12V Fuse Size Calculator

Work out a starting fuse size for a 12V or 24V DC circuit, and check it isn't bigger than your cable can carry.

The fuse protects the cable, not the device. This finds a standard fuse size above your load's normal current, then checks it against your cable's current rating. Everything is worked out in your browser. Nothing is saved or sent anywhere.

1. Your load

Use the highest current the device draws in normal use, from its label or manual. If the manual names a fuse size, use that instead.

Only used to turn watts into amps.
125% is a common starting point, so the fuse doesn't blow under normal load.

2. Your cable

Get the cable's current rating from the cable maker. It depends on the cable and how it's run, for example bundled, in conduit or in a hot engine bay.

Leave blank to skip the cable check.

3. Results

How it was worked out

    What this does and doesn't tell you

    • This gives a starting point, not a compliant circuit design. If the device's manual names a fuse, use that one.
    • The fuse must be above the load's normal current and no higher than the cable's current rating. If it can't be both, use a bigger cable.
    • Mount the fuse as close to the battery positive as you can, so as little cable as possible is left unprotected.
    • Use a fuse and holder rated for DC at your system voltage, with a cover. Devices with a high start-up current, such as inverters and compressors, may need the maker's recommended fuse type.
    • It doesn't check voltage drop. Use the 12V Voltage Drop Calculator or the Cable Size Estimator for that.
    • Work on 240V AC wiring, including inverter outputs and mains inlets, must be done by a licensed electrician. For high-current DC gear such as inverters, DC-DC chargers and battery links, follow the maker's installation manual and have an auto electrician check your work if you're unsure.

    Next steps

    Read the 12V Cable and Fuse Size Guide for the full method and a worked example. Check your cable with the 12V Cable Size Estimator and the 12V Voltage Drop Calculator.

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