Cable Size & Circuit Protection Calculator

Undersized cable is one of the most common mistakes in 12V, 24V and 48V DC systems — and one of the most dangerous. Too-thin cable causes excessive voltage drop, wasted power, and heat build-up that can damage equipment or create a fire risk. Use this calculator to find the right cable size, fuse rating and expected voltage drop for your setup.

 

Enter your voltage, load and cable run length, and the calculator will recommend a cable size that keeps voltage drop within safe limits while ensuring your circuit protection is correctly matched.

 

Load type

Continuous watt rating of your inverter

Continuous current draw of the DC load

One way — voltage drop uses round trip (2×)

3% recommended for most installs

NB. When calculating DC-DC charger input cable sizes, increase input load current by 10% compared to output charge current.

Max current

Recommended fuse

Next standard size above max current

Recommended cable

Cable comparison

Showing recommended cable plus 2 sizes smaller and 2 larger. Voltage drop calculated at max current × 1.25 safety factor. Ampacity checked against fuse size (the fuse protects the cable).

Cable Area (mm²) Rated amps Est. V drop Est. % drop Passes
Assumptions: Copper cable only. Resistance calculated at 0.01724 Ω·mm²/m (20°C). Ampacity ratings are a general guide — derate for bundled cables, high ambient temps, or runs inside conduit. Fuse protects the cable, not the load — always fit fuse as close to the power source as practical. Results are a guide only; verify by dropping us a line, with a qualified auto electrician or to AS/NZS 3001 for caravan and marine installations.

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