Thread: What thickness wire

6B&S

Voltage drop in automotive 12V DC circuitry is definitely an issue if the wrong size wiring is used.

The size of the wiring needed depends on the length of the run (i.e. the distance from the battery to whatever is being powered and the length of the earth return) and the current being supplied.

Here's a useful link. Voltage Drop and Cable Size Calculator

So, the positive lead from the battery to the starter/alternator is very thick because the currents being carried are high. Winch cabling is very thick for the same reason. On the other hand, a wire supplying current to an led instrument light can be very thin as the current will be only a fraction of an amp.

If you're going to run a 12V DC supply to a bank of fuses, that wire is likely to need to be thicker than any of the wires running from the individual fuses to whatever they're powering.

When running cable to your dual battery set-up, it's important to remember that your battery will not charge fully if there is too great a voltage drop between it and your cranking battery. So, do not skimp on this cable - it will prove to be false economy.

On earthing, if you use the car body as an earth, this would usually be considered as offering no resistance to current flow and hence you would only take the length of the positive wire from the battery to whatever you're powering when working out what size wire to use. However, there are times when this can bring you undone if the part of the body you choose to use as an earth has a poor earth connection to the rest of the vehicle.

The following wire size information might be useful :-

1 B&S = 45 sqr mm
2 B&S = 32.5 sqr mm
3 B&S = 26 sqr mm
6 B&S = 15 sqr mm
8 B&S = 8 sqr mm

what i did was go down to super cheap and got a set of jumper leads and just took the clips off , i think it was $9 for 2.5 meters long of red and black wire

for overkill battery connections, go to a welding specialist and get 50 mm2 welding cable (equivalent to 0ga). You'll need to get your hands on a big crimping tool to add end terminals. You can go smaller sizes but for the extra cost overkill is a good thing. I've found welding shops to be the cheapest for 50mm2 wire. Heatshrink for this size wire isn't the cheapest but again is very worthwhile. Welding cable is more flexible and has double insulation.

Wire back to the switches depends on current draw: see mellow yellow's post above.

First of all you need to ask yourself what you are going to do with the second battery - If you ask people what size cable you need and they assume a different use, you will get different answers!

Assuming that you want the ability to link the two batteries and start the vehicle from the second then you will need to use heafty cable. In a Defender where the batteries are next to each other you should use 35mm^2 cable. If the batteries are some distance appart then use 50mm^2 to cater for voltage drop.

If on the other hand you only want to run a fridge and some other ancillary devices then you can probably reduce the cable sizes that link the two together.

Dont forget to consider the maximum current draw of your redarc unit. Im not familiar with that particular system but I know some split charge systems will only handle 25A or so. I use a Swiss made IBS unit and that has a 200A relay to handle heavy current flow between batteries.

Earthing.
NEVER use the vehcile chassis as an earthing route for your auxillary circuits. Sharing the chassis for both batteries always leads to problems and earth leakage currents. The number of customers we had in the UK that did this was amazing!
Always link both battery eaarth terminals directly together with the same rated cable as you link the +12v with.

Cable Size:
Im not entirely convinced the above referenced cable size calculator is accurate. It certainly gives you cable on the heavy side. Bear in mind you can get "normal" cable and also nowadays (becoming much more available) "thin walled cable". The thin walled stuff is essentially the same as normal cable but the qty of strands are greater and each one is smaller in diameter. The difference I guess is not important, the real benefit though is that the same diameter cable can carry more current and is also more flexible - Ideal for 12V automotive applications!

Hope that helps!

Jon

Thanks for the advice guys,

I will not be using the second battery as a jump start source, but will still run decent cable between the batteries.
In the back of the car I will be running a fridge, lights, water pump and possible an inverter on the odd time.
The redarc unit can handle 100A constant and 400A max so it will do just fine.
I am thinking now I will run 1 twin core decent sized cable for a dedicated fridge power and another bigger twin core cable to the back to a 4 port fuse box and add plugs off that as i need to in the future.

Andrew

Hi,
I read some where that the wire gauge is a tad too big if you have to upgrade your suspension.
cheers

Hi Andrew, a good rule of thumb is if the two batteries are under the bonnet, 8B&S ( 7.9mm2 ) will be fine.

When the auxiliary battery is to be mounted anywhere else, like in the rear or in a caravan or camper trailer, then 6B&S ( 13.5mm2 )

If you have both batteries under the bonnet and you need to jump start, a single positive jumper lead is all you need.

For your power in the back, simply run a length of twin 8B&S to the rear and then split the cable at the rear and use thinner cable from there.

BTW, if you want to cater for the possibility that you may need to jump start from the auxiliary battery at some time, then just use 6B&S.

6B&S cable has a continuos rate of 90 amps when used in a dual battery set up, which is heaps more than you are ever likely to need and is rated at 250 to 300 amps when used as starter motor cable.

if you want to use thicker cable, go for broke but there is little to no advantage, just a bigger hole in your pocket.