Ampacity Calculator — Temperature & Bundling Derating | WireStandard

The ampacity calculator applies NEC 310.15(B) derating to give you the safe continuous current rating for a conductor. Starting from the base ampacity in NEC Table 310.16, it multiplies two correction factors: an ambient temperature correction (NEC 310.15(B)(1)) that reduces capacity when surroundings exceed 30 °C, and a bundling adjustment (NEC 310.15(C)(1)) that accounts for heat buildup when four or more current-carrying conductors share a conduit. The result is the maximum current the wire can carry continuously without exceeding its insulation temperature rating.

Derating Formula

I_derated = I_base × Ct × Cb. I_base is the NEC Table 310.16 ampacity at 30 °C ambient for the selected AWG and insulation column. Ct is the NEC 310.15(B)(1)(1) ambient temperature correction factor. Cb is the NEC 310.15(C)(1) bundling adjustment factor: 1.00 for 1–3 conductors, 0.80 for 4–6, 0.70 for 7–9, and 0.50 for 10 or more. For ambient temperatures above 40 °C use the 75 °C column. Service entrance conductors default to the 75 °C column per NEC 230.42.

Worked Example

AWG 8 THWN, 90 °C insulation, 6 conductors in conduit, ambient 40 °C. Base ampacity from NEC Table 310.16, 90 °C column: 55 A. Ambient correction Ct = 0.91 (40 °C per NEC 310.15(B)(1) table). Bundling factor Cb = 0.80 (4–6 conductors per NEC 310.15(C)(1)). I_derated = 55 × 0.91 × 0.80 = 40.0 A. Select a 40 A breaker, not the 55 A base value.

Frequently Asked Questions

Which insulation temperature column should I use?

For most new commercial and industrial work, use the 75 °C column. THWN-2 and XHHW-2 conductors are rated 90 °C but most breaker and equipment terminals are rated only 75 °C. Unless every termination in the circuit is listed for 90 °C, NEC 110.14(C)(1) limits you to the 75 °C ampacity value even with 90 °C insulation.

Does the neutral count as a current-carrying conductor for bundling?

For balanced three-phase wye systems, the neutral carries only unbalance current and does not count. However, NEC 310.15(B)(5) requires counting the neutral as current-carrying when the load is rich in harmonics — for example, circuits feeding LED driver panels, variable-frequency drives, or data center equipment where third-order harmonic currents add rather than cancel on the neutral.

Why is AWG 12 limited to 20 A even when using the 90 °C column?

NEC 240.4(D)(2) imposes a small-conductor rule that caps the overcurrent protection for AWG 14 at 15 A, AWG 12 at 20 A, and AWG 10 at 30 A regardless of the insulation rating or calculated ampacity. The rule exists because small conductors are easily damaged by sustained overcurrent before a standard breaker trips. The column value sets the wire's thermal limit; NEC 240.4(D) sets the breaker ceiling.