Why must ambient temperature and installation method be considered when sizing cables?

• A. Ambient temperature affects only insulation resistance, not ampacity.
• B. Temperature has no effect; only insulation matters.
• C. Temperature derating affects ampacity; installation method affects cooling and heat dissipation.
• D. Installation method affects color coding of conductors.

Answer: (C)

The key idea is that how much current a cable can safely carry (ampacity) depends on how much heat is produced by the current and how well that heat can be removed. The hotter the surroundings or the less effective the cooling, the less current the insulation can safely handle without overheating.

Ambient temperature matters because it sets the starting temperature the conductor sits at. If the air around the cable is warm, there’s less headroom for the conductor to rise before reaching the insulation’s temperature limit, so you must reduce the allowed current—this is a temperature derating.

Installation method matters because it changes how easily heat escapes. Cables in free air cool better than cables in conduits, trays, or buried in concrete, and having many conductors together in a raceway or bundle reduces heat dissipation. In those cases, you apply additional derating to the ampacity to account for reduced cooling.

So, sizing must consider ambient temperature and installation method to ensure the chosen conductor can carry the required current without overheating. The ideas in the other options don’t fit: temperature does affect ampacity, not just insulation resistance, and color coding isn’t related to current-carrying capacity.