Wire Derating Chart Repack -

At 55°C ambient, the factor is .

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.

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: 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.

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) . At 55°C ambient, the factor is

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

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. By the book

| 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 |