Cable

They’re two different ways of naming cable size, and only B&S tells you how much copper is inside.

B&S (Brown & Sharpe) is the same system as the American AWG gauge. Each number means a fixed amount of copper, so 6 B&S is about 13.5mm² whoever makes it. The numbers run backwards: the smaller the number, the thicker the cable. Past 1 B&S it goes to 0, 00, 000 and so on, getting thicker each time.

“mm” auto cable (3mm, 4mm, 5mm, 6mm) is an Australian trade name. It roughly describes how big the cable is including its insulation, not the copper inside. So 6mm auto cable has only about 4.6mm² of copper – roughly a third of 6 B&S – and the copper can vary between brands.

The true measure is copper area in mm², which is what our calculator works from:

Watch out: 6mm auto cable and 6 B&S are very different. If you need 6 B&S for a DC-DC charger, 6mm auto cable has only a third of the copper.

Measure one way, from the battery to the device, along the path the cable will actually take – not in a straight line. The calculator doubles it for the return (negative) cable. Add a little extra for bends, and round up to the next length we sell.

Either. Most devices list one or the other on a label or in the manual. If you only know watts, the calculator works out amps for you (amps = watts ÷ volts). For a DC-DC charger, enter its rated amps, e.g. 40A.

A DC-DC charger boosts its own output, so a bit of voltage lost on the input cable doesn’t stop your second battery charging at full voltage. It just means slightly more current drawn from the vehicle. That’s why a 40A charger on a typical 10m caravan run is fine on 6 B&S. On very long runs, or with a smart alternator, going a size up gives extra margin.

Yes. Current flows through both, so the negative needs to be just as heavy as the positive. Undersizing the negative is one of the most common reasons a setup underperforms. For caravans and trailers, run a dedicated negative rather than relying on the chassis or tow ball.

Dual core (twin core) has the positive and negative in one sheath – neater and tougher for long runs like the back of a ute or to an Anderson plug. Single core gives you separate red and black cables, which is easier to route in tight spots and is the usual choice for heavier sizes and battery-to-battery connections.

Always. A thicker cable has less voltage drop and runs cooler – the only downsides are cost, weight and fitting it into terminals. Never go smaller than the recommended size.

Fit a fuse or circuit breaker on the positive cable as close to the battery as possible. It should be rated above your device’s normal draw but no higher than the cable can safely carry – the fuse is there to protect the cable. If both ends of the cable connect to a battery, fuse both ends. Check your device’s manual, as many specify the fuse size.

Yes – choose the length you need and we cut it for you. Shipping on cable is free Australia-wide, or pick it up from our Geelong or Melton store. Need lugs, Anderson plugs or clips fitted? Use our Cable Builder or contact us.