Why the same 65 A load that needs #6 AWG in normal conditions needs #4 AWG when it is hot and bundled — and how the 90 °C column and the 75 °C terminals interact.
| Load | 65 A continuous design current (after any 125% factor) |
| Conductor | Copper THHN/THWN-2, 90 °C insulation |
| Terminals | Rated 75 °C |
| Ambient | 40 °C |
| Current-carrying conductors in the raceway | 6 |
Because the insulation is rated 90 °C, the factors are taken from the 90 °C column.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| #6 AWG Cu | ≥ 65 A | 54.6 A | ✗ FAIL |
| #4 AWG Cu | ≥ 65 A | 69.2 A | ✓ PASS |
Key insight: The 90 °C column gives head-room for derating, but the final value can never exceed the 75 °C terminal rating. Here derating governs (69.2 A < 85 A); in a cool, lightly filled raceway the terminal rating would govern instead.
Every input in this example is editable in the live calculator — no signup needed.
Open Wire Size / Ampacity Calculator (NEC) calculator →Only in some cases. A neutral carrying only the unbalanced current of a 3-phase 4-wire circuit is not counted, but it is counted when the major part of the load is non-linear (310.15(E)).
Only as the starting point for correction and adjustment. The final ampacity used with that breaker is limited to the 75 °C column value (110.14(C)).