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Wire Ampacity Calculator

Base ampacity, ambient correction, bundling adjustment and the governing result after 75 °C termination limits are applied.

Quick answer

Derated ampacity is base ampacity × ambient factor × bundling factor. The ambient factor is √((Tc − Ta) / (Tc − 30)); the bundling factor is 80% for 4–6 conductors, 70% for 7–9, 50% for 10–20. The final usable value is also capped by the termination temperature rating, commonly 75 °C.

Conditions

I = Ibase × √((Tc − Ta) / (Tc − 30)) × Fbundle

30°C ambient at 90°C insulation gives a 1.000 correction. 3 conductors give a 1.00 adjustment.

Result

Base at 90 °C

37A

Ambient factor

1.000

Bundling factor

1.00

Derated ampacity

37A

Governing value: 32 A — the lower of the derated 37 A and the 75 °C termination limit of 32 A for this size.

No derating applies at these conditions.

See the full spec sheet for 10 AWG.

Formula estimate — source: Power-law ampacity model with standard ambient and bundling factors

Bundling adjustment factors

Current-carrying conductorsAdjustment factor10 AWG copper 90 °C becomes
1–3100%37 A
4–680%30 A
7–970%26 A
10–2050%19 A
21–3045%17 A
31–4040%15 A
41+35%13 A

Source & method: Standard stepped adjustment schedule for conductors bundled or run together in a raceway longer than 24 inches.

Ambient correction across common temperatures

Size21 °C ambient30 °C ambient40 °C ambient50 °C ambient60 °C ambient
1421 A20 A18 A16 A14 A
1229 A27 A25 A22 A19 A
1040 A37 A34 A30 A26 A
853 A49 A45 A40 A35 A
672 A67 A61 A55 A47 A
496 A90 A82 A73 A64 A
3113 A105 A96 A86 A74 A
2131 A122 A111 A100 A86 A
1152 A142 A130 A116 A100 A
1/0177 A165 A151 A135 A117 A
2/0206 A192 A175 A157 A136 A
3/0239 A223 A204 A182 A158 A
4/0279 A260 A237 A212 A184 A
Estimated ampacity for copper at 90 °C insulation, three conductors.

Frequently asked

How do you derate wire ampacity?
Start from the base allowable ampacity for the conductor size, material and insulation rating, then multiply by the ambient temperature correction factor and by the adjustment factor for more than three current-carrying conductors. Both factors apply together.
What is the ambient temperature correction formula?
F = the square root of (Tc − Ta) divided by (Tc − 30), where Tc is the insulation temperature rating and Ta is the actual ambient temperature, both in Celsius. At 30 °C ambient the factor is 1.0.
How many conductors before derating applies?
Adjustment starts above three current-carrying conductors in a raceway or cable: 4 to 6 conductors 80%, 7 to 9 conductors 70%, 10 to 20 conductors 50%, 21 to 30 conductors 45%, 31 to 40 conductors 40%, and 41 or more 35%.
Do neutrals and grounds count for derating?
Equipment grounding conductors never count. A neutral carrying only unbalanced current in a normal multiwire circuit generally does not count, but a neutral carrying harmonic current from nonlinear loads does. This calculator uses whatever count you enter.
Why does the 90 °C rating not always help?
You may derate from the 90 °C column, but the final ampacity is still limited by the temperature rating of the terminations, which are commonly 75 °C. The higher insulation rating gives derating headroom, not a higher final ampacity.

Gear for derating checks

Gear for derating checks

Confirm ambient temperature and actual load.

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