Derating Wire -

NEC 310.15(B)(3)(c): Approx 0.96 factor 33.5A × 0.96 = 32.16A

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

is the process of reducing the current-carrying capacity (ampacity) of a conductor to account for operating conditions that increase its temperature. Since heat is the fundamental enemy of insulation, derating is not a suggestion—it is a thermodynamic necessity. NEC 310

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. What if four of those wires are bundled inside a conduit

Neutrals that carry only unbalanced current (e.g., in a 3-phase wye system) are not counted. Neutrals that carry full load (e.g., single-phase, or non-linear loads with triplen harmonics) are counted.

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