12V Cable Size & Voltage Drop Calculator
The right cable for a circuit has to pass two tests. It must carry the current without overheating ‐ its current rating ‐ and it must deliver the power without losing too much voltage along the run ‐ the voltage drop. Too much drop and winches run slowly, lights dim, and fridges or chargers cut out. Enter your circuit details below and the calculator checks both tests at once.
Nominal system voltage ‐ double it for a 24 V system.
The cable run from the power source to the accessory, along the route the cable actually takes. The return run is counted automatically.
Technical details
Cable resistance per metre: ‐ Ω
Total circuit resistance (incl. return path): ‐ Ω
Continuous current rating of selected cable: ‐ A
Based on a copper cable resistance of 0.0185 Ω·mm²/m. Current ratings are for a single thin-wall cable (flexible PVC battery cable in the larger sizes) in free air ‐ derate for bundled looms, trunking or hot engine bays. Heating effects on resistance are not accounted for.
A maximum voltage drop of around 3‐4% is generally acceptable. This calculator doubles the one-way length to allow for the return circuit, giving a worst-case figure to work with.
Cable Sizing Conversion Table
Metric cable is sized by conductor cross section in mm²; American Wire Gauge (AWG) is the US equivalent you'll see on imported kit. This table shows the nearest AWG size for each metric size.
Nearest AWG size by conductor area ‐ the match is never exact, so for critical circuits size up.