Cable sizing and voltage drop

Running a compressor fridge from a 12v leisure battery is straightforward, but the cable connecting them matters more than most people expect. Use a cable that is too thin or a run that is too long, and the fridge receives less voltage than it needs, works harder, performs worse, and is at greater risk of damage.

What voltage drop means

Voltage drop is the loss of voltage that occurs as current flows through the resistance of a cable. Every cable has resistance, and longer cables with smaller cross-sections have more of it. The result is that the voltage arriving at the fridge is lower than the voltage at the battery.

A fridge designed to run at 12v that receives 11v instead will draw higher current to compensate, generate more heat, and potentially trigger low-voltage protection or simply underperform. The effect is most noticeable under higher current draw, exactly the conditions a compressor fridge creates during startup.

How to get it right

The solution is to use correctly sized cable for the run length and the current being drawn. Shorter runs can use thinner cable. Longer runs, or higher current draws, need larger cable to keep voltage drop within acceptable limits.

As a general guide, use at minimum 2.5mm2 cable for runs up to around 3 metres, 4mm2 for runs up to 5 metres, and 6mm2 for longer runs. These are starting points; always check the fridge manufacturer's recommendations and use a proper voltage drop calculation for the specific current and run length involved.

Avoid running a compressor fridge from a cigarette lighter or 12v socket for extended use. These circuits are typically fused at 10 to 15 amps and the connectors are not designed for sustained high-current use. A direct connection to the leisure battery with correct cable and an appropriate fuse is the right approach.