Required ampacity = 45A continuous × 1.25 = 56.25A
Suddenly, the 30A wire becomes a 15A fire hazard. derating wire
12 current-carrying THHN #12 wires in a conduit. Base 90°C ampacity = 30A. 12 wires = 50% derate. Result = 15A. Suddenly, that 20A circuit is illegal. Pillar 3: Continuous Loads (>3 Hours) Even if a wire is sized perfectly for non-continuous load, running at 100% for hours allows heat to saturate the entire assembly (conduit, wall, junction boxes). Required ampacity = 45A continuous × 1
16A continuous load. Required ampacity = 16A × 1.25 = 20A. After derating for ambient and bundling, the wire’s final adjusted ampacity must be ≥20A. Part 4: Advanced Derating Scenarios 4.1 High Altitude (Above 2,000 m / 6,500 ft) At higher altitudes, air density decreases, reducing convective cooling. The NEC (310.15(B)(3)(c)) mandates a correction factor of 0.95 to 0.80 depending on altitude. IEC 60364-2-2 has similar provisions. 12 wires = 50% derate
Table 310.15(C)(1): 7–9 conductors = 70% 47.85A × 0.70 = 33.5A