The right cable size depends on the current it carries, the length of the run and the system voltage. Low-voltage DC systems need much thicker cable than higher-voltage ones for the same power, because voltage drop matters more at 12V. As a rule, keep voltage drop under about 3 per cent on power circuits and size battery-to-inverter runs generously.
Undersized cable is one of the most common and most dangerous mistakes in off-grid wiring. Too thin, and the cable overheats, wastes energy and can become a fire risk. Too thick, and you have spent money and made the install harder than it needs to be. Here is how to get it right.
The three things that decide cable size
- Current (amps): how much current the cable carries. Work it out from power and voltage: Amps = Watts / Volts.
- Run length (metres): the total there-and-back length of the cable, not just the one-way distance. Longer runs drop more voltage.
- System voltage: 12V, 24V or 48V. Lower voltage is far less forgiving of voltage drop.
Voltage drop: the figure that matters most
Voltage drop is the voltage lost to the cable's own resistance. A little is unavoidable, but too much starves your appliances. The usual targets are under 3 per cent for general power circuits and under 1 to 2 per cent for the run between your solar panels and charge controller, where every volt counts.
Voltage drop rises with current and length, and falls as you increase the cable's cross-sectional area (measured in mm squared). That is the trade-off the calculation balances.
Worked example
Say you are running a 12V circuit drawing 30A over a 5 metre one-way run (10 metres round trip). At 12V, holding voltage drop under 3 per cent (about 0.36V) needs a substantial cable, in the region of 16mm squared. Move the same 360W load to a 48V system and the current falls to just 7.5A, so a far thinner, cheaper cable does the job with less loss. That is the low-voltage penalty in action.
The high-current runs to watch
Battery to inverter
This carries the most current in the system. A 2000W inverter on a 12V bank can pull over 190A under load. That demands very thick, short cable and a correctly rated fuse close to the battery terminal.
Panels to charge controller
Keep voltage drop here especially low so you do not lose harvest. Wiring panels in series raises voltage and cuts current, which allows thinner cable over long roof or ground runs.
12V vs 24V vs 48V
If you are planning anything beyond a small system, higher voltage saves cable cost and cuts losses. Many van and cabin builds use 12V for appliance compatibility, while larger cabins and home systems increasingly use 48V precisely because the wiring is thinner, cheaper and safer for the same power.
Rather than work the maths by hand, enter your current, length and voltage into our free cable size calculator for the minimum safe cross-section in seconds.
