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The wrong wire size and poor connections can lead to disaster.


Many DIY 12-volt systems fail because of inadequate wire sizing, poor quality connections and inadequate earthing. 




It shouldn’t need to be said that if you’re not sure about your ability to wire up your 4WD accessories or camper circuits you shouldn’t be doing such jobs: leave it to the automotive electrical experts.

In some cases, you may be able to do a fair amount of the labour-intensive stuff and then get the pros to do the final assembly and testing.

The common automotive direct current voltage is a nominal 12 volts and that was derived back in the good ol’ days, when car and truck electrical circuits were very basic. The 12-volt system is well out of its depth these days, but we’re stuck with it. (European and Japanese trucks adopted 24V many years ago, to handle heavier-duty work.)

Household voltage is a nominal 240V, alternating current, which is a much more potent and dangerous beast.

One of our electrical engineer mates explained the 12-volt-DC circuit system by comparing the flow of amps with low-pressure water flowing through a number of hoses, diverters and taps. 

It’s ‘low-pressure’, because 12-volts is a much ‘lower-pressure’ electrical system than a 240-volt one. Because the 12-volt potential is so weak, the system can’t tolerate any leaks or restrictions, without serious performance compromises or failure to flow.




If it were a low-pressure water system the 12-volt circuit would need a relatively large diameter hose, to overcome friction resistance from the hose walls. A thinner hose has more wall area for a given water volume and, hence, more friction resistance. It’s exactly the same with a 12V DC system: the fatter the wire; the less internal resistance.

The lesson here is: use the biggest wire diameter that you can fit and afford.

As with our water-pressure analogy, any restrictions cause a drop in current flow and, importantly, pressure. The electrical equivalent of that is voltage drop and many electrical accessories won’t operate well, or at all, at voltage below 12V.

You can calculate voltage drop in any 12-volt circuit with a simple formula:  Voltage drop = 2 x L x I x R ÷ A, where ‘L’ is the length of the wire in metres between battery and electrical load; ‘I’ is the current in amps; ‘R’ is the resistance of the wire (copper is 0.017) and ‘A’ is the cross-sectional area of the wire.

The voltage-drop answer is expressed in a decimal, say 0.58 volts, and if that’s acceptable, you’ve chosen the right cable length for that application. If that’s too much voltage drop, up the size of the cable.

Wire sizes are a mish-mash of rival measurement systems: AWG (American Wire Gauge), for some reason, rates small wires with higher numbers than fat wires. There are also the CSA (Conductor Surface Area) range and metric cable sizes – in square-millimetre cross-sectional area.


AWG  Actual CSA (mm²) Closest equivalent metric cable size (mm²)
0000 (4/0) 107.16 120.00
000 (3/0) 84.97 95.00
00 (2/0) 67.40 70.00
0 (1/0) 53.46 50.00
1 42.39 40.00
2 33.61 35.00
3 26.65 25.00
4 21.14 20.00
5 16.76 16.00
6 13.29 16.00
7 10.55 10.00
8 8.36 8.50
9 6.63 7.00
10 5.26 6.00
11 4.17 4.00
12 3.31 3.00
13 2.63 2.50
14 2.08 2.00
15 1.65 1.50
16 1.31 1.50
17 1.04 1.00
18 0.82 1.00
19 0.65 0.75
20 0.52 0.50
21 0.41 0.35
22 0.33 0.35


If the significance of these different wire diameters doesn’t mean something to you; don’t do your own wiring!

As with a water system, a 12V DC circuit needs free-flowing connections. A joint that’s too small becomes a restriction. With 12V wiring there’s no point having low-resistance wire and poor quality or ill-fitted crimps or soldered joints that restrict the flow of current. 



Cheap crimping tools are the cause of much bush-breakdown grief, because they don’t produce a tight connection that eliminates restrictions. Blade connectors can work loose on rough roads and reduce the effective contact surface area, so screw connectors are a better idea.

The water-flow analogy makes it easy to understand that a good positive (red-wire) output from the battery needs to be matched with an equally good negative return (black wire). There’s no point having good current flow to the fridge, for example, and poor flow back to the battery.

The system needs to operate in free-flow and will be only as efficient as its most restrictive point. 


Screw connectors beat blade types


The usual bottleneck in the black wire circuit is where a common ‘ground’ – usually the chassis – is used instead of a wire. all the way back to the battery. The earth connection to the chassis, near the accessory that’s being powered and the connection from the chassis to the battery – are highly likely sources of ‘leaks’. ‘Bad earths’ are responsible for many vehicle malfunctions. 

In-vehicle computer systems are particularly sensitive to poor connections, because they operate at such low current levels.

Our real world experience of poor-earth drama happened at Fitzroy Crossing in WA, when one of our convoy started OK after an overnight camp, but had all the dash lights remain on. The bush-mechanic’s diagnosis was alternator trouble, because we’d done river crossings the day before and LandCruiser V8s are infamous for wet-alternator trouble. 

Two days and a tilt-tray trip later, the real cause was diagnosed in Broome: a loose battery-to-earth connection! 

A crucial part of pre-trip maintenance is a checking wiring suitability and the integrity of every single electrical connection in your vehicle, trailer, van or camper.