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.
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: $$P = I^2 \times R$$ derating wire
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. The wire’s ampacity table is a starting point
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. 16A continuous load
NEC 310.15(B)(3)(c): Approx 0.96 factor 33.5A × 0.96 = 32.16A