Which factor is used to determine conductor ampacity in relation to temperature?

• A. Ambient air temperature
• B. Conductor diameter
• C. Insulation color
• D. Voltage rating

Answer: (A)

Ampacity is the maximum current a conductor can carry without the insulation overheating. Whether the conductor stays within its temperature limit depends on how well heat is removed, which is strongly affected by the surrounding temperature. Higher ambient air temperature means less cooling, so you must derate the current—lowering the allowable ampacity to keep temperature rise within safe limits. Cooler environments allow more current, up to the insulation’s rating.

The other factors don’t determine the temperature-based limit in the same way. Conductor diameter influences resistance and heating, but the standard temperature derating uses ambient temperature, not size. Insulation color doesn’t affect thermal ratings, and voltage rating governs how much voltage the insulation can withstand, not how much current it can carry.