Wire Derating Chart < 2024-2026 >

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.

That’s the new maximum before insulation damage. But the breaker is 20A, so the circuit is limited to 20A anyway. wire derating chart

Multiply the wire’s original 90°C ampacity (30A for 12 AWG) by 0.71: At 20°C, that 12 AWG wire’s insulation can

Use the chart, or rewire the fire later. Find wire’s 90°C column ampacity → multiply by ambient temp factor → multiply by bundling factor (if >3 CCCs) → result must be ≥ load + 125% if continuous. If not, go up one wire gauge. Now the copper and insulation approach — the

Every electrician who skips derating because “it’ll never get that hot” or “the breaker will trip first” is gambling with insulation life. The chart’s story is simple:

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) .

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.