Continuous load must not exceed 80% of the derated ampacity (or conversely, the derated ampacity must be ≥ 125% of the continuous load).
Introduction: The Silent Killer of Electrical Systems Every year, fires, motor failures, and power supply meltdowns trace their root cause to a single, overlooked design step: failing to derate a wire. derating wire
| Number of Conductors | Percent of Ampacity | |----------------------|---------------------| | 1–3 | 100% | | 4–6 | 80% | | 7–9 | 70% | | 10–20 | 50% | | 21–30 | 45% | | 31–40 | 40% | Continuous load must not exceed 80% of the
Suddenly, the 30A wire becomes a 15A fire hazard. 12 current-carrying THHN #12 wires in a conduit
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).