From NEC 310.16, a 10 AWG THHN copper wire at 90°C has a base rating of .
Ambient 45°C → For 90°C wire, factor = (from temp table). 28 A × 0.82 = 22.96 A → 23 A
A derating chart is a graphical or tabular representation that shows the relationship between a component’s maximum allowable load and a specific stress factor—most commonly temperature. derating chart
| Condition | Typical Factor | Notes | |-----------|---------------|-------| | 4-6 CCCs in conduit | 0.80 | NEC 310.15(C)(1) | | 7-9 CCCs | 0.70 | | | Ambient 35°C (THHN) | 0.96 | Adjust per chart | | Ambient 45°C (THHN) | 0.82 | | | Continuous load (breaker) | 0.80 | NEC 210.20(A) | | Rooftop (above 3/4" from surface) | +33°C to ambient | Then use temp derate |
Go to the manufacturer's published derating chart—NEC tables are conservative but code-minimum. For safety-critical systems (cranes, data centers, hospitals), derate one extra step (e.g., use next lower ambient column). From NEC 310
20A breaker, THHN, 3 conductors (no bundling derate), ambient 48°C.
This guide is written for electricians, engineers, technicians, and hobbyists working with electrical systems, specifically wires, cables, and circuit breakers. | Condition | Typical Factor | Notes |
Derating is the practice of operating an electrical component or device at a lower stress level than its maximum rated capacity. This is done to improve reliability, reduce wear and tear, and extend the lifespan of the component. Derating can be applied to various parameters such as voltage, current, temperature, and power.