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).

Table 310.15(C)(1): 7–9 conductors = 70% 47.85A × 0.70 = 33.5A derating wire

NEC Table 310.15(B)(2)(a) for 45°C ambient, 90°C insulation = 0.87 55A × 0.87 = 47.85A 12 current-carrying THHN #12 wires in a conduit

Continuous load must not exceed 80% of the derated ampacity (or conversely, the derated ampacity must be ≥ 125% of the continuous load). derating wire