[extra Quality]: Derating Wire

12 current-carrying THHN #12 wires in a conduit. Base 90°C ampacity = 30A. 12 wires = 50% derate. Result = 15A. Suddenly, that 20A circuit is illegal. Pillar 3: Continuous Loads (>3 Hours) Even if a wire is sized perfectly for non-continuous load, running at 100% for hours allows heat to saturate the entire assembly (conduit, wall, junction boxes).

Required ampacity = 45A continuous × 1.25 = 56.25A

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

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.

Suddenly, the 30A wire becomes a 15A fire hazard. 12 current-carrying THHN #12 wires in a conduit

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? Result = 15A

| Number of Conductors | Percent of Ampacity | |----------------------|---------------------| | 1–3 | 100% | | 4–6 | 80% | | 7–9 | 70% | | 10–20 | 50% | | 21–30 | 45% | | 31–40 | 40% |