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.

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

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. By the book, 12 AWG + 20A breaker = safe, legal, functional

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. Use the chart, or rewire the fire later

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) .