Wire Derating Chart [cracked] ⏰ 🔖

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

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. wire derating chart

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: That’s the new maximum before insulation damage

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. But at 55°C, the wire starts hotter

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) .

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.

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.