices areusually required to be readily accessible, but what about a fused switch on anair-conditioning compressor high in the air? This is the reason for the special

allowance in 240.24(A)(4) It is understood that such equipment is not readily

accessible, and a special allowance permits it to be so Figure 100-3 showsother examples of these special allowances

Fig 100-1.

Handles of switches and CBs must be not morethan 6 ft 7 in above floor or platform.

This panel switch, CB, switchboard

MCC is not readily accessible

if crates or other obstaclesblock access to it

There is one other provision in the ready access definition that neatly tiessome of the key concepts together Readily accessible equipment must be

reachable quickly by those for whom ready access is requisite This pointedly

does not mean everyone A locked electrical room is a very well-understoodconcept, and perfectly acceptable as long as those who belong in the room have

a key

Ampacity:

Ampacity is the maximum amount of current in amperes that a conductor may

carry continuously under specific conditions of use without exceeding the perature rating of its insulation Refer to the discussion on NEC 310.10 and310.15 in Chap 3 of this book, together with coverage at the end of this book onAnnex D, Example D3(a), for a detailed analysis of ampacity calculations Thecalculation of conductor ampacities is one of the most important skills to belearned in the electrical trade, and unfortunately it is also one of the most com-plicated There are two key points to raise here, however, in terms of the actualcontent of the definition

tem-First, ampacity applies to electrical conductors Other parts of an electricalsystem may have current ratings, such as switches, circuit breakers, motor con-tactors, etc., but only electrical conductors have an ampacity Second, ampac-ity in its true sense cannot be defined by a table in a code book, or even ahundred tables Every condition of use defines a different ampacity And everytime a condition changes, such as when the ambient temperature changes, theapplicable ampacity changes For example, 12 AWG THHN has an allowableampacity of 30 A at 30°C with three (or fewer) current-carrying conductors in araceway Raise the number of current-carrying conductors in the raceway, orraise the ambient temperature, or both, and the ampacity will decrease by vary-ing degrees, all based on the conditions of actual use

The word approved means acceptable to the inspectional authority

[techni-cally, the authority having jurisdiction (AHJ)], and nothing more or less It doesnot mean “identified” unless the inspector chooses to use compliance with thedefinition of “identified” as the basis for his or her decision Similarly it doesnot mean “listed” unless the inspector chooses that standard as the basis forhis or her decision For this reason, any statements in product literature (andthey are common) that something is “approved” by some testing laboratory is

necessarily fallacious A product may be listed by a testing laboratory, butnever approved.

The word identified is routinely confused with the normal usage in the English language of the word marked It does not mean marked It means what

Art 100 says it means It means generally recognizable as suitable for the cific application called out in the NECrequirement This often comes from prod-uct literature generated by manufacturers This use of the term also correlates

spe-with the fine print note (FPN) in 100.3(A)(1), where suitability is explained first

in terms of “a description marked on or provided with a product to identify thesuitability of the product for a specific purpose, environment, or application.”The note goes on to indicate that suitability may also be evidenced by listing orlabeling, an additional possibility

For an example of correct usage of this term in a Code rule, the NECrequirestwo-winding transformers reconnected in the field as autotransformers to be

identified for use at elevated voltage in 450.4(B) These transformers are

fre-quently listed, but as two-winding transformers They could not be listed asautotransformers because they do not leave the factory this way, and they havewide application as two-winding transformers A listing would be excessivebecause the transformer manufacturers would have to run two production lineswith two different labels for the same product The installer needs to rely onproduct literature from the manufacturer to verify suitability for reconnection,and fortunately, these manufacturers all provide specific information on how tomake the reconnections so the transformers will buck or boost the voltage asdesired

The word listed covers the most specific method of product acceptance,

because it means that a qualified testing laboratory, usually with testingfacilities that an inspector could not possibly duplicate, has performedexhaustive tests to judge the performance of the product under the condi-tions contemplated in a specific Code rule The Code note that follows thedefinition needs some explanation as well Although the note is written in ageneral and explanatory manner, in fact, all qualified testing laboratoriesoperating under the current North American electrical safety system dorequire a label as evidence of the listing It follows, then, that if a label fallsoff, the product no longer has the status of being listed Further, the only way

a label can be reapplied is in the presence of an employee of the testing oratory Sending labels through the mail is not an option and will result indisciplinary action against the manufacturer by the testing laboratory Thetesting laboratories will all send personnel into the field to witness the reap-plication of labels

lab-Be aware that OSHA rules governing workplaces generally require a “listed,”

“labeled,” or otherwise “certified” product to be used in preference to the same

“kind” of product that is not recognized by a national testing lab (Fig 100-4)

Authority Having Jurisdiction:

This definition clarifies the meaning of this term, which is used repeatedlythroughout the Code As indicated by the FPN, the authority having jurisdiction