What happens to conductor ampacity when ambient temperature increases beyond the rating?

• A. Ampacity increases.
• B. Ampacity stays the same.
• C. Ampacity can become unpredictable.
• D. Ampacity decreases.

Answer: (D)

When the surrounding temperature is higher than what the conductor rating assumes, the amount of current the conductor can carry safely goes down. The insulation and conductor heat up as current flows, and a hotter ambient means there’s less headroom before the insulation reaches its maximum allowable temperature. NEC rules require applying temperature correction/derating factors to reduce ampacity when ambient temperature rises, so you end up with a smaller allowable current. That’s why higher ambient temperatures lead to decreased ampacity.