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?
Required ampacity = 45A continuous × 1.25 = 56.25A derating wire
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: $$P = I^2 \times R$$ At first glance, electrical wiring seems simple