Alternator to Leisure Battery Cable Size
This beginner-friendly UK guide explains how to choose the correct cable size between the alternator, starter battery and leisure battery in a campervan. Learn what cable size works for DC-DC chargers, split charge relays and longer rear battery installs.
Alternator to leisure battery cable size at a glance
This table shows typical campervan charging cable sizes based on charger current and general installation type.
| Charging Setup | Typical Cable Size | Best For |
|---|---|---|
| 20A DC-DC charger | 10mm² to 16mm² | Shorter cable runs |
| 30A DC-DC charger | 16mm² | Most campervan installs |
| 40A DC-DC charger | 16mm² to 25mm² | Longer runs or higher demand |
| 50A DC-DC charger | 25mm² | Heavy-duty charging setups |
| Basic split charge relay | 16mm² to 25mm² | Older campervan systems |
Main charging cable path
starter battery → fuse → cable run → DC-DC charger → fuse → leisure battery
Simple answer: what cable size do most campervans need?
For many campervan builds with a rear-mounted leisure battery and a 30A DC-DC charger, 16mm² cable is a common starting point.
- Suitable for many standard UK campervan conversions
- Works well with common 30A charging setups
- Offers better voltage drop performance than smaller cable
If the cable run is longer or the charger is larger, 25mm² may be the better choice.
Why cable size matters between starter battery and leisure battery
The cable between the vehicle battery side and the leisure battery charging side carries significant current. If the cable is too small, the system can suffer from excessive voltage drop, weaker charging and unnecessary heat.
- Long cable runs increase resistance
- Undersized cable reduces charging performance
- Higher current chargers need thicker cable
- Voltage drop is often worse than people expect in vans
What affects alternator to leisure battery cable size?
Choosing the right cable size is not only about charger rating. You also need to consider the full installation layout.
- DC-DC charger current rating
- Split charge relay or regulated charger setup
- Total cable run length
- Battery position inside the van
- Acceptable voltage drop
- Battery type and charging requirements
That is why two campervans with the same charger may still need different cable sizes.
Cable size vs DC-DC charger size
This quick reference table shows common cable size pairings for campervan DC-DC charger installations.
| DC-DC Charger Size | Typical Cable Size | Notes |
|---|---|---|
| 20A | 10mm² to 16mm² | Often acceptable on shorter runs |
| 30A | 16mm² | Most common campervan recommendation |
| 40A | 16mm² to 25mm² | Longer runs usually benefit from 25mm² |
| 50A | 25mm² | Typical heavy charging setup |
Cable size vs cable length in campervans
Longer battery-to-battery runs need thicker cable to keep voltage drop under control. This is especially important in panel vans where the starter battery is at the front and the leisure battery is mounted in the rear living area.
| Cable Run Length | Common Charger Range | More Typical Cable Choice |
|---|---|---|
| Short run | 20A to 30A | 10mm² to 16mm² |
| Medium run | 30A to 40A | 16mm² |
| Long rear install | 30A to 50A | 16mm² to 25mm² |
| Very long / higher current run | 40A to 50A+ | 25mm² or larger |
The longer the cable run, the more likely it is that 25mm² becomes the smarter option rather than 16mm².
Rear leisure battery installs need extra attention
Many campervans place the leisure battery near the rear bench, garage area or kitchen unit. This creates a longer charging path than many beginner wiring diagrams suggest.
- Front-to-rear cable runs are common
- Voltage drop becomes a bigger issue
- Charging performance can suffer with undersized cable
- Heavier cable often pays off over time
Voltage drop comparison
10mm² vs 16mm² vs 25mm² cable on a longer rear battery install
What cable size is used with split charge relays?
Older split charge relay systems often need similarly robust cable sizing because they can pass significant current directly between battery banks without the controlled output of a DC-DC charger.
| Charging Method | Typical Cable Range | Main Consideration |
|---|---|---|
| Split charge relay | 16mm² to 25mm² | Current can be high and less controlled |
| DC-DC charger | Matched to charger rating | More predictable current draw |
If your system uses a relay rather than a DC-DC charger, cable thickness becomes even more important.
DC-DC charger vs split charge relay cable sizing
Although both systems connect the starter battery side to the leisure battery side, the way current flows is different.
- DC-DC chargers have defined charging current ratings
- Split charge relays can allow much higher current flow
- Relay systems often need very robust cable sizing
- Modern smart alternator vans usually suit DC-DC chargers better
That is why many campervan builders now size cables around the charger specification rather than just copying older split relay installs.
Can 10mm² cable be enough?
10mm² cable can be enough in some lighter-duty campervan charging circuits, especially with smaller chargers and short cable runs.
- More realistic for 20A chargers
- Less suitable for longer rear battery installs
- Often marginal once voltage drop is considered
- Usually not ideal for larger charging systems
In many practical campervan layouts, builders move up to 16mm² to gain better charging performance.
When should you use 16mm² cable?
16mm² is one of the most common and useful cable sizes for campervan charging systems.
- Strong fit for many 30A DC-DC charger installs
- Suitable for typical front-to-rear cable runs
- Offers a good balance between flexibility and performance
- Often the best starting point for standard camper conversions
For many UK builds, this is the practical “default” size to check first.
When should you use 25mm² cable?
25mm² becomes a strong option when current is higher or the charging cable run is longer.
- 40A to 50A charging setups
- Long rear battery installs
- Reduced voltage drop over distance
- Better future-proofing for later upgrades
Don’t forget fuse size and cable protection
The charging cable should always be fused close to the battery. This applies whether the system uses a DC-DC charger or a split charge relay.
- Fuse near the starter battery side
- Fuse near the leisure battery side where required
- Fuse must match cable safety rating
- Do not oversize fuses beyond cable capability
Correct fuse protection is just as important as choosing the cable size itself.
Common cable sizing mistakes
- Choosing cable only by charger amps, not cable length
- Using the same logic as short appliance circuits
- Ignoring voltage drop on front-to-rear installations
- Using undersized cable with lithium charging setups
- Oversizing the fuse instead of increasing cable size
Helpful calculators for charging cable sizing
Related campervan electrical guides
Choose charging cable size before installation
The correct alternator-to-leisure battery cable size improves charging performance, reduces voltage drop and makes the whole campervan electrical system safer and more reliable.
Open Electrical CalculatorsElectrical installation safety notice
This guide and the associated calculators are provided for general educational purposes only. They show typical wiring layouts and estimation methods commonly used in UK 12V campervan electrical systems.
Every campervan installation is different. Cable sizing, fuse selection, charging current, battery chemistry and alternator behaviour should always be checked against manufacturer specifications and the requirements of your specific vehicle.
Where high-current battery systems, DC-DC chargers, inverters or 230V mains hook-up equipment are involved, installation should be completed or inspected by a suitably qualified installer.