The formula
The factor of 2 accounts for the current flowing out on the positive cable and back on the negative. If your return path is a vehicle chassis, the drop on that side is usually smaller, but poor chassis connections often add more than you'd expect.
How much voltage drop is acceptable?
- 3% or less for critical loads: battery charging, DC-DC chargers, inverters, compressor fridges, navigation electronics. This matches the 3% figure in the US ABYC boat standard.
- Up to 10% for non-critical loads like lighting.
- On a 12 V system, 3% is only 0.36 V, which is why 12 V cables are so much thicker than mains cables for the same power.
Worked example
A 20 A fridge circuit, 5 m from the battery, in 6 mm² cable: 20 × 2 × 5 × 0.01724 ÷ 6 = 0.57 V, or 4.8% at 12 V. That's too much for a fridge, so step up to 10 mm² (2.9%).
Want the calculator to choose the size for you? Use the DC cable size calculator.
Current ratings are guide figures for single-core copper cable. 'Standard PVC' uses conservative values for 70 °C insulation, bundled or enclosed; '105 °C' uses typical marine-table values for high-temperature cable run on its own. Always check the cable maker's rating, and derate for hot spaces such as engine bays.
Frequently asked questions
What is an acceptable voltage drop for 12V?
3% (0.36 V) or less for chargers, inverters, fridges and electronics; up to 10% for lighting.
Does cable length include both wires?
Enter the one-way distance. The calculator doubles it to include the return conductor.
Does temperature affect voltage drop?
Yes. Copper resistance rises about 0.4% per °C, so a hot cable has slightly more drop than this 20 °C calculation shows.