Wire Derating Chart Better -

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

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. 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. That’s the new maximum before insulation damage

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: The Setup: A Perfectly Rated Circuit An electrician

At 55°C ambient, the factor is .

| 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 |

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