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: $$P = I^2 \times R$$
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.
| Number of Conductors | Percent of Ampacity | |----------------------|---------------------| | 1–3 | 100% | | 4–6 | 80% | | 7–9 | 70% | | 10–20 | 50% | | 21–30 | 45% | | 31–40 | 40% | 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?
Required ampacity = 45A continuous × 1.25 = 56.25A | Number of Conductors | Percent of Ampacity
Continuous load must not exceed 80% of the derated ampacity (or conversely, the derated ampacity must be ≥ 125% of the continuous load).
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). What if four of those wires are bundled inside a conduit
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.