Wire Ampacity & Derating Calculator

Start from the NEC Table 310.15(B)(16) 75°C ampacity for a copper or aluminum conductor, then apply temperature-correction and conduit-fill adjustment factors to get the derated ampacity. Useful for hot ambient runs and conduits with more than three current-carrying conductors.

Inputs

Table 310.15(B)(1) — 30°C = 86°F base
Neutral counts only with harmonic currents
60°C and 90°C columns are pending — until they’re transcribed the derate uses the 75°C base, so the 90°C-derate + termination-limit upgrade isn’t applied yet.

Derate chain

Base Ampacity (75°C)25 A
× Ambient Correction1.00
× Adjustment (CCC)1.00
Termination Limit (75°C)25 A
Allowed Ampacity25 A
240.4(D): 12 AWG copper max OCPD = 20 A (small-conductor rule, regardless of ampacity).

base = min(derated, termination column). NEC Table 310.16 · 310.15(B)(1) · 310.15(C)(1) · 110.14(C).

Method: base ampacity from NEC Table 310.15(B)(16), 75°C column, derated by the temperature correction factor (Table 310.15(B)(2)(a)) and the conduit-fill adjustment factor (Table 310.15(B)(3)(a)). Terminations may further limit usable ampacity. Verify against the governing edition of the NEC for your jurisdiction.

Run your whole job on the same numbers

These NORDIX tools are a taste of the full platform — bid pipeline, estimating, and job costing that carry your numbers from the first bid to the final invoice.

See what NORDIX does →