NEC RULES FOR CONDUCTORS

NEC rules for the ends of a wire differ from those for the middle. (Adapted from Practical Electrical Wiring, 20th edition, © Park Publishing, 2008, all rights reserved).

The key to applying these rules, and the new NEC Example D3(a) in Annex D on this topic is to remember that the end of a wire is different from its middle. Special rules apply to calculating wire sizes based on how the terminations are expected to function.

Entirely different rules aim at assuring that wires, over their length, don’t overheat under prevailing loading and conditions of use. These two sets of rules have nothing to do with each other—they are based on entirely different thermodynamic considerations.

Some of the calculations use, purely by coincidence, identical multiplying factors. Sometimes it is the termination requirements that produce the largest wire, and sometimes it is the requirements to prevent conductor overheating.

You can’t tell until you complete all the calculations and then make a comparison. Until you are accustomed to doing these calculations, do them on separate pieces of paper.


Current is always related to heat.
Every conductor has some resistance and as you increase the current, you increase the amount of heat, all other things being equal. In fact, as is covered in Sec. 110 of Div. 1 and elsewhere, you increase the heat by the square of the current.

The ampacity tables in the NEC reflect heating in another way. As the reproduction of NEC Table 310.16 (see Table 18 in Div. 12) shows, the tables tell you how much current you can safely (meaning without overheating the insulation) and continuously draw through a conductor under the prevailing conditions—which is essentially the definition of ampacity in NEC Article 100: The current in amperes that a conductor can carry continuously under the conditions of use without exceeding its temperature rating.

Ampacity tables show how conductors respond to heat.
The ampacity tables (such as Table 18 in Div. 1) do much more than what is described in the previous paragraph. They show, by implication, a current value below which a wire will run at or below a certain temperature limit.

Remember, conductor heating comes from current flowing through metal arranged in a specified geometry (generally, a long flexible cylinder of specified diameter and metallic content). In other words, for the purposes of thinking about how hot a wire is going to be running, you can ignore the different insulation styles.

As a learning tool, let’s make this into a “rule” and then see how the NEC makes use of it: A conductor, regardless of its insulation type, runs at or below the temperature limit indicated in an ampacity column when, after adjustment for the conditions of use, it is carrying equal or less current than the ampacity limit in that column.

For example, a 90 C THHN 10 AWG conductor has an ampacity of 40 amps. Our “rule” tells us that when 10 AWG copper conductors carry 40 amps under normal-use conditions, they will reach a worst-case, steady-state temperature of 90 C just below the insulation.

Meanwhile, the ampacity definition tells us that no matter how long this temperature continues, it won’t damage the wire. That’s not true of the device, however. If a wire on a wiring device gets too hot for too long, it could lead to loss of temper of the metal parts inside, cause instability of nonmetallic parts, and result in unreliable performance of overcurrent devices due to calibration shift.

Termination rules protect devices.
Because of the risk to devices from overheating, manufacturers set temperature limits for the conductors you put on their terminals. Consider that a metal-to-metal connection that is sound in the electrical sense probably conducts heat as efficiently as it conducts current. If you terminate a 90 C conductor on a circuit breaker, and the conductor reaches 90 C (almost the boiling point of water), the inside of the breaker won’t be much below that temperature.

Expecting that breaker to perform reliably with even a 75 C heat source bolted to it is expecting a lot. Testing laboratories take into account the vulnerability of devices to overheating, and there have been listing restrictions for many, many years to prevent use of wires that would cause device overheating. These restrictions now appear in the NEC.

Smaller devices (generally 100 amp and lower, or with termination provisions for 1 AWG or smaller wire) historically weren’t assumed to operate with wires rated over 60 C such as TW. Higherrated equipment assumed 75 C conductors but generally no higher for 600-volt equipment and below. This is still true today for the larger equipment. (Note that medium-voltage equipment, over 600 volts, has larger internal spacings and the usual allowance is for 90 C, but that equipment will not be further considered at this point.)

Today, smaller equipment increasingly has a “60/75 C” rating, which means it will function properly even where the conductors are sized based on the 75 C column of Table 18, Div. 1.

ELECTRIC MOTOR STANDARDS BASIC INFORMATION AND TUTORIALS

Motors and generators are required to meet various industry and national standards and in some instances specific local codes and customer specifications. The more important of these standards may be briefly described as follows:

1. NEMA Standards are voluntary standards of the National Electrical Manufacturers Association and represent general practice in the industry. They define a product, process, or procedure with reference to nomenclature composition, construction, dimensions, tolerances, operating characteristics, performance, quality, rating, and testing. Specifically, they cover such matters as frame sizes, torque classifications, and basis of rating.

2. IEEE Standards (AIEE) concern fundamentals such as basic standards for temperature rise, rating methods, classification of insulating materials, and test codes.

3. USA Standards are national standards established by the United States of America Standards Institute, which represents manufacturers, distributors, consumers, and others concerned. USA Standards may be sponsored by any responsible body and may become national standards only if a consensus of those having substantial interest is reached.

Standards may cover a wide variety of subjects such as dimensions, specifications of materials, methods of test, performance, and definition of terms. USA Standards frequently are those previously adopted by and sponsored by NEMA, IEEE, etc. The chief motor and generator standard of USASI is C50, “Rotating Machinery,” which is substantially in agreement with current NEMA Standards.

4. National Electrical Code is a USA Standard sponsored by the National Fire Protection Association for the purpose of safeguarding persons and buildings from electrical hazards arising from the use of electricity for light, heat, power, and other purposes. It covers wiring methods and materials, protection of branch circuits, motors and control, grounding, and recommendations, regarding suitable equipment for each classification.

5. Underwriters’Laboratories, Inc. is an independent testing organization, which examines and tests devices, systems, and materials with particular reference to life, fire, and casualty hazards. It develops standards for motor and control for hazardous locations through cooperation with manufacturers.

It has several different services by which a manufacturer can indicate compliance with Underwriters’ Laboratories Standards. Such services are utilized on motors only in the case of explosion proof and dust-ignition proof motors where label service is used to indicate to code enforcing authorities that motors have been inspected to determine their adherence to Underwriters’ Laboratories Standards for motors for hazardous locations.

6. Federal Specification CC-M-641 for integral-horsepower ac motors has been issued by the federal government to cover standard motors for general government uses. Standard motors meet these specifications, but other Federal Specifications issued by various branches of the government for specific use may require special designs.

7. World Standards. Standards similar to our NEMA Standards have been established in other countries. The most significant are

a. IEC (International Electrochemical Commission) Standard 72-1, Part 1

b. German Standard DIN 42673

c. British Standard BSI-2960, Part 2

These standards specify dimensions, classes of insulation, and in some cases horsepower ratings.

IEEE STANDARDS 519 AND 1159 – POWER QUALITY STANDARDS

IEEE Standards are publications that provide acceptable design practice. IEEE Standards addressing power quality include those defining acceptable power quality (IEEE Standard 519) and another standard relating to the measurement of power-quality “events” (IEEE Standard 1159).

Both of these standards focus on AC systems and their harmonics (that is, multiples of the line frequency). IEEE Standard 519 [2.1] (denoted IEEE Std. 519-1992) is titled “IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems.”

The abstract of this standard notes that power conversion units are being used today in industrial and commercial facilities, and there are challenges associated with harmonics and reactive power control of such systems. The standard covers limits to the various disturbances recommended to the power distribution system.

The 1992 standard is a revision of an earlier IEEE work published in 1981 covering harmonic control. The basic themes of IEEE Standard 519 are twofold. First, the utility has the responsibility to produce good quality voltage sine waves.Secondly, end-use customers have the responsibility to limit the harmonic currents their circuits draw from the line.

Shown in Figure 2.1 is a utility system feeder serving two customers. The utility source has resistance R and line reactance jXs. The resistance and reactance model the impedances of the utility source, any transformers and switchgear, and power cabling.

Figure 2.1 Harmonic-generating load causing voltage distortion at the point of common coupling (PCC). The AC source is modeled as an ideal voltage source in series with a resistance Rs and a reactance jXs.

Customer #1 on the line draws harmonic current Ih, as shown, perhaps by operating adjustable speed drives, arc furnaces, or other harmonic-creating systems. The voltage Customer #2 sees at the service entrance is the voltage at the “point of common coupling,” often abbreviated as “PCC.” The harmonics drawn by Customer #1 cause voltage distortion at the PCC, due to the voltage drop in the line resistance and reactance due to the harmonic current.

The voltage harmonic distortion limits apply to the quality of the power the utility must deliver to the customer. For instance, for systems of less than 69 kV, IEEE 519 requires limits of 3 percent harmonic distortion for an individual frequency component and 5 percent for total harmonic.

The current harmonic distortion limits apply to limits of harmonics that loads should draw from the utility at the PCC. Note that the harmonic limits differ based on the ISC/IL rating, where ISC is the maximum short-circuit current at the PCC, and IL is the maximum demand load current at the PCC. IEEE Standard 1159 [2.2] is entitled “IEEE Recommended Practice for Monitoring Electric Power Quality,” and as its title suggests, this standard covers recommended methods of measuring power quality events.

Many different types of power-quality measurement devices exist and it is important for workers in different areas of power distribution, transmission, and processing to use the same language and measurement techniques. In future chapters, we draw extensively from IEEE Standards 519 and 1159. distortion.

ELECTRICAL PRODUCT AND WORK STANDARDS BASIC INFORMATION AND TUTORIALS

The generation, transmission, and distribution of electrical power are now deregulated, but many rules, regulations, and standards still apply to the manufacture of electrical equipment, the job site, and the installation of electrical systems. Many of these standards are focused on safety issues, such as the elimination or avoidance of hazards in working with or using electricity.

The installation of any inferior wiring devices or equipment, substandard workmanship, or inadequate test and maintenance procedures could be the cause of fires or explosions and result in the creation of electric shock hazards.

There is a need for standards that, when adopted, will serve as a basis for proper inspection and supervision. There are regulatory standards, national consensus standards, product standards, installation standards, and international standards.

The consensus standards include the National Electrical Code (NEC), the National Electrical Safety Code (NESC), National Fire Protection Association (NFPA) 70B and 70E, and other NFPA standards, as well as American National Standards Institute (ANSI) and the Institute of Electrical and Electronic Engineers (IEEE) standards.

Interested persons with requisite education, training, and background experience volunteer their time and expertise to develop these standards. Some might be employees of electrical product manufacturers, and others might be consultants or engineering professors.

Some standards were developed specifically for electrical applications. These include the National Electrical Code (NEC) (officially NFPA 70) and the National Electrical Installation Standards (NEIS).

In addition, the National Electrical Manufacturers Association (NEMA) standards cover equipment design and construction, while the Underwriters Laboratories Inc. (UL) standards cover safety provisions in the manufacture of electrical devices, products, and accessories.


The National Fire Protection Association (NFPA), publisher of both the NEC and NESC, has also developed standards related to electrical work including:  NFPA 79 Industrial Machinery  NFPA 780 Lightning Protection  NFPA Static Electricity

The NFPA has also developed standards for fire prevention, installation of sprinklers, stacking materials, and a standard building code. The following organizations have also developed standards that have a bearing on electrical work:

ACS: American Chemical Society
ACGIH: American Conference of Governmental Industrial Hygienists
AIChE: American Institute of Chemical Engineers
ASME: American Society of Mechanical Engineers
ASTM: American Society for Testing and Materials
ASSE: American Society of Safety Engineers
AWS: American Welding Society
CGA: Compressed Gas Association
CMA: Chemical Manufacturing Association
CMAA: Crane Manufacturer’s Association
GSA: General Services Administration Federal Supply Services
NSC: National Safety Council
OSHA: Occupational Safety and Health Administration

Individuals or organizations with a professional or business interest in these standards can join these organizations to help support them and gain access to their newsletters so that they can stay informed on any changes within the standards. Standards-making organizations may make changes to their standards between normal cycles that are not included in the printed text of the original issue of the standard.

The NFPA, for example, does this in the form of a Tentative Interim Amendment (TIA). Additionally, changes may take place without a formal notice of change, so it is important to stay current with any given standard. Interested parties can communicate with these standards organizations and suggest changes or revisions in standards.

It is the responsibility of all electrical contractors, electricians, and installers in the field to know which standards apply to any project taking place within any given location or job site. It is also important to remember that not all standards that might apply to every job site or location actually apply. A hazard assessment by the project supervisor or licensed electrician must determine which standards apply at each workplace and that they are followed.


The contractor has the responsibility for assuring that a workplace is free from recognized hazards and is a safe place for electricians and installers to work. This could apply to such factors as the quality of ladders or scaffolding at the site or the need for safety goggles or face masks when performing certain kinds of work.

For example, eye protection should be worn during any grinding or cutting operations that could result in flying chips of metal, and proper face masks should be worn by anyone performing burning or welding that could result in the release of toxic gases.

Designers, engineers, contractors, electricians, or equipment installers and all other persons whose work is governed by one or more standards should be familiar with and know how to apply the rules found in all of the applicable standards. These are the rules that relate to design, including safety considerations, for a particular project or task.

TURBINE GENERATOR STANDARD AND OPTIONAL EQUIPMENT

Standard Equipment

The manufacturer shall equip the turbine-generator unit with the following standard equipment:

1) Speed/Load-Control System.

A speed /load-control system capable of controlling and regulating the speed of the turbine in conformity with the performance characteristics hereinafter specified. The speed/load-control system should include means by which the steady-state speed regulation may be adjusted to values within the limits hereinafter specified.

Adjustment of the steady-state speed regulation, while the turbine is in operation, is not required by this recommended practice unless otherwise agreed upon between the manufacturer and the purchaser.

2) Speed/Load Reference Changer.

A speed/load changer by means of which the speed or power output of the turbine may be changed within the limits hereinafter specified while the turbine is in operation. The speed/load reference changer shall be equipped with means for manual adjustment and should be equipped to accept input(s) for remote control.

3) Valve Position Limiter (Load Limit).

For turbines rated over 10 MW, a valve position limiter manually adjustable to limit the degree of opening of the control valves to any value within the full range of valve travel while the turbine is in operation.

If this device is used for load-limiting purposes, the speed-control system will not necessarily control the overspeed of the turbine, if the speed/load reference changer is set at its high-speed stop.

4) Miscellaneous.

At the discretion of the manufacturer, any instruments, controls, or safety devices not specified as standard equipment in (1), (2), and (3) may be included.

Optional Equipment

The following devices or other optional devices may be specified by the purchaser:

1) Valve Position Limiter.

For turbines rated 10 MW or under, a valve position limiter.

2) Adjustment of Steady-State Regulation.

A means by which, in the speed/load-control system the steady-state speed regulation may be adjusted, within limits agreed to by the manufacturer and purchaser, while the turbine is operating at any power output.

3) Remote or Local Indication.

A means for remote or local indication, or both, of the positions of the control valves or any other element of the control system to be specified by the purchaser.

4) Remote Control of the Valve Position Limiter.

For turbines rated over 10 MW, a means for remote setting of the valve position limiter within the limits hereinafter specified.

5) Remote Control of Speed/Load Reference Changer.

For turbines rated over 10 MW, a means for remote control of the speed/load reference changer within the limits hereinafter specified.

6) Miscellaneous.

At the discretion of the manufacturer, any instruments, controls, or safety devices not previously specified as optional equipment may be included.

POWER QUALITY STANDARDS BY IIEE

10.1.1 Surge Protective Devices
32-1972 (R1997) IEEE Standard Requirements, Terminology, and Test Procedures
for Neutral Grounding Devices
C62.1-1989 (R1994) IEEE Standard for Gapped Silicon-Carbide Surge Arresters for
AC Power Circuits
C62.2-1987 (R1994) IEEE Guide for the Application of Gapped Silicon-Carbide
Surge Arresters for Alternating Current Systems
C62.11-1993 IEEE Standard for Metal-Oxide Surge Arresters for Alternating Current
Power Circuits
C62.22-1991 IEEE Guide for the Application of Metal-Oxide Surge Arresters for
Alternating-Current Systems
C62.23-1995 IEEE Standard Draft Application Guide for Surge Protection of Electric
Generating Plants
C62.31-1987 (R1998) IEEE Standard Test Specifications for Gas-Tube Surge-Protective
Devices
C62.32-1981 (R1997) IEEE Standard Test Specifications for Low-Voltage Air Gap
Surge-Protective Devices (Excluding Valve and Expulsion Type Devices)

C62.33-1982 (R1994) IEEE Standard Test Specifications for Varistor Surge-Protective
Devices
C62.35-1987 (R1993) IEEE Standard Test Specifications for Avalanche Junction
Semiconductor Surge Protective Devices
C62.36-1994 IEEE Standard Test Methods for Surge Protectors Used in Low-Voltage
Data, Communications, and Signaling Circuits
C62.38-1994 IEEE Guide on Electrostatic Discharge (EDS): ESD Withstand Capability
Evaluation Methods (for Electronic Equipment Subassemblies)
C62.41-1991 (R1995) IEEE Recommended Practice on Surge Voltages in Low-
Voltage AC Power Circuits
C62.42-1992 IEEE Guide for the Application of Gas Tube and Air Gap Arrester
Low-Voltage (Equal to or Less than 100 V rms or 1200 V dc) Surge-Protective
Devices
C62.45-1992 (R1997) IEEE Guide on Surge Testing for Equipment Connected to
Low-Voltage AC Power Circuits
C62.47-1992 IEEE Guide on Electrostatic Discharge (ESD): Characterization of the
ESD Environment
C62.92.1-1987 (R1993) IEEE Guide for the Application of Neutral Grounding in
Electrical Utility Systems, Part I-Introduction
C62.92.2-1989 (R1993) IEEE Guide for the Application of Neutral Grounding in
Electrical Utility Systems, Part II-Grounding of Synchronous Generator Systems
C62.92.3-1993 IEEE Guide for the Application of Neutral Grounding in Electrical
Utility Systems, Part III-Generator Auxiliary Systems
C62.92.4-1991 IEEE Guide for the Application of Neutral Grounding in Electric
Utility Systems, Part IV-Distribution
C62.92.5-1992 (R1997) IEEE Guide for the Application of Neutral Grounding in
Electric Utility Systems, Part V-Transmission Systems and Subtransmission
Systems
1299/C62.22.1-1996 IEEE Guide for the Connection of Surge Arresters to Protect
Insulated Shielded Electric Power Cable Systems
C62.34-1996 IEEE Standard for Performance of Low-Voltage Surge-Protective
Devices (Secondary Arresters)
C62.37-1996 IEEE Standard Test Specification for Thyristor Diode Surge Protective
Devices

C62.48-1995 IEEE Guide on Interactions Between Power System Disturbances and
Surge-Protective Devices
C62.64-1997 IEEE Standard Specifications for Surge Protectors Used in Low-Voltage
Data, Communications, and Signaling
10.1.2 Power Capacitors Standards
18-1992 IEEE Standard for Shunt Power Capacitors
824-1994 IEEE Standard for Series Capacitors in Power Systems
1036-1992 IEEE Guide for Application of Shunt Power Capacitors
C37.99-1990 (R1994) IEEE Guide for the Protection of Shunt Capacitor Banks
10.1.3 Protective Relaying
C37.90-1989 (R1994) IEEE Standard for Relays and Relay Systems Associated
with Electric Power Apparatus
C37.90.1-1989 (R1994) IEEE Standard Surge Withstand Capability (SWC) Tests
for Protective Relays and Relay Systems
C37.91-1985 (R1990) IEEE Guide for Protective Relay Applications to Power
Transformers
C37.95-1989 (R1994) IEEE Guide for Protective Relaying of Utility-Consumer
Interconnections
C37.97-1979 (R1990) IEEE Guide for Protective Relay Applications to Power System
Buses
C37.101-1993 IEEE Guide for Generator Ground Protection
C37.102-1987 (R1990) IEEE Guide for ac Generator Protection
C37.106-1987 (R1992) IEEE Guide for Abnormal Frequency Protection for Power
Generating Plants
C37.108-1989 (R1994) IEEE Guide for the Protection of Network Transformers
C37.109-1988 (R1993) IEEE Guide for the Protection of Shunt Reactors
C57.13.1-1981 (R1992) IEEE Guide for Field Testing of Relaying Current Transformers
C57.13.2-1991 IEEE Standard Conformance Test Procedures for Instrument Transformers
C57.13.3-1983 (R1990) IEEE Guide for the Grounding of Instrument Transformer
Secondary Circuits and Cases

C37.110-1996 IEEE Guide for the Application of Current Transformers Used for
Protective Relaying Purposes
C37.112-1996 IEEE Standard Inverse-Time Characteristic Equations for Overcurrent
Relays
10.1.4 Stationary Battery Standards
450-1987 IEEE Recommended Practice for Maintenance, Testing and Replacement
of Large Lead Batteries for Generating Stations and Substations
484-1987 IEEE Recommended Practice for Installation Design and Installation of
Large Lead Storage Batteries for Generating Stations and Substations
485-1983 IEEE Recommended Practice for Sizing Large Lead Storage Batteries for
Generating Stations and Substations.
937-1987 (R1993) IEEE Recommended Practice for Installation and Maintenance
of Lead-Acid Batteries for Photovoltaic (PV) Systems
946-1992 IEEE Recommended Practice for the Design of dc Auxiliary Power Systems
for Generating Stations
1013-1990 IEEE Recommended Practice for Sizing Lead-Acid Batteries for Photovoltaic
(PV) Systems
1106-1987 IEEE Recommended Practice for Maintenance, Testing, and Replacement
of Nickel-Cadmium Storage Batteries for Generating Stations and Substations
1115-1992 IEEE Recommended Practice for Sizing Nickel-Cadmium Batteries for
Stationary Applications
1145-1990 IEEE Recommended Practice for Installation and Maintenance of
Nickel-Cadmium Batteries for Photovoltaic (PV) Systems
1159-1995 IEEE Recommended Practice for Monitoring Electric Power Quality
1184-1994 IEEE Guide for the Selection and Sizing of Batteries for Uninterruptible
Power Systems (BCI/ANSI)