12V Install Tools & Testers

12V Cable Size Estimator (Voltage Drop Only)

Find the smallest listed copper size that keeps voltage drop under your target. It checks voltage drop only, not whether the cable can safely carry the current.

Voltage drop only. This is not a cable selection. The result is the smallest size that meets your voltage drop target under simplified assumptions. You still need to check the cable's current rating, the fuse and the installation, as explained below.

Enter your system voltage, current, one-way length and voltage drop target. Everything is worked out in your browser. Nothing is saved or sent anywhere.

1. Your cable run

The highest current the device draws. Watts ÷ volts gives amps.
Battery to device. The calculator doubles it for the return path.
Hot copper has more resistance. This is an approximate adjustment.
Around 3% is a common target. Many chargers and fridges need less, so follow their manual.

2. Results

How it was worked out

    Check a different cable size in the Voltage Drop Calculator →

    Before you buy cable

    • This is an educational estimate of voltage drop only, using an approximate copper resistance. It is not a compliant cable design and doesn't implement the Australian cable selection standard, AS/NZS 3008.1.1.
    • It does not check the cable's current rating, which depends on the cable and how it's run (bundling, insulation, heat, engine bays). Get that from the cable maker.
    • It does not check fuses, circuit breakers, terminals, lugs or short-circuit protection. The fuse protects the cable, so it must suit the cable you choose. See our 12V Cable and Fuse Size Guide.
    • Auto cable is often sold by its overall size (for example "6mm" or "8 B&S"), which isn't the copper area inside. Use the conductor area in mm² from the spec sheet.
    • Follow your equipment's manual where it gives a cable size or voltage drop limit. Chargers and fridges often need a tighter limit.
    • For low-voltage DC only. 240V AC wiring must be designed and installed by a licensed electrician. For a vehicle or caravan installation, check the Australian requirements that apply and have it checked by a qualified auto electrician.

    Next steps

    Check the cable's current rating with its maker, then choose the fuse using our 12V Cable and Fuse Size Guide. For inverter cables, see the cable, fuse and installation section of our Camping Inverter Guide. For a DC-DC charger, our DC-DC Charger Size Calculator estimates its input current.

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