Battery cable size calculator

Enter your current, the length of the run and your system voltage. Your cable has to pass two checks: voltage drop (is it thick enough for the length?) and heat (can it carry the fuse rating without overheating?). Whichever check needs the bigger cable sets your size.

Max current
A

The most the cable carries for more than a few minutes. For an inverter, use its rated watts ÷ the low battery voltage.

Length of the run, one way

Battery to the device, following the route. We double it for the round trip (positive and negative).

System voltage
Allowed voltage drop

Critical: battery to inverter or charger, main feeders, electronics. Non-critical: general lighting and appliances.

Cable insulation rating

Printed on the cable jacket. Not sure? Pick 90 °C.

Where it runs

Count the current-carrying cables tied together or in the same conduit. Bundled cables can't shed heat as well.

Fuse or breaker optional
A

Leave blank and we use the next standard size at or above 125 % of your current. The cable must be able to carry the fuse rating.

Next steps

All sizes for your run
Sizemm²CarriesDrop at your current

"Carries" is the allowable current for your insulation rating, bundling and hot space. Copper, stranded.

How we work this out

Voltage drop. The cable's resistance turns some of your voltage into heat. We find the smallest size whose cross-section (in circular mils) is at least 10.75 × current × round-trip feet ÷ allowed drop in volts. 10.75 is the resistance of copper per circular mil-foot, the same basis as the common marine wire charts.

Heat. The allowable current for each size comes from the US Coast Guard conductor table (33 CFR 183.425, Table 5): stranded copper in free air at 30 °C, by insulation rating. We reduce it for bundling (×0.70 for 3 cables, ×0.60 for 4 to 6, ×0.50 for 7 or more) and for a hot space (×0.85 at 105 °C down to ×0.58 at 60 °C), as the table's notes say. The result must be at least your fuse rating, so the fuse protects the cable.

Your size. The cable has to pass both checks, so the larger of the two sizes is your minimum. Going one size up costs a little more and runs cooler.

Cable size chart

Each block is the minimum size that passes both checks, with the fuse at 125 % of the current. Your numbers are outlined. Change the voltage, drop, insulation or bundling above and the chart follows.

Disclaimer: For reference only. Do proper engineering to determine your requirements and always follow applicable code (NEC, ABYC or your local code).
Assumes copper cable and clean, properly crimped and torqued lugs.