Master Electrician: National Electrical Code (NEC)

The code covers the installation of electrical equipment for houses, public buildings, mobile homes, recreational vehicles, floating buildings, outdoor areas such as parking lots and carnivals, and industrial substations.
The equipment covered is used for the supply and distribution of electricity, except those that are an integral part of a generating plant or a utility substation.
Also, not covered by the code are installations in vehicles such as ships, boats, railroad cars, airplanes, cars, trucks; in and around mines; those used exclusively for signaling and communications purposes; nor those operated by an electric utility.
The code applies to installations of equipment in any area provided the local regulations specify its use.  Local code requirements always supersede the national code and national code requirements can be waived by the authority having jurisdiction.
As applied to installed equipment, accessible means allowing approach and not isolation by locked doors.  Readily accessible is that which is capable of being reached quickly without requiring climbing , removal of obstacles, or the use of a ladder.  Exposed live parts are those that could be accidentally touched because they are not insulated or guarded in any way.  Enclosed means surrounded by a housing or fence that prevents a person from accidentally touching an energized circuit.  Guarded means covered or otherwise protected from human contact.
As applied to wiring methods: accessible is that which is not covered by the structure of the building and that can be exposed or removed without damaging the finish of the building.  Exposed means attached to the surface or behind panels that allow access. Concealed means inaccessible due to the structure or finish of the building.  Wires are concealed even if they are easily withdrawn.
 

A ground is a connection between an electrical circuit and the earth (either intentional or accidental).
Grounded means connected to the earth directly or indirectly through another conductor.  Generally, this implies a low resistance path to ground.  Earth, concrete, brick, and tile surfaces are considered to be grounded.

Effectively grounded means intentionally connected to the earth through a low resistance path that prevents the device from receiving a voltage that could be hazardous to the equipment or to people.
Solidly grounded means connected directly to ground with a minimum of resistance.
A grounded conductor is one that is intentionally grounded at the service head to limit its voltage to ground.  A <i>grounding</i> conductor is one that connects equipment to a grounding point.  Equipment grounding conductors do not carry current unless there is a circuit fault.
A dry location is one that is not normally exposed to moisture, except on a temporary basis during construction.  A damp location is one that is protected from weather but may be exposed to some moisture, such as under a canopy or porch, or in a basement.  A wet location is one encased in concrete, in direct contact with the earth, or otherwise potentially exposed to water.
Rainproof means built to operate in the presence of rain.  Raintight means built so that exposure to rain will not allow water to enter.  Watertight means built to keep all water out under any conditions.
Weatherproof means built to operate in any type of weather, such as rain, snow, ice, dust, and temperature extremes.  Rainproof, raintight, or watertight equipment is considered weatherproof if these additional weather conditions do not occur.

NEC Article—110 Requirements and Restrictions for Electrical Panels
For all electrical service equipment, specific clearances in depth, width, and height must be used as described in the appropriate code article.  This is to ensure the ready and safe operation and maintenance of the equipment.
For any circuits of 50 or more volts, live parts must be guarded to prevent a person from making accidental contact.  The protection may be from an enclosure, a locked room, or by installing them out of reach.
Higher voltage equipment (greater than 600 volts) must be accessible only to qualified personnel.  The protection must be from a locked cabinet or room.  Access to higher voltage outdoor installations, unless they are out of reach, must be limited by a wall or fence.

NEC Article 200—Proper Use and Identification of Grounded Conductors
One conductor must always be wired to ground.  The grounded conductor must be identified with a continuous white or gray outer covering or by three continuous white stripes on any background color other than green. 
For wires larger than AWG 6, the grounded conductor may be identified by white or gray markings at the ends rather than continuously.
On receptacles and plugs, the terminal for connection to the grounded conductor is identified by a white coating, the word “white”, or with the letter W located next to the terminal.
Direct current circuits also need to be grounded.  The code specifies that DC systems operating at greater than 50 volts must be grounded.

NEC Article 210—Locations Where GFCIs and 20 Amp Outlets Are Required
Ground Fault Circuit Interrupters (GFCIs) are required on any 125 volt single-phase 15 and 20 ampere circuits in residential or commercial location where ground faults are more likely to occur.  The locations are those where personal protection is needed because plug-in appliances may be used.  In homes, this includes all receptacles in bathrooms, garages, unfinished basements, outdoor locations, crawl spaces, kitchens, and laundry or utility rooms.  In commercial settings, GFCIs are also required in kitchens, restrooms, and outdoor public spaces.
The larger capacity 20 ampere circuits are needed for small appliances in the kitchen, pantry, and dining areas.  Two or more 20 amp branch circuits must be provided in these areas.  In the laundry room, at least one 20 amp branch circuit must be provided to supply the laundry receptacle outlets. 
In each bathroom, at least one 20 amp branch circuit must be provided for the bathroom receptacle outlets.

NEC Articles 210, 230, 240, 285—Other Branch Circuit Protective Devices
The Transient Voltage Surge Suppressor (TVSS) is a device for limiting transient voltages by diverting or limiting surge current.
An Arc Fault Circuit Interrupter (AFCI) is a special circuit breaker that detects sudden bursts of current that indicate an electrical arc is occurring.  It is able to distinguish the arcs from normal current spikes and quickly deenergize the circuit.  In bedrooms, all 120 volt single phase 15 and 20 ampere branch circuits supplying outlets must now be protected by an arc fault circuit interrupter.
Thermal relays and other devices not designed to open short circuits or ground faults do not fulfill the requirement for protection against overcurrent, but the use of these devices is permitted to protect motors and their branch circuit conductors from overload.
Surge protectors are electronic devices that prevent short term voltage spikes by limiting or removing the voltage.  These devices are often incorporated into plug-in extension cords but are also sometimes used in the electrical service.

NEC Article 210—Requirements on Branch Circuit Outlets in General Living Areas
In general, receptacles on an individual branch circuit must have an ampere rating at least as high as that of the branch circuit.  The maximum load on each circuit must be limited to a lower value than the circuit rating and receptacle rating as defined by tables in the code.
There are requirements on the number and spacing of outlets.
In most living areas, outlets must be located so that no point along the floor on any wall is more than 6 feet from a receptacle.  Hallways of 10 feet or more in length must have at least one receptacle outlet.
At least one wall switch-controlled light or lighting outlet must be installed in every habitable room and bathroom.

NEC Article 210—Requirements for Branch Circuit Outlets in Specific Areas of a House
Appliance outlets installed for specific appliances such as air conditioners and stoves must be located within 6 feet of the appliance.
Along wall counter spaces there must be an outlet on each wall that is 12 inches or wider.  Outlets are installed so that no point along the wall line is more than 24 inches from an outlet.
In bathrooms, at least one outlet must be installed within 3 feet of the edge of each basin.  The outlet must be located on a wall that is adjacent to the basin or countertop.
Outdoors there must be at least one outlet accessible from the ground level at the front and the back of the house.
There must be at least one outlet in the laundry area. In addition to this laundry equipment outlet, at least one outlet must be installed in each basement and in each garage.

NEC Article 230—Segments of an Electrical Service Connection
In a traditional overhead distribution system, power is supplied from the pole via a service drop.  The code specifies clearance requirements for the overhead wires and service feeders.
With an underground system, the power is supplied by the main through a service lateral.  There are depth of burial and protection requirements on the main and the lateral.
The service head is where the meter and main box are located.  It contains a means of disconnection and a connection to ground. 
Within the box are the fuses or breakers that distribute and protect each branch circuit.

NEC Article 215—Feeder Connections at a Service Entrance
Feeder connections are protected against overcurrent.  The rating of this feeder overcurrent device is calculated from the total of the individual branch circuit loads and should be greater than their sum.
The grounded conductor must be identified in the same way as the branch circuit wiring, with a continuous white or gray outer covering or by three continuous white stripes on any background color other than green.
The grounding conductor of a feeder is identified in the same way as the branch circuit wiring.  The wire may be bare, but if it is insulated it must be green or have yellow stripes.
The ungrounded conductors, if there is more than one, are identified where they are accessible with separate colors or a tag.
Open conductors in the service drop should be insulated or covered, not bare.

NEC Article 250—No Grounded Conductors
There are some AC systems that are not required to be grounded.  These include circuits that are used exclusively to supply industrial electric furnaces for melting, refining, etc.  Also excluded from the grounding requirement are systems used exclusively for rectifiers that supply direct current to adjustable speed industrial drives. Systems supplying transformers for control circuits where continuous DC control power is necessary are also not required to be grounded.
Circuits that are prohibited from being grounded include those used for electric cranes that operate near combustible materials, and circuits in health care facilities.

NEC Article 225—Requirements for Outside Overhead Wires
If any conductors are within 10 feet of any building, the individual overhead conductors must be insulated or covered.
Conductor size and clearance requirements depend on the voltage of the wires.  For lower voltage conductors (less than 600 Volts), 10
AWG copper or 8 AWG aluminum is used for spans up to 50 feet, and 8 AWG copper or 6 AWG aluminum is used for longer spans.
There are tables in the code for required clearances over buildings, roadways and walkways.
Vegetation such as trees cannot be used for the support of overhead service conductors.

NEC Article 230—Requirements for Service Overcurrent Protection
Every ungrounded service conductor must have overload protection that has a rating less than or equal to the ampacity of the conductor.  Service disconnection means must consist of not more than 6 switches.  Emergency power for a building is tapped on the line side of service.
No overcurrent device can be installed in a grounded service conductor except a circuit breaker that simultaneously opens all conductors of the circuit.
The service overcurrent device must be an integral part of the service disconnect or be located immediately adjacent to it.
There are specific values of energy per square foot required for various types of commercial and residential buildings.  These are given in tables in the code.  The values determine the appropriate number, wattage, and location of lights and are also used in determining the overall load for the service.  There are also formulas for loads of general use receptacles based on the square footage of the building.
The total service load is the weighted sum of lighting and appliance loads.  Individual load requirements for permanent appliances such as heaters, air conditioners, and stoves can be found on the device nameplates.

NEC Article 240—Overcurrent Protection Devices for Branch Circuits
Overcurrent protection may be provided by either fuses or circuit breakers.  The standard ampere ratings for electrical service fuses and circuit breakers are: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110,
125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000,
1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes.  Additional standard ratings for fuses are: 1, 3, 6, and 10.  The code permits the use of fuses and circuit breakers with ratings other than these standard values.
No overcurrent device can be connected in any grounded conductor, unless the overcurrent device opens all conductors of the circuit and it is designed so that no pole can operate independently.  An exception is for required motor overload protection.
Overcurrent devices are to be protected by installation in enclosures, must be isolated from moisture and flammable materials, and are accessible only to qualified personnel.

NEC Article 250—Different Ways the Electrical Service Can Be Grounded
Electrodes that are permitted for grounding the service include rod and pipe connections specifically installed for this purpose, or the use of existing water pipes, metal building frames, or devices called grounding rings.
Rod and pipe electrodes consist of a pipe or conduit specific for this purpose that must be at least 8 feet in length.  Metal underground water pipes may be used if they are in direct contact with the earth for 10 feet or more.  The metal frame of the building or structure may be used if there is adequate earth contact.
An electrode encased by concrete may be used.  It must be located near the bottom of a concrete foundation that is in direct contact with the earth, consist of at least 20 feet of bare or conductive reinforcing bar, or be a bare copper conductor of at least 4 AWG.  A ground ring encircling the building may be used if it is in direct contact with the earth and consists of at least 20 feet of bare copper conductor of at least 2 AWG.
Other local metal underground systems or structures such as piping systems, underground tanks, and underground metal well casings can be used.  Prohibited by the code are connections to underground gas piping system and aluminum electrodes.
The code requires that any lightning protection system ground terminals be bonded to whatever grounding electrode system is used.

NEC Article 300—Physically Protecting Conductors
The code specifies that, where they are subject to physical damage, conductors must be protected.  Wires are generally required to be insulated.  In some cases, the wire or cable needs to be installed in conduit or have an armored covering.  In underground installations, cable or conduit is to be installed at a depth to meet minimum cover requirements specified in the code tables.
Conductor housings such as raceways, boxes, and conduit are made of materials suitable for the environment in which they are installed. 
Steel devices are protected against corrosion, sometimes even on the threads of conduit and fittings.  The protection is provided by galvanizing or a coating of some other corrosion resistant material.  Stainless steel devices do not need protective coatings.

NEC Article 300.10—Box Requirements and Connection
For most types of wiring, a box must be installed at each outlet and switch.  It may be metallic or nonmetallic.  When using conduit, a box or conduit body is installed at each conductor splice point, outlet, switch, or pull point.  A box or terminal fitting must be used wherever a change is made from conduit or other enclosed wiring device to open wiring or to concealed knob and tube wiring.
All metal boxes are grounded, usually with a screw threaded into in a tapped hole on the box.  All metal enclosures for conductors must be joined together into a continuous electrical conductor and be connected to all other parts to provide electrical continuity for the grounding connection. 
These measures are to protect users against electrical shock.
A table is provided in the code for the allowable number of conductors of each size for each size of metal box.

NEC Article 300—Requirements for Using Wireways, Raceways and Gutters
The number and size of conductors in any raceway must be limited for proper dissipation of the heat generated, and to allow for easy installation and removal of the conductors without damage.
Where conductors are installed in metal enclosures or metal raceways, they must be arranged to avoid heating the surrounding metal by induction from their alternating current in the conductors.  To accomplish this, conductors for all phases and the grounded and grounding conductors are grouped together in order to cancel these effects from the individual wires.
Wireways and raceways may be used indoors, outdoors, and even in wet locations if they are so rated.

NEC Articles 360-380—Types of Wireway Enclosures
Metallic and nonmetallic wireways and auxiliary gutters are all enclosures with hinged or removable covers for housing and protecting electric wires, cable, and busbars. The conductors are laid in place after the wireway has been installed as a complete system.  The metallic variety is made of sheet metal and the nonmetallic ones are made of flame retardant materials.
A busway is a manufactured grounded metal enclosure that contains copper or aluminum conductors made of bars or rods.  A cablebus is a series of components consisting of insulated conductors and fittings that are connected to form an enclosed protected assembly.
Surface raceways are mounted to the surface of a structure, and use couplings and connectors.  They may be metallic or nonmetallic.
Cable tray systems are assemblies consisting of tray sections and fittings used to fasten and support electrical cables.

NEC Article 310—Markings Required on Cables and Location
All conductors and cables are marked to indicate their maximum rated voltage, the type of wire or cable, the manufacturer’s name or other identifying mark, and the wire gauge or area in circular mils.
The code specifies which types of cable can be used in various locations. For dry and damp locations, the permissible types are: FEP,
FEPB, MTW, PFA, RHH, RHW, RHW-2, SA, THHN, THW, THW-2, THHW, THHW-2, THWN,
THWN-2, TW, XHH, XHHW, XHHW-2, Z, or ZW. In wet locations, the specified types are: “moisture impervious metal-sheathed”, or MTW, RHW, RHW-2, TW, THW, THW-2,
THHW, THHW-2, THWN, THWN-2, XHHW, XHHW-2, or ZW.

NEC Article 400—Requirements for Flexible Cords
The code includes tables of allowable ampacities for flexible cords and cables.
Flexible cords and cables are to be marked with a printed tag attached to the cable or its packaging.
Flexible cords and cables may not be used as a substitute for fixed wiring.  They may only be used in continuous lengths without splices when they are initially installed.  The repair of cord of 14 AWG and larger is permitted if the completed splice retains the outer sheath properties of the original cord.
Flexible cords and cables must be protected by bushings or fittings when passing through holes in covers, outlet boxes, or similar enclosures.

NEC Article 400—Requirements for Flexible Cord
The flexible cord grounded circuit conductor must have a continuous marker or separator that is white or gray or light blue.  Braided wire may instead be tinned to indicate the grounded conductor.  One or more ridges, grooves, or white stripes located on the exterior of the cord may also be used to identify it.
A conductor that is used for equipment grounding must have a continuous marker that distinguishes it from the other conductors.  The identifying marker may be a braid or insulation with a continuous green color or green with one or more yellow stripes.

NEC Article 402—Requirements for Conductors Used in Light Fixtures
The allowable ampacity of fixture wires is specified in a code table.  Fixture wires are never allowed to be smaller than 18 AWG.
The number of fixture wires permitted in a single conduit or tubing is limited by the maximum percentage fill for the tubing, and that is specified in another code table.
Fixture wires that are used as grounded conductors are identified by one or more continuous white stripes on the insulation that must be of a color other than green.
Fixture wires cannot be used as branch circuit conductors.

NEC Article 404—Requirements for Connections to Switches
Switches are wired so that the switching is done only on the ungrounded circuit conductor, never on the grounded conductor of a circuit.
Switches and circuit breakers that are mounted in an enclosure must be clearly marked as to their ON and OFF positions.  If the switch or circuit breaker handles are operated vertically, the up position of the handle must be the ON position.
Metal enclosures for switches or circuit breakers are grounded.  If nonmetallic enclosures are used with metal raceways or metal armored cables, the ground continuity must be maintained to the switch by using a grounding wire.

NEC Article 406—Requirements for Replacement of a Faulty Non-Grounding
Type Receptacle
When a non-grounding receptacle fails, the code specifies the following rules for its replacement:
- If there is a way to ground the receptacle enclosure or if a grounding conductor is installed, a grounding type receptacle must be used and be connected to the grounding conductor.
- Where a means for grounding does not exist in the receptacle enclosure, a new non-grounding type receptacle can be used to replace the faulty one.  A grounding type receptacle can be used to replace the non-grounding receptacle when it is supplied through a ground-fault circuit interrupter.  If so, the receptacle must be marked “GFCI Protected” and “No
Equipment Ground.”
- A non-grounding receptacle may be replaced with a ground-fault circuit interrupter-type receptacle.  The receptacle must be marked “No
Equipment Ground.”  In this case, no equipment grounding conductor can be connected from this receptacle to any other outlet supplied from the ground-fault circuit-interrupter receptacle.

NEC Article 406—Requirements for Installing Outlet Receptacles
Receptacles are contact devices for the attachment of electrical plugs.  They are mounted in boxes or other assemblies designed for this purpose, and these assemblies must be securely fastened in place. 
Receptacle faceplates must completely cover the opening and seat against the mounting surface.  When the installation of a receptacle is completed, the plaster or drywall must have no air gaps greater than 1/8 inch.  Metal faceplates must be grounded.
Grounding type receptacles must have a means for connection of a grounding conductor.  The terminal for this connection must be a green hexagonal screw or nut, or another connection device that is green.  If the grounding terminal is not visible, the entrance for the wire will be marked with the word green or ground, the letters G or GR, a ground symbol, or some other mark that is green in color.
Receptacles that have an isolated grounding connection used to reduce electrical noise (such as in a medical facility) are identified by an orange triangle on the face of the receptacle.

NEC Article 408—Requirements for Electrical Panels
Conductors and busbars must be located so as to be protected from physical damage and must be held firmly in place.  Barriers are placed in all service switchboards, so that uninsulated and ungrounded surfaces of conductors and terminals are not subject to accidental contact from other equipment or by people servicing the panel.
Each circuit must be legibly marked as to its specific purpose.  The identification must include enough detail to distinguish each circuit from the others.  A circuit directory must be on the face or inside of the panel door.
Unused openings for circuit breakers and switches must be closed and marked as such.
Panels that have any exposed live parts must be in dry locations and accessible only to qualified persons.  They must also be located so as to be free of damage from equipment.
In addition to the main circuit breaker, not more than 42 overcurrent devices of lighting and appliance branch circuits may be installed in any one cabinet.
Metal cabinets and frames must be in contact with each other and be grounded.  If the panel is used with nonmetallic raceway or cable, a terminal bar for the grounding conductors is used to and bonded to the cabinet and panel frame.

NEC Article 410—Requirements for Live Parts of a Lighting Fixture
Fixtures and lampholders may not have live parts exposed to external contact.  Exposed terminals in lampholders and switches must not be installed in metal luminaires (fixtures) or in open bases of table or floor lamps.
No parts of a ceiling fan or a lighting fixture that is connected by a cord or is suspended can be located within 3 feet horizontally or 8 feet vertically from the top of a bathtub or shower stall.  Other types of lighting fixtures located in this area must be certified for damp or wet locations.
Fixtures must be wired so that the screw shells of all lampholders are electrically connected to the grounded conductor.

NEC Article 411—Requirements for Low Voltage Lighting Systems
Low voltage lighting systems are those that operate at 30 volts or less.  The secondary circuit of the transformer must be insulated from the branch circuit by using an isolating transformer, and the secondary side of the transformer is not grounded.
Exposed bare conductors and current carrying parts are permitted for these low voltage applications, but only for indoor installations.  The bare conductors must be installed at least 7 feet above the floor.
Lighting systems that operate at 30 volts or less may be supplied from a maximum 20 ampere branch circuit.
Even though the code recognizes the less dangerous nature of lower voltages, the electrician must always remember that even low voltages can be dangerous.

NEC Article 430—Data Available on the Nameplate of Motors
A motor is always marked with the following information:
- Manufacturer’s name.
- Rated voltage and full load current.  If it is a multispeed motor, the full load current for each speed is listed.
- Rated frequency and number of phases (AC motor).  “PH” refers to the number of phases.
- Type of winding (DC motor).
- Rated full-load speed.
- Rated temperature rise or the insulation system class and rated ambient temperature (normally 40 C.)
- Time rating (minutes of operation or continuous).
- Rated horsepower (if 1 HP or more).  If it is a multispeed motor, the horsepower for each speed is listed.
Other data may be also available on the nameplate.  A motor with a thermal protector containing heat sensing elements is marked “Thermally
Protected” or simply “T.P.”  A motor that is impedance protected is marked
“Impedance Protected” or “Z.P.”
If a motor is operated at higher than its rated voltage it will draw more current, use more power, and may have a shorter life.

NEC Article 430—Requirements for Branch Circuit Wiring to Motors
A method for connection of the grounding conductor is provided on the motor housing.  The connection may be located either inside or outside the housing.
Conductors that supply a single motor used in a continuous duty application must have an ampacity of at least 125 percent of the motor’s full-load current rating.
All continuous duty motors of more than 1 horsepower, or ones that are started automatically, must be protected by an overload device. 
If it is an overcurrent sensing device, it is set to trip at 115% to125% of the full load current rating, depending on the motor’s service factor and temperature rating.  If the motor contains a thermal protection device, the trip current will be 140% to 170% of the full load current, depending on the rated current.
Motors with less than 1 horsepower are sometimes impedance protected; that is, they cannot overheat if the motor fails to start because the impedance of the motor windings is high enough to prevent this.
Lights and motors should not be wired into the same circuit because overloads and shorts are more common on motors and these would disrupt lighting.  The lights would also tend to dim due to large starting currents on the motors.

NEC Chapter 5—Special Occupancies
The word “occupancy” refers to the intended use of a building.  Special occupancies are locations where the environment or use of the facilities poses unique hazards that warrant special or additional requirements in the code.
Special occupancies include those that are hazardous such as places where flammable liquids, gases, or dust are present; assembly areas where large numbers of people congregate; specific assembly areas such as theatres and carnivals; locales such as marinas, boatyards, and RV parks; and specific structures like manufactured buildings, agricultural buildings, mobile homes, floating buildings, and temporary installations.
Special occupancies may require special wiring methods, equipment, materials, or monitoring and alarm systems as specified in the code.

NEC Articles 511- 517—Specific Hazardous Locations
Specific locations with unique requirements include commercial garages used for service, repair, and storage of vehicles that use volatile fuel (like gasoline and diesel).  Also listed are hangars where aircraft undergo service, repairs, or alterations if temperatures can be above the flash point of the fuel.  All motor fuel dispensing facilities (gas stations) and bulk storage and processing facilities for marine or highway use are also listed.
Facilities such as paint shops with spray application, dipping, and coating processes using combustible liquids and powders are also listed.
All health care facilities (for humans) are also listed.

NEC Article 518—Assembly Occupancies
Assembly occupancies are those facilities that attract large numbers of people. They include all buildings designed or intended for the gathering of 100 or more people for such purposes as meetings, worship, entertainment, eating and drinking, amusement, and waiting for transportation.
Specifically listed are: armories, exhibition halls, assembly halls, gymnasiums, auditoriums, multipurpose rooms, chapels, bowling alleys, museums, conference rooms, courtrooms, places of worship, dance halls, pool rooms, restaurants, bars, and skating rinks.
Because of unique additional hazards, listed separately with other special requirements are theatres, studios, carnivals and circuses.

NEC Article 590—Criteria for Temporary Installations
The provisions of this article apply to temporary electrical power and lighting that is installed in buildings during construction, remodeling, maintenance or repair, or for demolition.  Temporary special occupancies may also be installed during emergencies or for short term tests and experiments.
Aside from these specific uses, installations qualify as temporary according to their intended duration, which is not to exceed 90 days.  This includes such purposes as holiday decorative lighting.
Temporary wiring must be removed immediately upon completion of construction or when the purpose for which they were installed has expired.

NEC Chapter 6—Special Equipment Requirements
The following is a list of special equipment that is listed in the code with special requirements:
- Electric signs
- Manufactured wiring systems which use field installed manufactured subassemblies
- Office furnishings (lighting accessories and wired office partitions)
- Cranes and Hoists
- Elevators, escalators, and other people conveyors
- Systems for electric vehicle charging, electrolytic cells, solar photovoltaic cells, and fuel cells
- Electric welders, electroplating, and induction and dielectric heating equipment
- Audio signal processing equipment
- Information technology equipment
- Sensitive electronic equipment
- Pipe Organs with a DC potential not exceeding 30 Volts
- X-Ray Equipment
- Industrial Machinery
- Electrical irrigation equipment and electrical equipment for use in swimming pools, fountains, and natural and artificial bodies of water
- Integrated Electrical Systems, where an orderly shutdown is required to minimize hazards
- Fire Pumps

NEC Article 647—Sensitive Electronic Equipment
This sensitive electronic equipment is instrumentation that has unique power requirements.  It is commercial or industrial equipment for specific applications that is used and controlled only by qualified personnel.
The equipment is wired to a special 120 volt single phase supply that is separate from other branch circuits.  It is a three wire system with 60 volts on each of two conductors that are ungrounded plus a grounded neutral conductor.  The equipment uses this unique wiring to reduce noise in the equipment.
This occupancy was included in previous versions of the NEC, but not under a separate article.  The original use for the special wiring was in movie and television studios, but by moving the requirements to a separate article it now covers additional uses.

NEC Article 670—Industrial Machinery
Industrial machinery is defined as power-driven machinery that is not portable while in use, and is used to for specific operations like cutting, forming with pressure or other means, laminating, or a combination of these processes.  It also includes associated equipment used to transfer materials or tooling to assemble, disassemble, inspect, test, or package.  Any associated electrical equipment such as controllers, software, and actuators and sensors, is considered part of the industrial machine.
This article covers the definition of, the nameplate data for, and the size and overcurrent protection of supply conductors to the industrial machinery.

NEC Chapter 7—Special Conditions
For special conditions, unique requirements are given which include minimum wire sizes, derating factors, overcurrent protection, insulation requirements, and wiring methods and materials.  Special conditions that are listed in the code are:
- Fire alarms, emergency and standby systems that supply electricity for illumination or power when the normal electrical supply is interrupted.
- Interconnected electric power production sources that operate in parallel with a primary source of electricity.
- Circuits and equipment operating at less than 50 volts, and remote control or signaling circuits whose use and power limitations distinguish them from normal light and power circuits.  These include programmed power distribution systems which are controlled by a signal between the supply and utilization equipment.
- Type ITC instrumentation tray cable, which is a factory assembly of two or more insulated conductors, with or without a grounding conductor, enclosed in a nonmetallic sheath.
- Optical fiber cables and raceways.

NEC Chapter 8—Communication Systems
Communication systems that are listed in the code with special requirements are:
- Telephone and telegraph wires, and wiring for fire alarms and burglar alarms and other central station equipment.
- Radio and television signal distribution systems, including antenna systems and the wiring and cabling that connects them to the transmitting and receiving equipment.  Also included are community antenna television and radio signal distribution systems that use coaxial cables to deliver programming.  These are sometimes designated as “CATV.”
- Network powered broadband communications systems that provide a combination of voice, audio, video, data, and interactive services through a network interface unit.

NEC Chapter 9—Types of Information Available in the NEC Tables
The National Electrical Code Chapter 9 consists of tables that contain the following types of data:
- Conduit and tubing cross sectional areas, allowable percentage of fill, and allowable bend radius.
- Copper and aluminum insulated power distribution conductor and fixture wire diameters and cross sectional areas, and AC and DC resistances and
AC reactances.
- AC and DC power source limitations in voltage, current, and similar data for Power Limited Fire Alarm (PFLA) circuits.
- Tubing and conduit fill tables are also provided in Annex C. 
These tables list the allowable number of conductors of various sizes that may be routed through conduit of various sizes.

NEC Annex B--“Application Information for Ampacity Calculations”
Annex B is entitled “Application Information for Ampacity Calculations.”  It is not a part of the requirements of the code book but it is included for information only.
This annex provides application information for the calculation of current carrying capacities (ampacities), which are to be calculated only under direct engineering supervision.  Thermal resistivity information is provided for various materials surrounding the conductors.
These calculations can be used in lieu of pre-calculated tabular values, which are also provided in this Annex.  These tables provide data for conductors in free air as well as in raceways, conduit, directly buried in the earth, and in underground ducts.  The data are in some cases given for various ambient air temperatures.

NEC Annex A, and the Cross Reference of Article Numbers, Annex F--“Product Safety Standards”
Annex A is entitled “Product Safety Standards.”  It is not a part of the requirements of the code book but it is included for information only.
This annex provides a list of product safety standards that are required by the code.  Most of the standards are those of the Underwriter’s
Laboratories (UL) and bear their numbers, and some are maintained by the Industry Standard Architecture (ISA) or the Institute of Electrical and Electronics Engineers (IEEE).
Annex F is a cross reference of article numbers between the current (2005) NEC and the two earlier versions.  It also is not a part of the requirements of the code book, but it is included for information.

NEC Annex D and the “Types of Construction,” Annex E
Annex D is entitled “Examples.”  It is not a part of the requirements of the code book but it is included for information only.
This annex provides examples of calculations used to determine the number and size of branch circuit and the main service size for different applications.
Annex E is entitled “Types of Construction.”  It also is not a part of the requirements of the code book but it is included for information.
This annex provides information on construction related data such as fire resistance ratings in hours and allowable number of stories for different types of construction.

NEC Annex G--“Administration and Enforcement”
Annex G is entitled “Administration and Enforcement.”  I t is not normally a part of the requirements of the code book and is included for informational, but this annex may be adopted by local jurisdictions, in which case the specifics are considered to be requirements.
The annex describes the duties and responsibilities of the local electrical inspector and the chief electrical inspector, and lists qualifications for these positions.  It also describes the plan review and inspection processes and specifies how and when a new service may be connected to the electrical supply.