How does ambient temperature affect conductor ampacity?

• A. It can reduce or increase the safe current rating depending on temperature.
• B. It has no effect on current rating.
• C. It only affects insulation color.
• D. It only affects voltage.

Answer: (A)

Ambient temperature changes how easily a conductor can shed the heat produced by current. Ampacity is the maximum current the conductor can carry without the insulation reaching its rated temperature. In a hotter surroundings, heat builds up more quickly and dissipation is reduced, so the safe current must be reduced (derated). In cooler surroundings, heat dissipates more efficiently, allowing a higher current before the insulation limit is reached. So ambient temperature can either reduce or permit a higher safe current, which is why ampacity depends on temperature and why derating charts exist.