Wire Derating Chart Instant

Multiply the wire’s original 90°C ampacity (30A for 12 AWG) by 0.71:

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: 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: Multiply the wire’s original 90°C ampacity (30A for

That’s the new maximum before insulation damage. But the breaker is 20A, so the circuit is limited to 20A anyway. It tells him: Every electrician who skips derating

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.

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.