Ampacity Derating

Category: infrastructure

The practice of reducing a conductor's maximum current rating to account for high ambient heat or multiple wires grouped in a single conduit.

Ampacity derating prevents insulation degradation. Per NEC guidelines, when more than three current-carrying conductors are bundled together or pass through areas exceeding 30°C (86°F), their allowable ampacity must be multiplied by a correction factor to prevent heat from trapping inside the raceway.

Common Examples

• We executed an ampacity derating calculation because eight current-carrying 12 AWG wires were pulled through the same attic conduit run.
• Failing to apply the proper ampacity derating multiplier can cause wire insulation to melt, inducing a severe short-to-ground fire hazard.