Audel Guide to the 2005 National Electrical Code pot

Audel
™
Guide to the 2005
National Electrical
Code
®
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Audel
™
Guide to the 2005
National Electrical
Code
®
All New Edition
Paul Rosenberg
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Vice President and Executive Group Publisher: Richard Swadley
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Library of Congress Control Number:
Printed in the United States of America
10 98765432
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eISBN: 0-7645-7903-7
Contents
Foreword xv
Introduction xvii
Article Chapter 1 General 1
100 Definitions 1
110 Requirements for Electrical Installations 1
General 3
600 Volts, Nominal or Less 12
Over 600 Volts, Nominal 16
Tunnel Installations Over 600 Volts, Nominal 21
Chapter 2 Wiring and Protection 25
200 Use and Identification of Grounded Conductors 25
210 Branch Circuits 30
General Provisions 30
Branch-Circuit Ratings 43
Required Outlets 48
215 Feeders 55
220 Branch-Circuit, Feeder, and Service Calculations 60
General 60
Feeders 63
Optional Calculations for Computing Feeder
and Service Loads 71
Method for Computing Farm Loads 75
225 Outside Branch Circuits and Feeders 75

More Than One Building or Structure 84
Over 600 Volts 88
230 Services 88
General 88
Overhead Services 92
Underground Service—Lateral Conductors 97
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iv Contents
Service-Entrance Conductors 98
Service Equipment—General 107
Service Equipment—Disconnecting Means 107
Service Equipment—Overcurrent Protection 115
Service Exceeding 600 Volts, Nominal 120
240 Overcurrent Protection 123
General 123
Location 127
Enclosures 134
Disconnecting and Guarding 135
Plug Fuses, Fuseholders, and Adapters 136
Cartridge Fuses and Fuseholders 138
Circuit Breakers 139
Supervised Industrial Installations 140
Overcurrent Protection Over 600
Volts, Nominal 142
250 Grounding 143
General 143
Circuit and System Grounding 147
Grounding Electrode System and
Grounding Electrode Conductor 162

Grounding Conductors 166
Enclosure and Raceway Grounding 174
Bonding 175
Equipment Grounding and
Equipment-Grounding Conductors 183
Methods of Equipment Grounding 190
Direct Current Systems 197
Instruments, Meters, and Relays 198
Grounding of Systems and Circuits
of 1 kV and Over (High Voltage) 200
280 Surge Arresters 202
General 202
Installation 203
Connecting Surge Arresters 204
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Chapter 3 Wiring Methods and Materials 207
300 Wiring Methods 207
General Requirements 207
Requirements for Over 600 Volts, Nominal 225
310 Conductors for General Wiring 227
312 Cabinets and Cutout Boxes 237
Installation 237
Construction Specifications 243
314 Outlet, Device, Pull, and Junction Boxes,
Conduit Bodies, Fittings, and Handhole
Enclosures 244
General 244
Installation 245
Construction Specifications 261
Manholes and Other Electric

Enclosures Intended for Personnel Entry 262
Pull and Junction Boxes for Use on
Systems Over 600 Volts, Nominal 263
320 Armored Cable: Type AC 265
General 265
Installation 265
Construction 267
322 Flat Cable Assemblies: Type FC 268
Installation 269
Construction 270
324 Flat Conductor Cable: Type FCC 270
General 270
Installation 272
Construction 274
326 Integrated Gas Spacer Cable: Type IGS 274
328 Medium Voltage Cable: Type MV 274
330 Metal-Clad Cable: Type MC 275
General 275
Installation 275
Construction Specifications 278
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332 Mineral-Insulated, Metal-Sheathed Cable:
Type MI 278
General 278
Installation 280
Construction Specifications 282
334 Nonmetallic-Sheathed Cable: Types NM
and NMC 283

General 283
Installation 284
Construction Specifications 287
336 Power and Control Tray Cable: Type TC 287
Installation 287
Construction Specifications 288
338 Service-Entrance Cable: Types SE and USE 289
340 Underground Feeder and Branch-Circuit
Cable: Type UF 292
Installation 293
342 Intermediate Metal Conduit 294
General 294
Installation 294
Construction Specifications 297
344 Rigid Metal Conduit 297
Installation 298
Construction Specifications 302
348 Flexible Metal Conduit 303
Installation 303
Installation 305
352 Rigid Nonmetallic Conduit 308
Installation 308
354 Nonmetallic Underground Conduit
with Conductors 311
General 311
Installation 311
Construction 313
356 Liquid-Tight Flexible Nonmetallic Conduit 313
General 313
Installation 313

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358 Electrical Metallic Tubing 315
Installation 315
360 Flexible Metallic Tubing 317
Installation 317
362 Electrical Nonmetallic Tubing 319
General 319
Installation 319
366 Auxiliary Gutters 322
368 Busways 326
General Requirements 326
Requirements for over 600 Volts, Nominal 330
370 Cablebus 331
372 Cellular Concrete Floor Raceways 333
374 Cellular Metal Floor Raceways 334
Installation 335
376 Metal Wireways 336
378 Nonmetallic Wireways 339
380 Multioutlet Assembly 341
Installation 341
382 Nonmetallic Extensions 343
Installation 343
386 Surface Metal Raceways 348
Installation 348
388 Surface Nonmetallic Raceways 350
Installation 350
Construction Specifications 351
390 Underfloor Raceways 351
392 Cable Trays 357
394 Concealed Knob-and-Tube Wiring 372

396 Messenger-Supported Wiring 375
398 Open Wiring on Insulators 377
Chapter 4 Equipment for General Use 381
400 Flexible Cords and Cables 381
General 381
Construction Specifications 385
Portable Cables Over 600 Volts, Nominal 386
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402 Fixture Wires 387
404 Switches 388
Installation 388
Construction Specifications 396
408 Switchboards and Panelboards 397
Switchboards 399
Panelboards 400
Construction Specifications 404
409 Industrial Control Panels 405
410 Lighting Fixtures, Lampholders, Lamps,
Receptacles, and Rosettes 406
General 406
Fixture Locations 407
Provisions at Fixture Outlet Boxes,
Canopies, and Pans 411
Fixture Supports 412
Grounding 414
Wiring of Fixtures 415
Construction of Fixtures 420
Installation of Lampholders 420

Construction of Lampholders 420
Lamps and Auxiliary Equipment 420
Receptacles, Cord Connectors, and
Attachment Plugs (Caps) 421
Special Provisions for Flush and
Recessed Fixtures 424
Construction of Flush and Recessed Fixtures 426
Special Provisions for Electric-Discharge Lighting
Systems of 1000 Volts or Less 426
Special Provisions for Electric-Discharge Lighting
Systems of More Than 1000 Volts 429
Lighting Track 432
411 Lighting Systems Operating
at 30 Volts or Less 433
422 Appliances 434
General 434
Branch-Circuit Requirements 435
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Installation of Appliances 435
Control and Protection of Appliances 440
424 Fixed Electrical Space-Heating Equipment 444
General 444
Installation 445
Control and Protection of Fixed Electric
Space-Heating Equipment 446
Marking of Heating Equipment 450
Electric Space-Heating Cables 450
Duct Heaters 456
Resistance-Type Boilers 458
Electrode-Type Boilers 460

Electric Radiant Heating Panels and
Heating Panel Sets 462
426 Fixed Outdoor Electric De-icing and
Snow-Melting Equipment 466
General 466
Installation 467
Resistance Heating Elements 468
Impedance Heating 471
Skin-Effect Heating 472
Control and Protection 472
427 Fixed Electric Heating Equipment for
Pipelines and Vessels 473
General 473
Installation 475
Resistance Heating Elements 476
Impedance Heating 477
Induction Heating 478
Skin-Effect Heating 479
Control and Protection 479
430 Motors, Motor Circuits, and Controllers 480
General 480
Motor Circuit Conductors 489
Motor and Branch-Circuit Overload Protection 494
Motor Branch-Circuit, Short-Circuit, and
Ground-Fault Protection 501
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Motor Feeder Short-Circuit and
Ground-Fault Protection 506

Motor Control Circuits 507
Motor Controllers 511
Motor Control Centers 515
Disconnecting Means 516
Over 600 Volts, Nominal 522
Protection of Live Parts—All Voltages 524
Grounding All Voltages 525
440 Air-Conditioning and Refrigerating
Equipment 527
General 527
Disconnecting Means 531
Branch-Circuit, Short-Circuit,
and Ground-Fault Protection 534
Branch-Circuit Conductors 536
Controllers for Motor-Compressors 537
Motor-Compressor and Branch-Circuit
Overload Protection 538
Provisions for Room Air Conditioners 540
445 Generators 542
450 Transformers and Transformer
Vaults (Including Secondary Ties) 544
General Provisions 545
Specific Provision Applicable to Different
Types of Transformers 558
Transformer Vaults 561
460 Capacitors 564
600 Volts, Nominal, and Under 564
470 Resistors and Reactors 569
480 Storage Batteries 569
490 Equipment Over 600 Volts, Nominal 570

General 570
Equipment—Specific Provisions 572
Equipment—Metal-Enclosed Power Switchgear
and Industrial Control Assemblies 577
Mobile and Portable Equipment 577
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Chapter 5 Special Occupancies 579
500 (Classified) Locations 579
501 Class I Locations 588
502 Class II Locations 608
503 Class III Locations 623
504 Intrinsically Safe Systems 630
505 Class I, Zone 0, 1, and 2 Locations 632
510 Hazardous (Classified) Locations—Specific 632
511 Commercial Garages, Repair, and Storage 633
513 Aircraft Hangars 640
514 Gasoline-Dispensing and Service Stations 645
515 Bulk Storage Plants 650
516 Spray Application, Dipping, and Coating
Processes 654
517 Health Care Facilities 659
General 659
Wiring Design and Protection 664
Essential Electrical System 668
Inhalation Anesthetizing Locations 679
X-Ray Equipment 682
Communications, Signaling Systems,
Data Systems, Fire Protective Signaling
Systems, and Systems Less Than 120 Volts,
Nominal 685

Isolated Power Systems 686
518 Places of Assembly 688
520 Theaters and Similar Locations 689
General 689
Fixed Stage Switchboard 691
Stage Equipment—Fixed 693
Portable Switchboards on Stage 695
Stage Equipment—Portable 696
Dressing Rooms 696
Grounding 696
525 Carnivals, Circuses, Fairs, and Similar Events 696
General Requirements 696
Power Sources 697
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Wiring Methods 697
Grounding and Bonding 699
527 Temporary Wiring 699
530 Motion Picture and Television Studios
and Similar Locations 702
540 Motion-Picture Projectors 702
545 Manufactured Building 702
General 702
547 Agricultural Buildings 705
550 Mobile Homes and Mobile Home Parks 710
General 710
Mobile Homes 715
Services and Feeders 716
551 Recreational Vehicles and Recreational

Vehicle Parks 718
553 Floating Buildings 719
555 Marinas and Boatyards 719
Chapter 6 Special Equipment 721
600 Electric Signs and Outline Lighting 721
604 Manufactured Wiring Systems 721
605 Office Furnishings (Consisting of Lighting
Accessories and Wired Partitions) 722
610 Cranes and Hoists 725
620 Elevators, Dumbwaiters, Escalators,
and Moving Walks 725
630 Electric Welders 725
640 Sound-Recording and Similar Equipment 725
645 Information Technology Equipment 725
650 Organs 728
660 X-Ray Equipment 728
665 Induction and Dielectric Heating Equipment 728
668 Electrolytic Cells 729
669 Electroplating 729
670 Industrial Machinery 729
675 Electrically Driven and Controlled Irrigation
Machines 729
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680 Swimming Pools, Fountains, and Similar
Installations 729
General 729
Permanently Installed Pools 735
Storable Pools 745
Spas and Hot Tubs 745
Fountains 748

Therapeutic Pools and Tubs in Health Care
Facilities 751
Hydromassage Bathtubs 752
685 Integrated Electrical Systems 753
General 753
Orderly Shutdown 753
690 Solar Photovoltaic Systems 753
Chapter 7 Special Conditions 755
700 Emergency Systems 755
General 755
Circuit Wiring 758
Sources of Power 759
Emergency Circuits for Lighting and Power 762
Control Emergency Lighting Circuits 763
Overcurrent Protection 764
701 Legally Required Standby Systems 764
General 764
Circuit Wiring 767
Sources of Power 767
Overcurrent Protection 767
702 Optional Standby Systems 767
General 767
Circuit Wiring 769
705 Interconnected Electric Power Production
Sources 769
720 Circuits and Equipment Operating at Less
Than 50 Volts 770
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725 Class 1, Class 2, Class 3, and Class 4
Remote-Control, Signaling, and
Power-Limited Circuits 770
General 770
Class 1 Circuits 771
Class 2 and Class 3 Circuits 774
760 Fire Protective Signaling Systems 777
Scope and General 777
770 Optical Fiber Cables and Raceways 778
780 Closed-Loop and Programmed Power
Distribution 782
Chapter 8 Communications Systems 783
800 Communication Circuits 783
General 783
Conductors Outside and Entering Buildings 784
Protection 786
Grounding Methods 788
Communications Wires and Cables within
Buildings 789
810 Radio and Television Equipment 791
820 Community Antenna Television and Radio
Distribution Systems 791
General 791
Outdoor Cables Entering Buildings 792
Protection 793
Grounding Methods 793
Cables Within Buildings. 795
830 Network-Powered Broadband
Communications Systems 796
Chapter 9 Tables and Examples 797

Index 799
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xv
Foreword
I think that almost everyone who has been required to use the
National Electrical Code (NEC)* on a regular basis has often
wished that it were easier to understand. Often, it seems that it lacks
sufficient clarity and detail; other times, it seems to be overflowing
with useless information. The purpose of this book is to help the
reader sort through the voluminous code regulations and find the
information he or she needs, with a minimum of effort. Perhaps it
would help to understand where this code book comes from.
The National Electrical Code is one of many codes and standards
published by the National Fire Protection Association (NFPA), a
not-for-profit corporation. The code is revised every three years in
order to keep up with new materials, tools, and methods that are
constantly being developed. This work is performed by 21 separate
committees, each consisting of approximately 10 to 15 persons, the
majority of them engineers. Members of each committee meet sev-
eral times, discuss all proposed changes, accepting some and reject-
ing others, and rewrite (as required) the sections of the Code that
were assigned to their committee. Then, they circulate the changes
among the various committees, coordinate the changes, and rewrite
again. So, obviously, the updating of the NEC is no small chore.
But the real difficulty is that it must remain applicable to all types
of electrical installations, leaving no gaps. Because of this, it
becomes rather difficult to interpret in many instances.
The purpose of this book is to arrange all of the pertinent require-
ments of the National Electrical Code in a manner that is user-
friendly, allowing the reader to find the needed information

painlessly and quickly. The challenge with the NEC is that many
communities use it as law, and as such, it must be written accord-
ingly. Every possible facet of every type of electrical installation
must be covered. Because of this, the NEC is full of engineering
requirements, installation requirements, and manufacturing require-
ments—all in engineering lingo and legalese. It’s not hard to see why
it is such a difficult document to comprehend. In order to make the
NEC more easily understood and applicable, a number of guides
have been written, most of which have a legitimate place. These
guides serve to make all parts of the NEC understandable. They are
written for engineers, designers, installers, and inspectors.
*National Electrical Code
®
and (NEC
®
) are registered trademarks of
the National Fire Protection Association, Inc., Quincy, MA.
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xvi Contents
The book you now hold in your hands is substantially different
from standard NEC guidebooks. Rather than covering everything in
the NEC, we concentrated only on the requirements for electrical
installations. By omitting the engineering and manufacturing require-
ments, much of the confusion of the NEC is eliminated in one stroke.
This leaves only the rules that actually apply to installing electrical
wiring—which is the reason the Code is referred to 99 percent of the
time.
This book is designed exclusively for the installer of electrical
wiring, and is the result of many years of supervising and instruct-
ing electricians in the requirements of the NEC. Every effort has

been made to make this book as easy to use as possible, both for the
professional electrician and for the homeowner who wishes to do
his or her own electrical work safely and efficiently, avoiding has-
sles with the local electrical inspector.
For actually installing electrical wiring, this book should be more
useful than the standard NEC handbooks. For engineering ques-
tions, however, the National Electrical Code should be consulted.
xvi Foreword
As you go through both this book and the Code, you will find
numerous references to other codes and standards. These various
codes and standards are useful but must always be used in conjunc-
tion with the NEC, not separate from it. It is critical to remember
that codes are generally adopted as law by local municipalities.,
while standards are not. So, codes contain mandatory requirements
and standards contain suggested methods.
Finally, please remember that good workmanship and safety-
consciousness are essential ingredients for any good electrical
installation. Like fire, electricity can be the best of friends or the
worst of foes. Without careful workmanship and an overriding
concern for the safety of the installation and the installer, no electri-
cal installation is worthwhile.
My sincere thanks go to all of the fine people I’ve worked with
down through the years—I have had the good fortune of working
with some of the finest people in the industry.
Paul Rosenberg
Throughout this book, you will see substantive changes for
the 2005 NEC highlighted. Bear in mind that these changes will
have the force of law once the 2005 Code is adopted in your
jurisdiction.
2005

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xvii
Introduction
The National Electrical Code is written as a minimum standard for
electrical installation for the protection of life and property. It does
not necessarily define the best installation methods, merely the min-
imum safety standards. Many purchasers of electrical installations
will want to surpass the code.
When reading and interpreting the NEC there are certain words
that you must pay attention to. These key words are:
Shall. Any time you see the word shall in the NEC, it means
that you must do something a certain way. You have no choice
at all; either you do it that specific way, or you are in violation
of the code.
May. The word may gives you an option. You can do it the
certain way that is stated, or you can do it another way; it is
your choice.
Grounded Conductor. This is almost always the neutral con-
ductor, although not necessarily. Take care not to let the word
grounded confuse you; “grounded conductor” does not refer
to a green wire.
Grounding Conductor. This is the green wire, more correctly
called the “equipment grounding conductor,” because it is
used to connect equipment to ground.
You will find these ideas expressed in section 90.5 of the NEC, dis-
cussed below. They are defined as Mandatory Rules (shall), Permissive
Rules (may), and Explanatory Material (Fine Print Notes). Special
care must also be taken to differentiate between similar terms, such
as “grounded conductor” (a neutral wire), and the “grounding con-
ductor” (the green equipment grounding conductor). These terms are

almost identical, and if you do not carefully examine each word, you
could very easily make a wrong interpretation.
In addition to these terms, there are other, less-common terms
(identification, listing, supervised, and so on) that can also be con-
fusing. Remember that the NEC cannot be read casually. In order
to make correct interpretations, every word must be considered.
This requires extra work and effort.
Before getting to the main body of the NEC (starting with Article
100), it is important to cover two other sections that precede. The
more important of these is Article 90, which explains what the Code
is and what it applies to. The other, Article 80, is relatively new, and
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xviii Contents
serves as a model local ordinance for the legal adoption of the
NEC.Article 80—Administration and Enforcement.
Article 80 is a model ordinance for the administration and enforce-
ment of the NEC.
Whether this section of the NEC will be adopted by most munic-
ipalities is still an unanswered question. You should definitely check
with your local government to see whether these requirements have
been adopted or not. Most municipalities have covered these con-
cerns with local ordinances for a long time; some may choose to
keep their own ordinances, and others may prefer simply to adopt
the NEC rules as a package.
The rules of Article 80 should have no bearing on how you install
electrical wiring, though it may mean slight changes in how your
installations are inspected. So, while checking on its adoption is a
good idea, don’t expect it to change any of your installations.
Article 90—Introduction
This article lays the groundwork for the writing and application of

the National Electrical Code. It begins by stating the purpose of the
document, “the practical safeguarding of persons and property
from hazards arising from the use of electricity,” and goes on to
explain that the NEC is written to provide safe installations,
though not necessarily efficient ones.
Section 90.2 is especially important, as it identifies what sorts of
installations are, or are not, covered by the NEC. Note that almost
all wiring owned by utilities or mines, and in boats, aircraft, and
automobiles are excluded.
90.1: Purpose
(1) Electricity can be dangerous if not used properly. The Code
is written to provide a set of rules for the safe installation
of electrical wiring.
(2) This Code’s provisions are those essential for safety, and
compliance with these rules may not always result in the
most efficient, convenient, or least expensive installations;
neither does it necessarily provide for the future expansion
of electrical usage. It is however essentially free from haz-
ards that may be encountered. Nonconformity to the rules
of the NEC may result in hazards or overloading of wiring
systems. Most of these problems result from not taking
into consideration the increasing usages of electricity. If
xviii Introduction
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future needs are taken into consideration at the time of the
original installation and adequate measures are taken to
provide for the increased usage of electricity, these hazards
and overloading may be greatly eliminated.
(3) In no manner is this Code intended to be used for design
specifications or as an instruction manual for untrained per-

sons. The rules of this Code will, however, add materially to
proper design. It is also adopted as the regulations governing
wiring installations by most government agencies. There
may be additional requirements by the local agencies and
these should be checked out.
90.2: Scope
(A) Covered. This Code covers:
(1) Electric conductors and equipment installed in or on: public
or private buildings or other structures, mobile homes and
recreational vehicles, floating buildings, and other premises,
such as yards, carnivals, parking and other lots, and indus-
trial substations.
Additional information concerning installations in multibuilding
complexes or industrial buildings is found in the National Electrical
Safety Code, ANSI C2-1997.
(2) The installation of conductors on the exterior of a premise
is covered.
(3) The installation of conductors outside of a premise is cov-
ered.
(4) The installation of optical fiber cables and raceways. The
inclusion of optical fiber cables in the NEC is odd, since
these cables carry no electricity at all. They are included in
the National Electrical Code for two primary reasons: (1)
because they are usually installed by the same persons who
install electrical wiring and (2) because optical fiber sys-
tems interact with, and depend upon, electrical and elec-
tronic systems.
The code’s reference to “optical raceway” refers to special
raceways whose use is dedicated to the optical cables they
house. These are special inner ducts and possibly tubes

associated with air-blown fiber. This is not defined clearly
in the code, so check with your local inspector if you have
any questions. Also, see 770.6 for details.
Introduction xix
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xx Introduction
(5) Wiring in of offices, warehouses, or other buildings owned
by electric utilities but not part a generating facility, substa-
tion, or control facility.
(B) Not Covered. This Code does not cover:
(1)
Ships, watercraft, trains, aircraft, automobiles, or trucks,
although mobile homes and recreational vehicles are covered.
(2) Installation of conductors is not covered in the NEC for
underground mines. This does not exempt the above-ground
installation of wiring, although self-propelled surface min-
ing machinery and its trailing cables are excluded.
(3) Railroad generation, transformation, and transmission or
distribution, if used only for signaling devices, and railroad
trains are not covered in the NEC.
(4) Communication equipment located outdoors or indoors, if
used exclusively by utilities, is not covered in the NEC.
(5) Electric utility wiring exclusively under the utility com-
pany’s control, used for communication, metering, genera-
tion, transformation, and distribution of electricity, whether
indoors or outdoors on property owned or leased by the
utility, whether out of doors by established rights on private
property and public highways, streets, or roads, are not
covered by the NEC.
(6) Any metering, wiring, buildings or structures on any premise

that is not owned or leased by the utility company is covered
by the NEC. The NEC does cover all wiring other than util-
ity metering equipment ahead of service equipment through
building structures or any other place not owned or leased
by the utility.
(C) Special Permission. Conditions and usages vary in different
localities; therefore, the authority having jurisdiction for the
enforcement of the Code must be able to grant exemptions for
the installation of the wiring system equipment not under the
control of the utilities. This occurs whenever utilities are con-
necting service-entrance conductors of the building or struc-
ture that they are serving. If such installations are outside the
building or terminate just inside the building, special permis-
sion should be granted in writing.
There has been an abundance of work done by utilities, and often
the work becomes a part of the Code. Should the installation of ser-
vice laterals, for example, be deemed good engineering practice by
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utilities and acceptable by the enforcing authority, this practice may,
by special permission, be permitted under the Code. This special
permission does not eliminate the Special Permission under Article
100; it applies only to Section 90.2.
90.3: Code Arrangement
The Code is divided into an introduction and nine chapters. Chapters
1 through 4 deal with general applications of the Code to wiring and
installations. Chapters 5, 6, and 7 supplement or amend the first four
chapters, and deal with special occupancies and installations that
involve special equipment or special conditions. Chapter 8 deals with
communication circuits, and with the equipment and installation of
radio and television. Chapter 9 deals with tables not included in, but

to be used in conjunction with, the first eight chapters. Also included
are examples for figuring requirements for installation. These exam-
ples are extremely valuable in the understanding of the preceding
chapters.
Familiarity with the various Code chapters makes it easy to find
what you want in the Code. Chapters 4 through 9 are special chapters
and refer back to the first three chapters.
90.4: Enforcement
The NEC is written so that it can be enforced when adopted by
agencies having the rights of inspection. The Code’s enforcement
and interpretation is placed in the hands of the enforcing agency or
authority. These authorities are the ones who make the final deci-
sions, hopefully using the good judgment that is essential in such
interpretations. In many instances, the Code puts the entire respon-
sibility of interpretation on the enforcing authority. For example,
you will often find the phrase by special permission; this means
special permission, in writing, by the Code-enforcing authority.
The enforcing authority is vested with the right to decide on the
approval of equipment and materials. However, listings from the
Underwriters’ Laboratory, the CSA, or other independent testing
laboratories are used for this purpose in many instances. One of the
deterrents to Code understanding can be lack of communication
between the inspector and the installer. Actually the inspector is the
installer’s friend, and all the inspector wants is a good safe job. The
best advice to offer in this respect is to get acquainted with your
inspector; he or she will be understanding and helpful in most cases.
Many industries have established procedures for installation and
maintenance that are very effective and in many cases far more
safety-oriented than the Code installations. This gives the enforcing
authority the latitude to okay such installations.

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xxii Introduction
90.5: Mandatory Rules, Permissive Rules
and Explanatory Material
The Code includes both mandatory and advisory rules. The manda-
tory rules are characterized by the word “shall” This means that
the rules must be strictly followed. Any time you see the word shall
in the NEC it means that you must do something in a certain way.
You have no choice at all; either you do it that specific way, or you
are in violation of the Code. Permissive rules are characterized by
the word may. The word may gives you an option. You can do it
the specific way that is stated, or you can do it another way; it is
your choice.itemizes the types of rules given in the NEC.
Explanatory material in the NEC is placed in Fine Print Notes
(FPN). These notes are important for you to read, but they are not
enforceable.
90.6: Formal Interpretations
An NEC committee is set up to render official Code interpretations
when these are necessary. In the majority of questions arising on the
Code, the interpretations are under the inspector’s jurisdiction, as
will be seen in the next section. However, there may be instances
when official interpretations are required. No official interpreta-
tions will be made unless the Formal Interpretation Procedures out-
lined in the Code are followed.
90.7: Examination of Equipment for Safety
Most equipment and materials have been tested by electrical testing
laboratories such as Underwriters’ Laboratories (UL), and carry
their label. However, the rates that UL charge equipment makers
can be prohibitively high. (They are somewhat of a monopoly.) To

work around this problem, some municipalities have experimented
with allowing consulting engineers to certify the equipment as
being safe. If UL rates remain as high as they are now (or possibly
go even higher), this method may become far more common.
Extreme care must be taken by any inspection authority or testing
service in judging the safety of any equipment, device, or material.
Care must also be taken to assure that the equipment, device, and
so on, will be used only in the way intended. Section 110.3 and
Article 100 cover examination of equipment and the meaning of
“Listed.”
90.8:Wiring Planning
This section is unusual in that it mentions planning for future
expansion, but does not require anything specific. It has long
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been good trade practice to oversize electrical components.
However, this is not required by the NEC. Oversizing is a design
issue, not an installation issue. Nonetheless, responsible installers
should oversize the electrical equipment they are responsible for
providing, if at all possible. Conduits should not be filled to
capacity, and distribution equipment should have plenty of
empty space.
In the design of electrical systems by electrical engineers, ample
provision should be made in the raceways for adequate wiring, as
well as distribution and load centers which should be laid out in
practical locations, keeping in mind their accessibility. The number of
wires in enclosures and boxes should adhere to Code requirements in
order to avoid fires and breakdowns and the inconveniences that
accompany such troubles.
In reaching the goal of good wiring and installation, there is one
requirement—good workmanship. Insulation damage, too many

wires, and overfusing are points that must be carefully watched.
Regardless of how good the design of the installation, cutting cor-
ners will defeat the intended product.
(A) Future Expansion and Convenience. Since the invention of the
electric light, the amounts of electricity used in both home
and industry have continually increased. Therefore, in design-
ing wiring systems consideration should be given to large
enough raceways and in some cases spare raceways to accom-
modate the changes—future uses of electricity or expansion of
operations—that are certain to come. During the design
phase, it would be a good idea to review Sections 110.16 and
240.24, which describe the necessary clearance distances and
accessibility for future additions.
(B) Number of Circuits in Enclosure. You will find later in the
NEC that there is a maximum number of conductors and cir-
cuits that you can put in a single enclosure such as raceways,
boxes, and so on. These limitations for single raceways and
boxes will reduce problems with short circuits and ground
faults in a circuit.
Severe damage could be done to conductor insulation by pulling
too many conductors in to raceways, or by pulling around too many
bends. There are even times, when pulling large sizes of conductors,
that the 360 degrees in total bends between pull boxes and the like
could be too many. Since the Code is not intended to be a design
manual, it is up to the designer and the inspection authority to
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