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) .
Multiply the wire’s original 90°C ampacity (30A for 12 AWG) by 0.71: wire derating chart
At 20°C, that 12 AWG wire’s insulation can shed heat easily. But at 55°C, the wire starts hotter. It heats up more under load. Now the copper and insulation approach — the insulation’s long-term limit. The mixer draws 16 amps under full load
| Ambient Temp | Adjustment Factor for 90°C-rated wire | |--------------|----------------------------------------| | 21–25°C | 1.00 | | 26–30°C | 0.96 | | 31–35°C | 0.91 | | 36–40°C | 0.87 | | 41–45°C | 0.82 | | 46–50°C | 0.76 | | | 0.71 | | 56–60°C | 0.65 | Heat from ovens and griddles rises to the
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:
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.
