Voltage drop calculator
Long or thin cables lose voltage. Check your DC cable run and find the right size.
Battery to the device. We allow for the return cable.
Voltage drop
0.30 V
As a percentage
2.5%
Voltage at the device
11.70 V
Good
Under the 2.5% we recommend for battery and DC wiring.
Smallest cable to keep the drop at 2.5% or less: 35 mm²
Shop cablesCheck the cable's current rating too
This calculator only works out voltage drop. A cable also has a maximum current it can safely carry, and that depends on how it's installed: in open air, enclosed in conduit or a cabinet, bunched with other cables, or in a hot space. Check the cable maker's rating for your install and fit the right fuse. If you're not sure, ask us or your electrician.
How we work it out
Voltage drop = 2 × current × one-way length × 0.017241 ÷ cable size (mm²). The 2 covers the positive and negative cables, and 0.017241 is the resistance of copper at 20 °C.
We aim for 2.5% or less, as Victron recommends for battery and DC wiring. AS/NZS 3000 sets a 10% maximum for extra-low-voltage installs. Hot cable has slightly more resistance, so allow some extra in engine bays and roof spaces.
Still have questions?
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