Hence, copper wire is used almost exclusively in branch circuitry. A ground fault circuit interrupter GFCI is required for all receptacles in wet locations defined in the Code.
The NEC also has rules about how many circuits and receptacles should be placed in a given residential dwelling, and how far apart they can be in a given type of room, based upon the typical cord length of small appliances. As of the NEC required that new Volt household receptacle outlets, for general purpose use, be both grounded and polarized. NEMA connectors implement these requirements.
The Code required that new volt receptacles be grounded also, which necessitates a fourth slot in their faces. Changes in standards often create problems for new work in old buildings. Unlike circuit breakers and fuses, which only open the circuit when the current exceeds a fixed value for a fixed time, a GFCI device will interrupt electrical service when more than 4 to 6 milliamperes of current in either conductor leaks to ground.
A GFCI detects an imbalance between the current in the 'hot' side and the current in the 'neutral' side. One GFCI receptacle can serve as protection for several downstream conventional receptacles. GFCI devices come in many configurations including circuit-breakers, portable devices and receptacles.
Another safety device introduced with the code is the arc-fault circuit interrupter AFCI. This device detects arcs from hot to neutral that can develop when insulation between wires becomes frayed or damaged.
While arcs from hot to neutral would not trip a GFCI device since current is still balanced, circuitry in an AFCI device detects those arcs and will shut down a circuit.
AFCI devices generally replace the circuit breaker in the circuit. As of the National Electrical Code, AFCI protection is required in new construction on all and amp, volt circuits to bedrooms. In home construction, wiring is commonly allowed directly in walls without additional protection. However, in commercial and industrial buildings, wiring must be protected from damage, so it is more commonly installed inside metal or plastic conduit or ductwork, or passageways cast in concrete.
While some types of cable are protected by flexible spiraled metal armor, it is more common to install conduit and ductwork and pull the wire in later. The NEC spends considerable time documenting safe methods of installing cable in conduit, the primary concerns being the abrading of insulation, damage to the wire or insulation caused by sharp bends, kinking, and damage due to excess pulling strain.
A wire pulled with excessive force may break inside the conduit, requiring costly removal and replacement. However, a wire pulled with enough force to stretch the wire, but not break it, creates a hazard of future failure or fire. The stretched wire section will have a thinner cross section and higher resistance than other parts of the cable, and may have damaged insulation.
Breaks may form in the stretched insulation, which may not be discovered until the circuit is powered and damage from arcing or shorting has occurred. Wires must be protected from sharp metal edges of cut conduits or cabinet holes.
The NEC specifies measures to protect wire insulation from damage by these edges during installation and use. For example, insulated cables may not be inserted directly through knockouts, due to the sharp edge around nearly all knockout holes.
Clamping and other wire protection is often not required for plastic conduit parts, as plastic is not likely to damage insulation in contact with it. In hazardous locations, more robust cable protection may be necessary.
Common conduit and ductwork protects against direct physical abuse, but is neither air nor water tight. In wet locations, conduit may resemble standard threaded pipe in appearance, with gasketed box openings to keep moisture out.
Areas with potentially explosive gases need further protection to prevent electrical sparks from igniting the gases, and internal conduit gas-tight barriers to prevent potentially ignited gases from traveling inside the conduit to other parts of the building. The temperature rating of a wire or cable is generally the maximum safe ambient temperature that the wire can carry full-load power without the cable insulation melting, oxidizing, or self-igniting.
A full-load wire does heat up slightly due to the metallic resistance of the wire, but this wire heating is factored into the cable's temperature rating. NEC The NEC specifies acceptable numbers of conductors in crowded areas such as inside conduit, referred to as the fill rating.
If the accepted fill rating is exceeded, then all the cables in the conduit are derated , lowering their acceptable maximum ambient operating temperature. Derating is necessary because multiple conductors carrying full-load power generate heat that may exceed the normal insulation temperature rating.
The NEC also specifies adjustments of the ampacity for wires in circular raceways exposed to sunlight on rooftops, due to the heating effects of solar radiation. In certain situations, temperature rating can be higher than normal, such as for knob-and-tube wiring where two or more load-carrying wires are never likely to be in close proximity. This must-have on-the-job reference helps your work pass inspection the first time.
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