Temperature Rating
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 310.10)
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. (NEC 310.16)
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. This section is expected to be modified to include cables in future editions.
In construction situations where future expansion is highly likely, it is sometimes more economical to install a slightly larger diameter conduit than is necessary for the initial building construction. Larger conduit costs more, but has a greater fill rating. This may eliminate the need for additional conduit installation while maintaining the conduit's overall temperature rating. Furthermore, it is more energy-efficient and often economically justified to install larger conductors than Code-required minimums due to decreased heating losses, and decreased voltage drop. Additionally, larger conductors than minimum allow some capacity for possible future load changes.
In certain situations, the 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. A knob and tube installation uses wires that are suspended in air. This gives them a greater heat dissipation rating than standard 3-wire NM-2 cable which includes two tightly bundled load and return wires.
Read more about this topic: National Electrical Code, Details of Selected NEC Requirements
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