That DC cable might look thick enough, but how can you actually tell?

This is the most common place people make a mistake, stopping your system from working properly. Even the professionals can get it wrong.

What a “big enough” cable is actually doing

Take a 12V system pulling 100A, with 3 metres of cable from the battery to the inverter. On 16mm² that looks like plenty of copper.

It looks chunky, it’s the size the last person used, and it’s quietly costing you every time the inverter is loaded.

Here’s what it’s actually doing:

12V, 100A, 3 metre run, 16mm² cable

  • Lost in the cable: 0.65 V
  • Arriving at the inverter: 11.36 V
  • Burnt as heat in the cable: 64 W

Work out your cable run

That 64W of loss is on the whole time the inverter is working. It comes straight off your battery voltage, it just warms up the cable, and causes your inverter to cut out.

Why the rule of thumb fails once the cable is hot

There’s a second more important problem with that run and it’s the one most people miss.

The common rule of thumb is 10 amps per mm² for copper at 20°C, which makes 16mm² a 160A cable. Once that cable is hot and sitting at 70°C the safe figure is closer to 5 A/mm², so it’s really an 80A max cable. At 100A you’re past it, and that isn’t inefficiency any more, you’ve hit the start of thermal runaway.

Unsafe and ineffective are two different problems, and the same cable can have both.

Why there is no single rule for cable size

I’ve put a calculator on the site. Put in your system voltage, your current, the one way run length, how many pairs you’re running and whether the cable is cold or hot and loaded. And most importantly, you tell it what that part of the circuit is doing. You won’t find that anywhere else.

Different parts of your circuit have different voltage loss sensitivities. There is not one global rule to rule them all. Some connections are highly voltage-sensitive, and some are very tolerant of large amounts of voltage loss. Use the selector on the calculator to choose what that part of your system is doing, whether that’s powering a winch motor or charging a battery.

It gives you what you lose, what arrives, and what it’s costing you in watts. It also lays out every conductor size from 1.5mm² to 180mm² with the AWG equivalent, so you can see what the next size up actually buys you before you pay for it.

If it’s a whole system rather than one run, contact us and we’ll size it for you.