Example: #12 THHN (90°C) rated 30A at 30°C ambient. At 45°C ambient → 30A × 0.82 = .
Derating means reducing the allowable current-carrying capacity (ampacity) of a wire below its standard-rated value because of . Goal: Prevent overheating and insulation damage. wire derating
Whether you are a DIYer running Ethernet cables or an apprentice electrician learning the trade, understanding derating is essential for a safe and code-compliant installation. Example: #12 THHN (90°C) rated 30A at 30°C ambient
If the surrounding air is already hot (e.g., in an attic), the wire cannot release heat as effectively. Goal: Prevent overheating and insulation damage
Even if you use 90°C wire, if terminals are rated 75°C, you for final ampacity selection after derating . Derating calculations still start from 90°C base, but final ampacity ≤ 75°C column value.
However, you are allowed to use the 90°C rating for the derating calculation itself (to get a higher starting number before applying the penalties), provided the final result does not exceed the termination temperature rating.