Derating Wire -

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

At first glance, electrical wiring seems simple. You look up a wire gauge (e.g., 10 AWG) on an ampacity chart, see it handles 30 amps, and select a 30A breaker. But what happens when that wire is run through a 140°F attic? What if four of those wires are bundled inside a conduit? What if the equipment is installed at 10,000 feet of altitude? derating wire

The wire’s ampacity table is a starting point , not an ending one. Ambient temperature, bundling, altitude, solar gain, and continuous operation all steal from the wire’s limited temperature budget. Your job as an engineer is to account for every thief. Continuous load must not exceed 80% of the

Required ampacity = 45A continuous × 1.25 = 56.25A But what happens when that wire is run

NEC Table 310.15(B)(2)(a) for 45°C ambient, 90°C insulation = 0.87 55A × 0.87 = 47.85A