How to Design a Campervan Electrical System (12V Wiring Layout UK Guide)
Designing a campervan electrical system becomes much easier when you break it into battery size, charging method, fuse layout and cable selection. This guide shows the typical structure used in UK 12V campervan installations and helps you plan each stage safely.
Quick campervan electrical system design chart
This table gives a simple starting point for planning a typical campervan setup before you move on to exact cable sizes, fuse ratings and charging details.
| System Part | What You Need to Decide | Typical Example |
|---|---|---|
| Battery bank | How much power you use each day | 100Ah to 200Ah leisure battery |
| Charging method | How the battery will recharge | DC-DC charger, solar, mains charger |
| Main protection | Fuse near leisure battery positive | 40A to 150A depending on system |
| Distribution | How 12V circuits are split | Fuse box with separate branch fuses |
| Cable size | Current, cable length and voltage drop | 1.0mm² to 25mm² depending on circuit |
| 230V appliances | Whether you need an inverter | 300W to 2000W inverter |
Campervan electrical system overview diagram
Most campervan electrical systems follow the same structure. Once you understand this layout, planning your own setup becomes far more straightforward.
Typical campervan electrical system structure
This is the normal order of major components in a 12V campervan build.
| Component | Purpose |
|---|---|
| Starter battery | Starts the engine |
| DC-DC charger or split charge relay | Transfers charging power to the leisure battery |
| Leisure battery | Stores power for lights, fridge and accessories |
| Main fuse | Protects the main positive cable |
| Fuse box / distribution panel | Splits power into protected circuits |
| Branch circuits | Supply lights, pump, fan, fridge and sockets |
Starter battery vs leisure battery explained
Campervans usually use two separate batteries so you can run appliances without risking a flat starter battery.
- Starter battery — used mainly for engine starting
- Leisure battery — powers the living area electrics
Step 1 — List everything you want to power
Before choosing batteries or cable sizes, list every device you expect to run in the van.
- LED lighting
- Water pump
- USB sockets
- Diesel heater
- 12V fridge
- Roof fan
- Phone and laptop charging
- Any inverter-powered appliances
Step 2 — Estimate daily power usage
A quick daily power estimate helps you choose a realistic battery size.
| Device | Typical Daily Usage |
|---|---|
| LED lights | 5Ah |
| 12V fridge | 35Ah |
| Water pump | 4Ah |
| Diesel heater | 10Ah |
| USB charging | 4Ah |
Total example daily use: 58Ah
Step 3 — Choose your leisure battery type
The leisure battery is the core of the electrical system, so choosing the right type matters.
| Battery Type | Main Advantage | Typical Use |
|---|---|---|
| AGM | Lower upfront cost | Simple budget builds |
| Gel | Good deep-cycle performance | Traditional leisure battery setups |
| LiFePO4 | High usable capacity and lower weight | Modern off-grid campervan systems |
Lithium batteries often give the best usable capacity, but they need suitable charging equipment and protection.
Step 4 — Decide how the leisure battery will charge
Most campervan systems use one or more of these charging sources.
- Alternator charging while driving
- Solar charging when parked
- Mains charger on hook-up
Modern vehicles with smart alternators usually need a DC-DC charger rather than a basic split charge relay.
DC-DC charger vs split charge relay
Both systems move charging power from the starter battery side to the leisure battery, but they are not the same.
| Option | Best For | Key Point |
|---|---|---|
| Split charge relay | Older, simpler vehicles | Lower cost but more basic charging control |
| DC-DC charger | Modern vans and smart alternators | Better charging performance and controlled output |
See related guides: Split Charge Relay Wiring Guide and DC-DC Charger Wiring Diagram.
Step 5 — Choose cable size between batteries
Cable size depends on current, total cable run and acceptable voltage drop.
| Cable Size | Typical Use |
|---|---|
| 6mm² | Short, lower-current charging runs |
| 10mm² | Common general charging installations |
| 16mm² | Higher-current DC-DC chargers |
| 25mm² | Larger battery banks and heavy-current runs |
Work out the correct size using the Cable Size Calculator.
Step 6 — Plan main fuse protection
Every positive cable connected to a battery should be protected with a correctly sized fuse close to the power source.
- Main fuse near the leisure battery positive terminal
- Fuse on the starter battery charging feed
- Separate fuse for inverter cable
- Protection for solar controller and other charging circuits
Estimate the correct rating with the Fuse Size Calculator.
Typical 12V fuse box layout
The fuse box distributes power from the leisure battery into smaller protected circuits.
Step 7 — Split the system into branch circuits
Once the leisure battery feed reaches the distribution panel, each circuit should be protected individually.
| Branch Circuit | Typical Example |
|---|---|
| Lighting | LED strips, reading lights |
| Water system | Pump and controls |
| Ventilation | Roof fan |
| Cold storage | 12V fridge |
| Sockets and charging | USB ports and 12V sockets |
Step 8 — Decide whether you need an inverter
An inverter is needed only if you want to run 230V appliances from the leisure battery.
- Laptop chargers
- Camera chargers
- Small kitchen appliances
- Other mains-powered devices
See full guidance here: How to Wire an Inverter in a Campervan.
Step 9 — Add solar if you want longer off-grid runtime
Solar is one of the best upgrades for extending time away from hook-up and reducing reliance on alternator charging.
- Keeps the leisure battery topped up while parked
- Supports fridges, lights, fans and device charging
- Works especially well with lithium battery systems
Estimate suitable panel size with the Solar Panel Sizing Guide.
Step 10 — Check voltage drop before installation
Voltage drop can reduce charging performance and appliance efficiency, especially on longer cable runs.
Check cable performance using the Voltage Drop Calculator.
Recommended cable types for campervan electrical systems
Useful calculators for campervan electrical system planning
Related campervan electrical guides
Plan your campervan electrical system before you buy cable
Getting the design right first helps avoid voltage drop problems, bad charging performance and expensive rewiring later.
Open Electrical Calculators