Conductor Ampacity
Pick the gauge (AWG/mm²), material and insulation and I compute the ampacity corrected for ambient temperature and conductor grouping, per NEC 310.16, and whether it carries your current.
Calculation breakdown
| Item | Value | Detail |
|---|
How it is computed · NEC 310.16
1. Base ampacity from the gauge and the
insulation column (60/75/90 °C) of NEC table 310.16, copper.
Aluminum is estimated as ≈ 78 % of copper.
2. Ambient temperature factor (base 30 °C):
21–25=1.05 · 26–30=1.00 · 31–35=0.94 · 36–40=0.88 · 41–45=0.82 · 46–50=0.75.
3. Grouping factor for current-carrying conductors
[NEC 310.15(B)]: 1–3=1.00 · 4–6=0.80 · 7–9=0.70 · 10–20=0.50.
4. Corrected ampacity = base × f_temp × f_group.
If you enter the load current, it is compared against this value.
! Reference values under a simplified model of 310.16. A real design may also require the terminal limit (60/75 °C), adjustments for more than 20 conductors, correction for sun exposure or burial, and local rules. Always verify against the current code and with a professional.
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How it works
The calculator returns the ampacity (maximum allowable current) of a conductor based on its AWG gauge (14 through 4/0), material (copper or aluminum) and insulation column (60, 75 or 90 °C), using the NEC 310.16 table for copper as the base; aluminum is estimated as 78 % of the copper value. If you also enter your load current, it tells you whether the conductor can carry it and with how much margin.
Two correction factors are applied on top of the base ampacity: one for ambient temperature other than 30 °C (from ×1.05 at 25 °C down to ×0.75 at 50 °C) and one for bundling of current-carrying conductors in the same conduit or tray per NEC 310.15(B) (×1.00 up to 3 conductors, ×0.80 for 4-6, ×0.70 for 7-9 and ×0.50 for 10-20). The final formula is corrected ampacity = base × f_temp × f_group. It is a simplified reference model: a real project must also check terminal ratings and the applicable local code.
Example: 6 AWG copper THWN at 40 °C with 5 conductors in the conduit
- Base ampacity of 6 AWG Cu in the 75 °C column: 65 A.
- Ambient temperature factor for 40 °C (36-40 °C band):
×0.88. - Bundling factor for 5 current-carrying conductors (4-6 band):
×0.80. - Corrected ampacity:
65 × 0.88 × 0.80 = 45.8 A. A 40 A load is supported with roughly 5.8 A of margin.