Voltage Drop & Cable Size Calculator
Undersized cable is the most common reason a lithium system underperforms. Pop in your system voltage, current and cable run and this calculator tells you how much voltage you'll lose, and what size cable you actually need. It works for 12V, 24V and 48V DC systems in vans, boats, cabins and homes.
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.
What is voltage drop?
Every cable has a little resistance, so some voltage is lost between the battery and whatever you're powering. Lose too much and inverters trip on surge, chargers undercharge your battery, and fridges and pumps struggle. The fix is cable that's big enough for the current and the length of the run.
How much voltage drop is OK?
- Battery to inverter: 1 to 2%. Inverters pull big current on surge, and this is where undersized cable causes the most head scratching.
- Charging circuits (alternator, DC to DC, MPPT, mains charger): 2% or better, or the charger thinks the battery is fuller than it is.
- Lighting and small loads: 3 to 5% is fine.
- Solar panels to MPPT: panel voltage is high, so a couple of percent is neither here nor there.
The calculator's suggested size aims 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.
Working it out by hand
Cable size (mm²) = 0.0172 × round trip length (m) × current (A) ÷ allowed voltage drop (V)
0.0172 is the resistance of copper. Example: a 50A load, 4 metres from a 12.8V lithium battery (8 m round trip), allowing 3% (0.38V). 0.0172 × 8 × 50 ÷ 0.38 = 18.1 mm², so round up to 25 mm². Then check 25 mm² can carry 50A comfortably, which it can.
Watch out for
- Copper clad aluminium (CCA): cheap cable sold as copper with roughly 60% of the conductivity. Your 50 mm² behaves like 30 mm². Our silicone insulated cable and lugs are tinned copper.
- Bundled or hot cable: runs in conduit, bundles or engine bays carry less current, so knock 20 to 30% off the rating.
- Round trip: always count the length of both the positive and negative cable.
Get the right cable
Common questions
Around 170A at full load. For a short run of 1 to 2 metres that's usually 70 to 95 mm²; use the calculator with your actual length.
Yes. The same power at 48V is a quarter of the current at 12V, so cable, fuses and lugs all get smaller.
A common target of no more than 3% voltage loss on a DC circuit. Tighter for inverters and chargers, looser for lights.
No, it's for DC battery systems. Fixed AC wiring in NZ must be done by a registered electrician.
Still have questions?
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