Short answer: a typical campervan wiring diagram runs from the leisure battery through a main fuse and isolator to positive and negative busbars. Chargers (DC-DC from the engine, solar via a controller, mains on hook-up) feed in, and a fuse box, the inverter and other loads feed out. Every circuit has its own fuse on the positive cable, close to where it takes power.
The first campervan wiring diagram we drew was a mess of arrows on the back of an envelope. It still helped, because it forced the questions that matter: where does each cable start, how long is it, and what protects it. Below is a tidy version of the layout most UK van builds end up with, then each part in turn. Your build may leave parts out (plenty of vans have no inverter or no solar), but the shape stays the same.
The campervan wiring diagram
Leisure battery, main fuse and isolator
The leisure battery is the heart of the system, separate from the van's starter battery. Straight off its positive terminal goes the main fuse, then a battery isolator switch so you can cut everything off when working on the van or leaving it. Victron's Wiring Unlimited describes exactly this shape: a main battery fuse, after which the system branches off to individual consumers, each with its own fuse.
The main fuse protects the thick cable between the battery and the busbar, so it's sized for everything that runs through that cable at once. In Roam's planner that's whichever is bigger, all the loads together going out or all the charging coming in. For big systems this is usually a MEGA or ANL fuse, or a fused battery terminal. Lithium batteries can deliver very high currents into a short circuit, and Wiring Unlimited has a section on choosing fuses with a high enough interrupt rating for lithium systems, so check that on the fuse's datasheet.
Busbars and the shunt
Busbars are solid metal bars with studs, one positive and one negative, that give every cable a proper place to land instead of piling lugs onto battery terminals. Everything connects to the busbars, not the battery.
A shunt sits in the main negative cable between the battery and the negative busbar. All the current in and out of the battery passes through it, which is how a battery monitor knows your state of charge. If you fit one, nothing else should connect directly to the battery negative, or the monitor will miss it.
Charging: DC-DC, solar and mains
DC-DC charger. It takes power from the starter battery and alternator while the engine runs. It has a fuse at the starter battery end of its input cable (that cable is often the longest in the van, running from under the bonnet or seat to the back), and another on its output cable at the busbar end. Roam sizes the input cable for about 1.2 times the output current, because the charger draws more going in than it puts out. Victron's Orion XS manual notes it detects whether the engine is running from the alternator voltage, without extra wiring, and that Euro 6 vans with smart alternators may need that setting fine-tuned. More in the DC-DC charger size guide.
Solar. Panels connect to a charge controller (usually MPPT), which connects to the busbar. Wiring Unlimited says a fuse needs to be located between the PV array and the solar charger and notes that rules vary by country, so check what applies. Many builds use a DC breaker there so it doubles as an isolator. The controller's output cable to the busbar is fused at the busbar end.
Mains charger. On hook-up, a mains battery charger (or the charger inside an inverter-charger) feeds the busbar through its own fuse. Its 230 V supply comes from the consumer unit.
Fuse box and 12 V loads
Most of what you run is small: lights, water pump, fridge, roof fan, USB sockets, heater. Rather than give each one its own fuse holder at the busbar, they go through a blade fuse box, fed by one fused cable from the positive busbar. Inside the box, each circuit has its own blade fuse sized for that circuit's cable. That follows Victron's rule that every consumer gets its own fuse, however small.
Each load's cable size depends on its current and length. The cable size calculator works out the cable, fuse and voltage drop for each run, using 3% drop for things that need a steady voltage and 10% for lights and USB.
Inverter
The inverter is usually the biggest load in the van and gets its own heavy cable straight from the busbars, with its own fuse (often MIDI or MEGA) at the busbar end. Keep the run as short as you can: a 1,000 W inverter on 12 V draws about 93 A, and over just 1 m our calculator gives 16 mm² cable and a 125 A MIDI fuse. Wiring Unlimited also suggests keeping the positive and negative cables of high current runs, like battery to inverter, close together. See the inverter size guide for choosing one.
The 230 V hook-up side
When you plug into a campsite bollard, mains comes in through an inlet on the van to a consumer unit with an RCD and circuit breakers, which then feeds the mains charger and any 230 V sockets. If you have an inverter-charger, it sits in this path too, switching between hook-up and inverter. This side is covered by BS 7671 Section 721 for caravans and motor caravans, which the IET summarises as including requirements for RCDs, electrical separation and protective bonding. We'd have a qualified electrician install or inspect and test anything on this side. Our guide to electric hook-up covers using it on site.
Fuse placement, all in one place
| Cable | Where the fuse goes |
|---|---|
| Leisure battery to busbar | Main fuse at the battery positive, then the isolator |
| Starter battery to DC-DC | At the starter battery end |
| DC-DC to busbar | At the busbar end |
| Solar panels to controller | Fuse or DC breaker between panels and controller |
| Controller to busbar | At the busbar end |
| Mains charger to busbar | At the busbar end |
| Busbar to fuse box | At the busbar end |
| Fuse box to each load | A blade fuse per circuit in the box |
| Busbar to inverter | At the busbar end |
The rule behind all of these: a fuse goes on the positive, at the end the power comes from, as close to it as you practically can, so the whole cable is protected. Roam's planner suggests fitting holders within 200 mm of the battery or busbar. Where a cable can be fed from both ends, such as a charger output, put it at the battery or busbar end.
Roam's Build your electrics tool draws a live version of this diagram from your own parts and cable runs, sizes every cable and fuse, and turns it into a shopping list. It's in the app now, in early access. The full planning order is on the campervan electrics page.
Questions
Where should the main fuse go on a leisure battery?
On the positive cable, as close to the battery terminal as you can, before the isolator and busbar. It protects the main cable, so it's sized for the most current that cable carries.
Do I need a busbar in a campervan?
For anything beyond a couple of loads, yes. Busbars give each cable a solid connection and keep the battery terminals clear, and they make the system much easier to fault find.
Does every 12 V device need its own fuse?
Yes. Victron's Wiring Unlimited says every consumer connected to a battery needs its own fuse, whatever its power. Small loads usually share a fuse box, with one blade fuse per circuit.
Where does the negative go?
Every device's negative goes back to the negative busbar, and from there through the shunt (if you have one) to the battery negative.