12V Install Tools & Testers

12V Voltage Drop Calculator

Estimate how much voltage you lose along a DC copper cable run, and what voltage reaches your device.

Enter your system voltage, the current, the one-way cable length and the copper size, and this estimates the voltage drop and power lost in the cable. It's an educational estimate for low-voltage DC runs, not a cable selection. 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.
The copper area from the cable spec sheet, not the auto cable size name.

2. Results

How it was worked out

    Find the smallest copper size for a voltage drop target with these figures →

    Limits of this calculator

    • 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

    Work through the full method, including the fuse, in our 12V Cable and Fuse Size Guide, or see how cabling fits into a whole setup in the wiring and electrical protection section of our Build Your System guide. Wiring a dual battery? See our Dual Battery Wiring Diagram.

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