DC cable size calculator

The right cable for a battery, inverter or solar panels at 12, 24 or 48 V, and the fuse to go with it.

ULIPF
Letters on the drawing match the letters next to the inputs.
V
W
m
%
Cable cross-section35 mm²

Shopping list

  • Cable, both wires6 m · 35 mm²
  • Fuse125 A

How we got there

Current
83.3 A
Minimum for the voltage drop
24.31 mm²
Minimum to carry the fuse current
35 mm²
Voltage drop
0.25 V
Voltage drop, share
2.1 %
Power lost in the cable
21 W

Results are estimates. Always check the consumption printed on the product you buy.

Calculated on planicalc.com · https://planicalc.com/en/dc-cable-size-calculator/

How it is calculated

At low voltage the current is high, and even a short cable loses a noticeable part of the voltage. The calculator checks two things and takes the larger cable. First the voltage drop: the current flows through both wires, so the length counts twice. Second the heating: the fuse is chosen at 1.25 times the working current, and the cable must carry the fuse current continuously. The result is rounded up to a standard size.

FormulaCurrent I = power ÷ voltageCross-section (mm²) = 2 × length × I × 0.0175 ÷ allowed drop (V)Fuse ≥ 1.25 × I; cable must carry the fuse currentVoltage drop = 2 × length × I × 0.0175 ÷ cross-section

Example

A 1,000 W inverter at 12 V draws 83 A. With 3 m of cable to the battery and 3 % allowed drop (0.36 V) the drop needs 24.3 mm². The fuse is 125 A, and a cable that carries 125 A in a wall conduit is 35 mm², so choose 35 mm²: the drop is then 0.25 V (2.1 %). At 24 V the same inverter needs only 16 mm², at 48 V 4 mm².

Tips from practice

  • For solar panels and battery-to-inverter cables keep the drop at 1–3 %; for lighting 3 % is the usual limit, and for less sensitive loads up to 5 % is fine.
  • Use flexible copper cable with crimped lugs that match the cross-section, and tighten the terminals to the torque the maker gives. A loose lug heats up.
  • The fuse protects the cable, so it goes at the start of the cable, as close to the battery as possible.
  • Work on batteries and solar systems can be dangerous. The result is a guide: let a qualified electrician check the installation.

Questions and answers

What size cable for a 2,000 W inverter at 12 V?

It draws about 170 A. With 2 m of cable that needs a 250 A fuse and about 95 mm² copper cable in a conduit (70 mm² can be enough in free air if the maker allows). At 24 V the current halves, so 35–50 mm² is enough.

Why does the length count twice?

Current goes to the device along one wire and comes back along the other. Both wires lose voltage, so the total length of the circuit is twice the distance.

What voltage drop is acceptable?

For DC systems 3 % is a common limit. Between solar panels and the charge controller, and between battery and inverter, aim for 1–2 % to lose less energy.

Found a mistake in a formula? Write to [email protected]