12V electrical system in your camper van: getting started

A complete 12V camper van system consists of these components:

1. Battery bank: stores the energy. Lithium (LiFePO4) or AGM (absorbent glass mat, a sealed lead battery).
2. Charge controller (MPPT, maximum power point tracking): converts solar panel voltage to the correct charging voltage.
3. B2B (battery to battery, a charger between the starter and house battery) charger: charges the leisure battery via the starter battery while driving.
4. Shore power charger: charges the battery when on a campsite.
5. Inverter: converts 12V DC to 230V AC.
6. Fuse box: distributes and protects all outgoing circuits.
7. Switches: for lighting, water pump, etc.
8. Wiring: connects everything with the correct gauge.

The core of your system is the battery. All charging sources (solar panels via MPPT, B2B charger, shore power charger) come together on the positive and negative terminals of the battery.

From the battery, a thick cable goes to the fuse box. From there, separate circuits go to your consumers, each protected by its own fuse.

The connection:Battery (+) → main fuse → busbar (+) → fuse box → consumersBattery (-) → busbar (-) → ground connection consumers

Keep the cables between battery and fuse box as short as possible (max 1-1.5 meters) and use the correct gauge. A cable that is too thin is the biggest safety risk.

A fuse protects the cable, not the device. If a short circuit occurs or the appliance draws too much current, the fuse blows before the cable overheats. So you pick the rating to suit the cable hanging off it, not the power of whatever sits at the far end.

How much a copper conductor can carry depends on its cross-section, on the temperature and on how many cables share a bundle. This is what is practically left for a cable running behind the panelling in a bundle of three, at 40 degrees:

• 1.5 mm²: around 19 A
• 2.5 mm²: around 23 A
• 4 mm²: around 31 A
• 6 mm²: around 42 A
• 10 mm²: around 54 A
• 16 mm²: around 77 A

Run that same cable loose and cool and it carries more: 1.5 mm² reaches 25 A and 6 mm² reaches 55 A. That quarter of a difference is exactly why a fixed table lets you down. The fuse calculator asks for the temperature and the number of cables in the bundle and works it out for your circuit.

The fuse goes at or below that current, and the cable also has to be thick enough to keep the voltage drop in check. On long runs it is usually that second requirement that decides.

For the main cable between battery and fuse box, use an ANL or MEGA fuse, typically 100 to 200 A depending on the cable gauge.

Always place the fuse as close as possible to the positive terminal of the battery. If the cable shorts further along, it is protected from the first moment.

The five most common mistakes in 12V installations:

1. No fuse on the battery cable: if this cable short-circuits, the full battery current (hundreds of amps) can flow through the cable. This causes fire.
2. Cables too thin: especially cables to the inverter are often underestimated. A 1000 W inverter draws around 100 A at 12 V, because its efficiency of roughly 85 percent counts too. Over 2 metres that means 25 mm², over 3 metres 35 mm². On 24 V the same inverter draws half as much and 6 mm² is enough.
3. Poor connections: loose lugs or badly crimped connections cause resistance, heat and eventually fire. Use a proper crimping tool and check every connection.
4. No main disconnect switch: you must be able to fully shut down the system in emergencies. Install a main switch on the battery cable.
5. Chassis ground: in a camper van you should use a return cable, not the chassis as ground. The chassis is not designed for the currents of a living system.

Most people size their cable on the current going through it. In a camper that is usually not the deciding factor, because 12 volts is not much and every tenth of a volt lost on the way counts.

An example from the cable calculator, at its default setting of 20 degrees. A circuit of 10 A over 4 metres:

• On heat alone 1.5 mm² would be plenty, since that conductor handles some 22 A
• But the voltage drop is then 7.9 percent, and less than 11 volts arrives
• To stay under the usual 3 percent you need 4 mm²

The same 10 A over 8 metres already calls for 10 mm², because 1.5 mm² would lose almost 16 percent there. And 20 A over 5 metres also comes out at 10 mm², while 4 mm² would be plenty on ampacity alone.

Why that 3 percent? Below 11.5 volts many appliances start to complain: a water pump runs slower, an inverter shuts down, LED lighting dims visibly. At 3 percent you keep enough margin for a battery that is already half empty.

Always work with the one-way length; the calculator adds the return conductor itself.

Most self-builders choose 12 volts because almost every camper part is available for it. As soon as a larger inverter goes in, that choice gets expensive.

Look at what the same inverter costs in cable:

• 1000 W at 12 V draws around 100 A and calls for 25 mm² over 2 metres
• 1000 W at 24 V draws half that and is done with 6 mm²
• 2000 W at 12 V draws almost 200 A and calls for 50 mm²
• 2000 W at 24 V calls for 16 mm²

From about 2000 W upwards, 12 volts becomes awkward: 50 mm² cable does not bend round a corner easily, the lugs get large and expensive, and so does a fuse in that class.

What 24 volts costs you: less choice in small appliances, and a converter down to 12 V for the parts that need it. Our converter calculator works out how heavy that has to be.

If you are building a camper with induction cooking, air conditioning or a water heater, 24 volts is nearly always the wiser start. If you stay with lighting, a fridge and charging laptops, 12 volts is simpler.

Wiring a camper goes wrong when you do it at the wrong moment. The cables have to be in before the walls close up, but the layout has to be settled before you pull any cable.

A workable order:

1. Put your layout on paper and decide where the battery, the fuse box and every socket go.
2. Work the circuits out: current per appliance, cable size and fuse.
3. Lay the cables while the walls are still open, with some spare length at each end.
4. Insulate and panel.
5. Mount the battery, the fuse box and the chargers, and only then connect.
6. Test every circuit through before you put power on it.

Always pull an extra cable or two to places where you might want something later. An unused cable costs a few euros; adding it afterwards costs a day of tearing out.

Label both ends of every cable the moment you lay it. After three weeks you will not recognise them, and once the panelling is on you cannot follow where they run.

With some care you can lay the 12 V side of a camper yourself. There are two parts where that is different.

The first is 230 volts. As soon as shore power comes into the van you are dealing with a residual current device, an earth connection and the risk that a mistake ends fatally rather than annoyingly. Have that part connected or checked by someone qualified.

The second is gas. A gas installation in a motorhome has to meet EN 1949, and fitting one is work for a certified installer. It is also the first thing an insurer asks for after a claim.

For the 12 V work you need:

• A proper crimping tool for lugs, not a pair of pliers with a notch in it
• A multimeter to test every circuit before you use it
• Adhesive-lined heat shrink for every connection
• Lugs that match the cable size, not roughly match it

The crimping tool is the purchase people most often skimp on and the source of most faults. A badly crimped connection works for months and then burns.