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 either melting, oxidizing, or self-igniting. A full-load wire does heat up slightly due to the metallic resistance to the current, 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. This is necessary because when multiple conductors are carrying full-load power, there is a combined heating of all the cables that may exceed the normal insulation temperature rating. (NEC 310.16)
In construction situations where future expansion is highly likely to occur, it is sometimes economical to install a slightly larger diameter conduit than is necessary for the initial building construction. Larger conduit costs more money, but has a greater fill rating than smaller conduit, so that in the future no additional conduit installations may be required in order to add additional circuits while maintaining the conduit's overall temperature rating.
In certain special 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. The single lone load wires suspended in midair in knob and tube wiring will have a greater heat-dissipation rate than the standard 3-wire NM-2 cable which includes two tightly bundled load and return wires.
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