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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 fusing and earthing. 



We’re all allowed to do 12-volt wiring work, but anything that’s 230V must be done by a licensed electrician. This clear distinction was set into law many years ago, before the advent of lightweight yet powerful lithium batteries, solar panels and inverters.

Yesterday’s caravans and super-flash camper trailers had 240V circuits – now 230V since an international agreement with the EU in 2000 – that worked only when the van was plugged into mains power at home or in a camp ground. There was sometimes a second power supply in the form of a small 12 volt lead-acid battery that powered an auxiliary light or two inside the van.

Those of us who camped off-grid in those days – we’re going back only as recently as the 1990s – had vehicles or camper trailers with no 240V circuitry at all. One or two lead-acid batteries powered a small portable fridge and were charged when the vehicle’s alternator was running. Longer stays than overnight camping meant powering up the petrol generator, to keep the fridge and lights working.

Then, in rapid succession, came solar panels, lithium batteries and on-board inverters. Solar panels could keep batteries topped up, to run camping power demands, provided the sun shone; lithium batteries offered around twice the usable power as a similarly-rated lead-acid battery, at around one third the weight; and an inverter could convert that considerable 12V battery power into 230V power.



Suddenly, you could have home-grade power when you were camped, without having to be on-grid.

What isn’t obvious to the average caravan or camper trailer owner is that the increase in electrical power makes today’s vans much trickier to work on than older ones.

The old law about anyone being able to wire up 12V circuits, but only licensed electricians being able to do 230V work hasn’t caught up with modern practice.

For example, it’s legal to buy a couple of 100 amp-hour lithium batteries and a 2000-watt inverter and install them yourself in your van or trailer. You’re legally able to wire the batteries and the 12V inverter input together, but you’re not legally entitled to wire the van from the inverter on its 230V side.

At OTA rethink that’s dangerous, because when that inverter is switched on, it draws sufficient current from the batteries to deliver 2000 watts of power on its 230V output side. The current on the 230V output side is only 8.7 amps, but between batteries and inverter it’s a whopping 160 amps or more. (You get the 2000W you need either through high-voltage/low current, or low-voltage/high-current means.)

Calculating the correct wiring and fuse capacity for that connection is a far cry from wiring the old van for a 12-volt globe or two – especially if the inverter 12V power wire feed has to pass through a metal bulkhead.



Ever seen a fire caused by high-current 12V DC wiring chafing on an inadequately-grommeted bulkhead hole?

What makes the potential fire situation even worse is that most vans and camper trailers have gas supply lines in close proximity to electrical wiring.

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 230V, 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 230-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.




























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