How does ambient temperature affect conductor ampacity in NEC tables?

• A. Higher ambient temperatures reduce ampacity; derating factors may be required.
• B. Higher ambient temperatures increase ampacity; no derating needed.
• C. Ambient temperature has no effect on ampacity in NEC tables.
• D. Derating applies only to indoor installations, not ambient temperature.

Answer: (A)

The main idea is that conductor ampacity is defined for a specific, standard ambient temperature and insulation rating, and the surrounding temperature directly affects how much current a conductor can safely carry. When ambient temperature rises, heat builds up more and dissipates less efficiently, so the conductor tends to run hotter for the same current. To keep equipment within safe limits, the NEC uses temperature correction/derating factors to reduce the listed ampacity as ambient temperature increases. In practice, the tables assume a reference ambient (often 30°C); if the actual ambient is higher, you apply those correction factors, and sometimes additional derating is needed if many current-carrying conductors are in the same raceway or enclosure. That’s why the correct choice states that higher ambient temperatures reduce ampacity and derating factors may be required.

The other ideas don’t fit: ampacity does not increase with higher ambient, and ambient temperature does affect ampacity. Derating isn’t restricted to indoor installations; it applies based on temperature and installation conditions, whether indoors or outdoors.