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.
Required ampacity = 45A continuous × 1.25 = 56.25A derating wire
NEC 310.15(B)(3)(c): Approx 0.96 factor 33.5A × 0.96 = 32.16A Neutrals that carry only unbalanced current (e
Continuous load must not exceed 80% of the derated ampacity (or conversely, the derated ampacity must be ≥ 125% of the continuous load). Table 310
Table 310.15(C)(1): 7–9 conductors = 70% 47.85A × 0.70 = 33.5A
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?
This article explores the physics, the code-mandated calculations (NEC, IEC), the environmental variables, and the common traps engineers fall into when derating conductors. 1.1 The Joule Heating Equation When current ($I$) flows through a conductor of resistance ($R$), power is dissipated as heat: $$P = I^2 \times R$$