Cable ampacity derating calculator

Derate a cable’s current rating for real installation conditions — ambient temperature above 30 °C and bundling with other current-carrying conductors.

Installation conditions

Base ampacity at 30 °C

The catalog current rating of the cable

Ambient temperature

90 °C insulation table — NEC 310.15(B)(1)

Bundled current-carrying conductors

NEC 310.15(B)(3)(a) adjustment factor

Derated ampacity

Derated ampacity: 29.1A
Ambient factor: 0.91(30 °C → 40 °C)
Bundling factor: 0.80(4–6 conductors)
Effective rating: 73%

The derated value is what the wire may actually carry in your installation. If it falls below your load, pick the next larger gauge — a 12 AWG rated at 30 A is the next reference size above this result.

Common 90 °C copper ampacity

Nominal ampacity at 30 °C ambient, single conductor (NEC Table 310.16 style).

Conductor	Ampacity at 30 °C
14 AWG	25 A
12 AWG	30 A
10 AWG	40 A
8 AWG	55 A
6 AWG	75 A
4 AWG	95 A
2 AWG	130 A
1/0	170 A
2/0	195 A
4/0	260 A

These are reference values for copper with 90 °C insulation. Always size from the actual manufacturer ampacity tables in the latest edition of your local code.

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Frequently asked questions

Why must cable ampacity be derated?

Published ampacity assumes a single conductor in 30 °C ambient air. Bundled conductors trap heat and warmer air raises the conductor temperature — NEC 310.15 applies two multipliers (ambient + bundling) so the insulation stays within its temperature rating.

How much do bundled conductors reduce capacity?

NEC 310.15(B)(3)(a) derates bundles to 80% for 4–6 conductors, 70% for 7–9, 50% for 10–20 and 45% for 21–30 current-carrying conductors. A 12-conductor harness at 40 °C can run a 90 °C-rated 40 A conductor at only ~18 A.

Should I derate for ambient temperature too?

Yes — for 90 °C insulation the factor is 0.91 at 40 °C, 0.82 at 50 °C and 0.71 at 60 °C. Apply ambient and bundling factors together: derated = base × ambient × bundle.

How it works

NEC 310.15 applies two multipliers to a cable’s base ampacity (the rating published at 30 °C ambient with a single conductor): derated = base × ambient factor × bundling factor. The ambient factor drops as the air temperature around the cable rises, and the bundling factor drops as more current-carrying conductors are packed together, because heat has less room to escape. The result is the current the wire may actually carry in your installation. IC Source Direct publishes these multipliers as a quick reference — always confirm against the latest local electrical code before specifying a conductor.

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IC Source Direct provides this tool for reference only.

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