Wire Ampacity Calculator
Start from the NEC 310.16 ampacity for your conductor, then apply ambient-temperature correction and conductor-bundle adjustment to get the real, derated capacity.
How derating works
The Table 310.16 number is a starting point for a run of no more than three conductors in a 30 °C ambient. Two adjustments bring it to reality:
Ampacity = Base × Temp correction × Bundle adjustment
- Temperature correction (NEC 310.15(B)(1)) — hotter surroundings lower capacity; the factor depends on ambient temperature and the insulation column.
- Bundle adjustment (NEC 310.15(C)(1)) — packing more than three current-carrying conductors together traps heat: 4–6 → 80%, 7–9 → 70%, 10–20 → 50%, 21–30 → 45%, 31–40 → 40%, 41+ → 35%.
Terminations also cap ampacity: per 110.14(C), most equipment limits you to the 60 °C or 75 °C column even when the wire itself is rated 90 °C. The 90 °C column is generally used only as the starting point for derating.
NEC 310.16 base ampacity
| Size | Cu 60°C | Cu 75°C | Cu 90°C | Al 75°C |
|---|
Common questions
Which temperature column should I use?
Use the lowest of: the wire's insulation rating, and the temperature rating of the breakers/lugs it lands on. For most branch circuits that's the 60 °C or 75 °C column. Higher columns are typically only for the derating start point.
What counts as a current-carrying conductor?
Ungrounded (hot) conductors and, in some cases, the neutral. Equipment grounding conductors do not count. Grounded neutrals count only when they carry unbalanced or harmonic current per 310.15(E).