Wire Derating

Start with wire’s insulation temperature rating (e.g., 90°C THHN). Multiply ampacity by factor below.

#12 THHN, 9 conductors in conduit, ambient 45°C. Base @90°C = 30A Bundling (9 wires) = 0.70 Temp (45°C) = 0.82 Final = 30 × 0.70 × 0.82 = 17.22A wire derating

We look at NEC Table 310.16.

Example: #12 THHN (90°C) rated 30A at 30°C ambient. At 45°C ambient → 30A × 0.82 = . Start with wire’s insulation temperature rating (e

Bundling multiple wires together causes them to heat each other. 9 conductors in conduit