Without derating, that 16-amp load would push conductor temperature over the limit. Insulation hardens, cracks, and eventually shorts. Alex opens the NEC (National Electrical Code) Table 310.16 — the standard wire derating chart. It tells him:
At 55°C ambient, the factor is .
The mixer draws 16 amps under full load. Well within limit. The kitchen’s exhaust fan breaks. Heat from ovens and griddles rises to the ceiling, where Alex’s wire passes through fiberglass insulation and a roof conduit. Ambient temperature inside the ceiling: 55°C (131°F) . wire derating chart
1. The Setup: A Perfectly Rated Circuit An electrician named Alex finishes wiring a new commercial kitchen. He runs a 12 AWG copper wire — rated for 20 amps at room temperature — from the breaker panel to a dedicated outlet for a large mixer. By the book, 12 AWG + 20A breaker = safe, legal, functional. Without derating, that 16-amp load would push conductor
That’s the new maximum before insulation damage. But the breaker is 20A, so the circuit is limited to 20A anyway. It tells him: At 55°C ambient, the factor is
However, if the mixer draws 16A continuously (more than 3 hours), the wire must be derated again by 125% — but here, 16A is fine. But Alex realizes: — because the breaker is thermal and also affected by ambient heat. 4. The Multiplier Trap Alex also has four current-carrying conductors in the same conduit (not just two). Table 310.15(C)(1) says: 4–6 conductors → derate by 0.80.
Multiply the wire’s original 90°C ampacity (30A for 12 AWG) by 0.71: