Wire harness current capacity calculator

Estimate the safe current-carrying capacity of wires bundled in a harness: enter the gauge, number of conductors and ambient temperature, and read the derated capacity.

Harness conditions

Wire size

Base ampacity auto-fills below

Base capacity (free air)

Single-conductor rating for the chosen gauge

Current-carrying conductors in bundle

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

Ambient temperature

NEC 310.15(B)(1) ambient factor

Insulation temperature rating

Your load current (optional)

Compare against the derated capacity below

Derated capacity

Base capacity (18 AWG, 30 °C): 16A
Bundling factor: 0.8(4–6 conductors)
Ambient factor: 0.91(30 °C → 40 °C, 90 °C insulation)
Derated capacity: 11.648A

The derated value is what the wire may actually carry in your bundle. Enter your load current to check it against this result.

Heat has nowhere to escape in a bundle, which is why NEC 310.15 derates grouped conductors. In tight automotive harnesses, engineers usually apply a further margin beyond the derated value for conduit and routing.

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Frequently asked questions

How is the safe current of bundled wires calculated?

The calculator follows NEC 310.15: derated = base ampacity × bundling factor × ambient factor. Base ampacity is the single-conductor rating (e.g. 16 A for 18 AWG copper with 90 °C insulation), the bundling factor is NEC 310.15(B)(3)(a) and the ambient factor is NEC 310.15(B)(1).

How much does bundling derate the current capacity of wires?

Heat has nowhere to escape in a bundle, so each current-carrying conductor cuts the usable capacity: 4–6 conductors drop to 80%, 7–9 to 70%, 10–20 to 50% and 21–30 to 45%. With the calculator's defaults — 18 AWG, 4–6 conductors at 40 °C with 90 °C insulation — the 16 A base becomes about 11.6 A.

Do I need to derate for ambient temperature in a harness?

Yes — the ambient factor is applied on top of bundling. For 90 °C insulation it is 0.91 at 40 °C, 0.82 at 50 °C and 0.71 at 60 °C (75 °C insulation derates slightly harder). In tight automotive harnesses engineers usually add a further margin beyond the derated value for conduit and routing.

How it works

IC Source Direct follows NEC 310.15, the same derating rules used to size bundled conductors in panel and machine wiring. Heat has nowhere to escape in a bundle, so each additional current-carrying conductor reduces the usable capacity. The calculator multiplies the base ampacity by a bundling factor (310.15(B)(3)(a)) and an ambient-temperature factor (310.15(B)(1)) to give the current the wire may actually carry. In tight automotive harnesses, engineers usually add a further margin on top of the derated value for conduit and routing.

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IC Source Direct provides this tool for reference only.

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