A wiring diagram is the single most useful document in a self-build campervan. It turns a pile of parts into a plan you can check, cost and follow, and it’s what you (or an electrician, or a future buyer) will reach for when something stops working. This guide shows you how to draw one for a simple 12V leisure system, what order the parts go in, and the rules from manufacturer guidance that your diagram should show clearly.
If the terms volts, amps, amp-hours, DC-DC charger or shunt are new to you, read our beginner’s guide to campervan electrics first. This post builds directly on it.
Important: this is a general planning guide, not a substitute for the manuals of the products you buy. Every manufacturer specifies the cable size and fuse for its own equipment, and those instructions take priority over any generic diagram, including the one below.
Why draw a wiring diagram before you buy anything
It’s tempting to buy a battery, a charger and a fuse box and work it out as you go. Drawing the diagram first saves money and mistakes, because it forces you to answer the questions that decide what you buy.
- It sizes your cables. Each line on the diagram has a current and a length, and those two numbers decide the cable cross-section.
- It places your fuses. You can see at a glance whether every positive cable leaving the battery is protected.
- It fixes your layout. Short heavy-current runs mean thinner, cheaper cable, so the diagram pushes you to group the battery, chargers and fuse box together.
- It becomes your shopping list. Every box and line on the page is a part, a length of cable or a terminal.
- It helps later. Insurers, buyers and anyone fault-finding will understand your van far quicker with a clear diagram. Our guide to self-build campervan insurance explains why documenting the conversion is worthwhile.
A simple 12V campervan wiring layout
Below is a typical layout for a basic leisure system with a DC-DC charger, solar and a mains charger. It’s a simplified block diagram to show the order of parts, not a build drawing, and it leaves out the 230V side, which is covered further down.
POSITIVE (+) SIDE
Starter battery (+) --[fuse]-- DC-DC charger --[fuse]--+
Solar panels --[fuse]-- Solar charge controller --[fuse]--+
Mains charger (from hook-up) --[fuse]------------------+
|
Leisure battery (+) --[MAIN FUSE]--[ISOLATOR]-- Positive busbar
|
Fuse box (one fuse per circuit)
|-- Lights
|-- Water pump
|-- Fridge
|-- USB / 12V sockets
|-- Roof fan
|-- Heater
NEGATIVE (-) SIDE
Leisure battery (-) -- SHUNT -- Negative busbar -- every load and charger negative
Read it from the battery outwards. Everything that charges the battery and everything that draws from it meets at a common positive point (a busbar or the charger’s own terminals), and every negative returns to a common negative busbar that connects to the battery through the shunt.
The parts in order
| Position | Part | What the diagram should show |
|---|---|---|
| 1 | Leisure battery | Type (AGM or lithium), capacity in Ah, mounting location |
| 2 | Main fuse | Rating, fuse type and its position close to the battery positive |
| 3 | Isolator switch | Its DC current rating and that it’s in the positive cable |
| 4 | Positive busbar | Which cables land on it |
| 5 | Charging devices | DC-DC charger, solar controller, mains charger, each with its own fuse |
| 6 | Fuse box | Each circuit, its fuse rating and what it feeds |
| 7 | Loads | Each appliance and its current draw |
| 8 | Shunt and negative busbar | Shunt rating, and that all negatives connect after it |
Fuses: what your diagram must show
Victron’s Wiring Unlimited guide is clear on fuse placement: each consumer that connects to a battery needs its own fuse, placed in the positive cable, no matter how big or small its power rating. A DC circuit usually has a main battery fuse, after which it branches off to the individual consumers, each with an individual fuse (Victron: Wiring Unlimited, DC wiring).
The same guide says a fuse needs to be located between a solar array and the solar charger, and that local regulations vary. When choosing any fuse, it lists four criteria: current rating, voltage rating, interrupt rating and speed.
On your diagram, write the rating next to every fuse symbol. If you can’t fill in a number, you haven’t finished the design.
Lithium batteries and interrupt rating
This catches out many self-builders. Victron warns that lithium batteries can deliver very high short-circuit currents, far higher than comparable lead-acid systems. The interrupt rating (or breaking capacity) is the maximum fault current a fuse can safely interrupt, and if it’s lower than the available short-circuit current, the fuse may fail catastrophically. Victron states that in every lithium battery installation, at least one fuse in the DC line must have an interrupt rating equal to or greater than the maximum prospective short-circuit current of the battery bank (Victron: Wiring Unlimited, DC wiring).
Victron’s guide gives these interrupt ratings for common fuse types:
| Fuse type (as listed by Victron) | Max DC voltage | Interrupt rating |
|---|---|---|
| Class T fuse (Eaton Bussmann) | 160V | 200kA |
| Class T fuses (various manufacturers) | 125 to 300V | 20kA |
| Blue Sea ANL fuse | 80V | 6,000A |
| AMX(L) fuses (Eaton Bussmann) | 125V | 3,000A |
| Littelfuse MEGA fuse (70V) | 70V | 2,500A |
| Blue Sea MRBF terminal fuse | 58V | 2,000A |
| Littelfuse MEGA fuse (58V) | 58V | 1,000A |
Source: Victron Energy, Wiring Unlimited. To use the table, you need the battery manufacturer’s figure for maximum short-circuit current; check its datasheet, and ask the manufacturer if it isn’t listed.
The isolator switch
An isolator lets you switch the leisure system off for storage or maintenance. Victron recommends that, if battery isolation is needed, only the positive battery cable is isolated, and that the switch is rated for the current expected at full load, as DC current. It also points out that some isolator switches can’t break current under load, so check the specification, and that a cheap switch adds resistance and voltage drop. Victron even notes an isolator may not be strictly necessary, because removing the main fuse also breaks the circuit (Victron: Wiring Unlimited). Most self-builders still fit one for convenience.
The shunt and the negative side
A shunt measures the current flowing in and out of the battery so a battery monitor can work out the state of charge. According to Victron, the shunt is typically placed in the negative cable for safety reasons and should be the last component before the battery or battery busbar, with all DC consumers and supplies connected after it (Victron: Wiring Unlimited, DC wiring).
This is the most common diagram error: if a single negative wire bypasses the shunt and goes straight to the battery terminal, the monitor won’t see that current and its readings drift. On your diagram, draw only one cable on the battery negative terminal, going to the shunt.
Victron also says the shunt must be rated for the maximum DC current that will flow, which matters if you add an inverter, since its peak draw can be very large.
What about the vehicle chassis?
Victron’s guide explains that in a mobile installation such as a vehicle, which has no earth stake, all touchable metal parts must be connected together to create a local earth, and it discusses this mainly in the context of the AC (230V) side (Victron: Wiring Unlimited). How and where the leisure negative connects to the chassis depends on your equipment manuals and, for the 230V side, on the electrician installing it, so mark it on your diagram as a decision to confirm rather than guessing.
Cable sizes and labels on the diagram
Every line on your diagram should carry three pieces of information: the maximum current, the total length (positive plus negative) and the cable cross-section in mm². Our electrics guide includes Victron’s table of maximum current per cable size and explains why longer runs need thicker cable.
Victron’s Wiring Unlimited also gives practical advice worth writing as notes on the drawing:
- Keep heavy-current runs short, and group the battery and its main devices close together.
- Leave some slack in cables, because tight cables plus vehicle vibration stress the terminals until they work loose.
- Use strain relief so heavy cables don’t hang off battery or device terminals.
- Allow heat to escape: cables in enclosed conduit may not cool properly, so Victron suggests conduit open at the top or thicker cable.
Crimp terminals
If your diagram lists terminals for your shopping list, match the colour to the wire. Victron gives these ranges for insulated crimp terminals:
| Terminal colour | Wire size |
|---|---|
| Red | 0.5 to 1.5mm² |
| Blue | 1.5 to 2.5mm² |
| Yellow | 2.5 to 6mm² |
Victron recommends a professional ratcheting crimp tool, and says it does not recommend bullet or butt-splice terminals because they often make bad contact (Victron: Wiring Unlimited, DC wiring).
Where the 230V hook-up fits in
If you want mains hook-up, show it on a separate diagram: inlet, consumer unit with RCD protection, sockets, and the mains battery charger that links it to the 12V side. Mains installations in caravans and motor caravans are covered by Section 721 of BS 7671, the UK wiring regulations (IET Wiring Matters). Unless you’re qualified, have a competent electrician design or at least inspect and test this part. Your 12V diagram only needs to show the charger’s DC output and its fuse.
A checklist for your finished diagram
- ☐ Every component is labelled with its make, model or rating
- ☐ Every positive cable leaving the battery or busbar has a fuse, with its rating written beside it
- ☐ The main fuse sits as close to the battery positive as possible
- ☐ For lithium, at least one fuse has an interrupt rating at or above the battery’s short-circuit current
- ☐ The isolator is in the positive cable and rated for full-load DC current
- ☐ Only the shunt connects to the battery negative; every other negative lands after it
- ☐ Every cable shows its current, total length and cross-section
- ☐ Cable and fuse sizes match each product’s manual
- ☐ Solar has a fuse between the panels and the charge controller
- ☐ 230V is on its own diagram, for an electrician to install or check
- ☐ Cable routes avoid sharp edges and the gas locker, with grommets where cables pass through metal
- ☐ A copy is kept in the van, with any changes marked up
FAQ
Is there a standard campervan wiring diagram I can copy?
There’s no single diagram that suits every van, because battery type, charger and loads all change the cable and fuse sizes. Many equipment makers publish example schematics for their own products, which are a good starting point, but always check them against the manuals for the exact parts you buy.
Does the fuse go on the positive or negative?
Victron’s guidance is to place the fuse in the positive cable, with a main fuse near the battery and an individual fuse for every consumer.
Where does the shunt go in a campervan wiring diagram?
In the negative cable, as the last component before the battery, with every load and charger negative connected after it, according to Victron.
Do I need an isolator switch?
Most builders fit one for convenience. Victron notes it may not be strictly necessary because removing the main fuse also isolates the battery, but if you fit one it should be in the positive cable and rated for the full-load DC current.
Can I draw the diagram by hand?
Yes. A neat hand drawing that shows every part, fuse rating and cable size is far better than none. What matters is that it’s complete and kept up to date.
Next steps
Start with the load list from our electrics guide, sketch the layout above with your own parts, then fill in every fuse rating and cable size from the manuals. If you’re working through a full build, our step-by-step van conversion guide shows where wiring sits in the build order, before insulation and panelling close up the walls. Next in this series, we’ll look at campervan insulation: materials, costs and the mistakes to avoid.