16A continuous load. Required ampacity = 16A × 1.25 = 20A. After derating for ambient and bundling, the wire’s final adjusted ampacity must be ≥20A. Part 4: Advanced Derating Scenarios 4.1 High Altitude (Above 2,000 m / 6,500 ft) At higher altitudes, air density decreases, reducing convective cooling. The NEC (310.15(B)(3)(c)) mandates a correction factor of 0.95 to 0.80 depending on altitude. IEC 60364-2-2 has similar provisions.
NEC 310.15(B)(3)(c): Approx 0.96 factor 33.5A × 0.96 = 32.16A
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. derating wire
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?
Table 310.15(C)(1): 7–9 conductors = 70% 47.85A × 0.70 = 33.5A 16A continuous load
NEC Table 310.15(B)(2)(a) for 45°C ambient, 90°C insulation = 0.87 55A × 0.87 = 47.85A
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: $$P = I^2 \times R$$ Part 4: Advanced Derating Scenarios 4
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.