How do ambient temperature and weather affect conductor ampacity and why?

• A. Higher ambient temperatures increase ampacity; derating factors account for humidity to prevent overcooling.
• B. Higher ambient temperatures reduce ampacity; derating factors account for temperature and installation conditions to prevent overheating.
• C. Ambient temperature has no effect on ampacity; derating factors are only for insulation material.
• D. Ampacity depends solely on conductor size; temperature is not a factor.

Answer: (B)

Ampacity is the maximum current a conductor can carry continuously without its insulation temperature rating being exceeded. The conductor heats as current flows (I^2R heating), and heat must be dumped into the surrounding environment. When ambient air is hot or heat dissipation is restricted by installation (in a crowded raceway, in a conduit, in a closed cabinet, or with little airflow), the conductor’s temperature rises more for a given current, so the allowable current must be reduced. Derating factors account for both temperature and installation conditions to keep the conductor from overheating. Weather can also affect cooling—windy or open-air conditions help remove heat, while still air or enclosed spaces hinder it—so ampacity decreases in hotter ambient temperatures and with less favorable cooling.