Complete guide: Wiring a 12V campervan system

The cable gauge thing is crucial, and it gets worse with 12V than higher voltages because you're dealing with such high currents. I learned this the hard way on the boat—ran 4mm² cable to the bow fridge thinking I'd save a few quid, and the voltage drop was killing it.

What @DailySolar's getting at is worth hammering home: a 3-metre run at 12V demands proper sizing. I now use an online volt drop calculator before any install. For reference, even a modest 20A circuit over 5 metres wants 6mm² minimum if you're not accepting more than 3% drop.

The real issue is that poor cable sizing doesn't just waste energy—it damages equipment slowly. Your charger works harder, your fridge cycles constantly, and batteries discharge faster. Not dramatic failures, just a system that feels sluggish and expensive to run.

@QuietTrekker's right about battery placement, but I'd add: once you've got the battery positioned, run your main positive and negative busses before anything else. Treat those as the backbone—everything branches from there. Learned that when

The cable gauge obsession is justified though—I've seen too many systems running warm cables and wondering why their voltage is dropping to 10V at the inverter. What @ExFirefighter's alluding to is the amperage issue: 12V systems pull massive currents compared to 24V or 48V setups for the same wattage. A 3kW load at 12V is 250 amps, at 24V it's 125 amps. That's why my off-grid setup runs 24V for the heavy lifting.

That said, campervans are often constrained to 12V because of existing automotive infrastructure. If you're retrofitting, seriously consider the battery-to-inverter run distance before specifying cable. I used 25mm² from my leisure battery to the Victron Smart BMS and still saw 0.3V drop under load—and that's only 2 metres.

Worth noting: don't just calculate based on continuous rating. Size for the largest single draw you'll have. Kitchen induction hob? Diesel heater startup? That's where cheap undersized runs fail. Use

Spot on about battery placement. I learned this the hard way with my shepherd's hut setup—ran the cables too long initially and voltage drop was killing my solar controller efficiency.

The cable gauge point @ExFirefighter's making is dead right, but worth adding: at 12V you're looking at potentially double the current compared to 24V systems for the same power draw. So a 3kW load at 12V is roughly 250A, at 24V it's 125A. That's why my motorhome conversion uses 10mm² for the main runs between battery and distribution, even though some online calculators suggested 6mm² would "work."

What's equally important nobody's mentioned yet is the fusing strategy. You want a hefty ANL or mega fuse right at the battery terminal—literally within 30cm. I use a Victron Cyrix-Li to manage my lithium pack, but that's only possible because the main circuit's properly protected first.

Also, cable routing matters more than people think. Keep your high-current positive and negative runs separated, and away from sensitive signal lines (solar input, temperature sensors).

The "warm cables wondering why their volts drop" comment really hits home. I've got a Victron setup in my tiny house that taught me this lesson the hard way—watched my 12V system struggle to run the kettle because I'd routed cables through half the garden to the boat.

@ExFirefighter11's got it right about high current systems being the

Mate, cable gauge is the difference between "my system works" and "my system works but slowly sets itself on fire" — copper's cheap, rewiring isn't.

@Paddy's spot on about warm cables being a silent battery killer. I've got dual 200Ah LiFePO₄ sitting maybe 60cm from my Victron fusebox, and even then I'm running 35mm² from the batteries. Cheap out here and you're literally burning energy as heat instead of using it.

The real trick nobody mentions: get your battery placement sorted before you install the leisure fridge and water tank in the exact spot you needed. Ask me how I know. Relocating 200kg of batteries is peak comedy.

If you're doing campervan specifically, factor in vibration on cable terminations — I've seen dodgy crimps fail spectacularly on forest tracks. Quality Anderson connectors and marine-grade terminals, not the dodgy automotive tat.

The battery-to-fusebox distance is absolutely critical, and @OldSailer's not joking about the fire risk. I learned this the hard way fitting a 200Ah LiFePO4 bank in my motorhome after initially running 4-gauge cable nearly 3 metres from the battery.

What nobody mentions often enough is that voltage drop compounds when you're drawing high amperage through a leisure charger or inverter. I was getting 11.2V at the fusebox despite the battery sitting at 13.8V—completely defeating the point of lithium.

The real solution is brutal honesty about amperage. If you're pulling 100A for an inverter, that cable run needs to be short and properly sized. Use a voltage drop calculator—Victron's is solid—and assume worst case. I ended up relocating my battery box forward, added a 150A ANL fuse right at the terminals, and suddenly everything behaved.

Segregate your wiring too. Keep your high-draw stuff (inverter, charger) on separate cable runs from sensitive kit like Victron controllers. Your future self will thank you when troubleshooting.

Spot on about the distance — I've got a static caravan setup and learned this the hard way. Ran the battery cables too long initially and voltage drop was killing my fridge efficiency. Dropped nearly 2V by the time it reached the fusebox.

What nobody mentions enough is the return path. You need proper gauge cable back to the battery negative, not just relying on chassis/caravan frame. Sounds obvious but I've seen dodgy installations everywhere. That's where the actual fire risk lives — high resistance return circuits.

Cable routing matters too. Keep power and load wires separated if you're running any comms (CAN bus stuff, phone chargers). Learned that when my Victron monitoring kept glitching until I rerouted everything properly.

For a motorhome specifically, consider a split-charging relay if you haven't already. Keeps your starter battery separate from your leisure bank. Stops that dead battery nightmare when you've been stationary a week.

@OldSailor's absolutely right — this isn't theoretical stuff.

The cable gauge conversation is spot on, but I'd add that voltage drop is the sneaky killer most people ignore. I've got a 200Ah LiFePO4 setup in my motorhome with about 2.5m runs from battery to fusebox, and even that distance matters when you're pulling 100+ amps through a charger or inverter.

Get yourself a decent voltage drop calculator — aim for under 3% on your main runs. For a 12V system with serious current draw, that often means 16-25mm² cable depending on length. Yeah, it's pricey compared to 6mm², but the difference in performance is genuinely noticeable. Your alternator charges quicker, your Victron gear runs more efficiently, and you're not burning through cables.

Also worth mentioning: use a dedicated battery monitor (I run a Victron BMV) so you can actually see what voltage is reaching your loads. Sorted us out on a previous trip when I thought my batteries were dying — turned out to be a dodgy connection causing a 0.8V drop across 3m of cable.

Cable

Voltage drop's the real culprit nobody talks about until it's too late. I've got a shepherd's hut setup running about 8 metres of cable from battery to consumer box, and even with proper 6mm² it was still dropping to 11.2V at the far end under load.

The trick is thinking about your actual usage. If you're just running lights and a fridge, you might get away with it. But throw in an inverter or water heater and suddenly you're looking at serious losses. @SimonKelly's right to flag this.

What helped my setup was running the positive and negative runs separately in conduit rather than bundled — keeps temps down and makes troubleshooting easier. And honestly, if you're over 5 metres, seriously consider bumping up a cable size. The cost difference between 6mm² and 10mm² is marginal compared to rewiring it all later.

Also worth calculating your actual peak draw while planning. That's what really determines what gauge you need, not just "what the campervan forum says."

Proper cable sizing is an absolute game-changer — I learned this the expensive way when my motorhome's solar charge controller kept throttling back because of voltage drop over a dodgy run of undersized cable. Cost me nearly a grand in wasted solar potential before I cottoned on.

@SimonKelly's spot on about the sneaky nature of it. Calculate your voltage drop before you buy a single metre of cable — there's decent calculators online. For anything beyond 3 metres at 12V, you'll want thicker gauge than you'd think. I'm running 35mm² from battery to distribution board now and the difference is noticeable.

Also worth mentioning: fused disconnect right at the battery. Sounds obvious but saves your cables (and potentially your rig) if something goes catastrophically wrong. Victron or equivalent, nothing dodgy.

The other thing nobody mentions is temperature. Cold batteries are grumpy batteries, so wherever you put it, avoid the outside wall of the motorhome if you can manage it.

The battery placement point @QuietTrekker raises is spot on, though I'd add the why matters more than people realise. When I first wired my array setup, I had the leisure battery tucked away "for safety" – massive mistake. Every metre of cable between battery and fusebox is resistance working against you.

What @SaltyHiker and @SimonKelly are getting at with voltage drop is genuinely the difference between your lights being properly bright and your fridge compressor struggling to kick in. I learned this when my Victron MPPT wasn't charging properly – turned out the 4mm² cable run was costing me 0.8V on a 12V system. That's nearly 7% loss before anything else happens.

The practical bit nobody mentions: calculate your voltage drop for each circuit. A 20A draw through thin cable to your water heater behaves completely differently from your 5A leisure lighting. I use a simple online calculator now – takes thirty seconds and saves headaches.

Battery to fusebox distance matters. Battery to each load matters more. Cable gauge isn't just about cost – it's about system performance.

The cable sizing comment from @RetiredNurse49 is spot on — I made exactly that mistake on my narrowboat. Undersized cables running from the battery bank to the main distribution point meant my Victron MPPT was throttling itself, losing about 400W of potential solar generation on decent days.

What's worth adding: once you've sorted cable runs and battery placement, your fusebox location becomes critical. Mount it within arm's reach of the battery (ideally under 1.5m), not tucked away in a cupboard. You'll need regular access for troubleshooting and it makes testing voltage drops far easier with a multimeter.

Also — and I don't see this mentioned enough — consider your future upgrades. If you're starting with a 100Ah LiFePO4, don't use 50mm² cable "for now" planning to upsize later. Just bite the bullet and run proper gauge from day one. Rewiring is a nightmare compared to getting it right initially.

The geometry of your layout matters more than the square footage of your van or narrowboat. Tight, direct runs always trump theoretical calculations.

One thing nobody's mentioned yet — voltage drop calculations. If your battery's more than a few metres from your fusebox, you need to work out whether your cable gauge is actually adequate for your loads.

I've got a static caravan setup where the leisure battery sits about 4 metres from the main distribution point. Running an underrated cable there cost me roughly 0.5V drop under load, which sounds minor until you're trying to charge an EV trickle charger or run a decent inverter. Swapped to 10mm² and the difference was immediately noticeable.

The formula's straightforward enough — voltage drop in volts equals (2 × cable length × current) divided by (conductor area × conductivity). Most folks can grab an online calculator, but the principle matters: thicker cable = less loss = better efficiency, particularly on 12V where voltage margins are already tight.

@RetiredTrekker's point about proximity is absolutely right, but if distance is unavoidable, don't skimp on cable cross-section. It's the one upgrade that pays for itself through better charging and load performance.

Voltage drop's the killer that nobody talks about until it's too late. I learned this the hard way with my shepherd's hut solar setup — had my battery bank about 8 metres from the main panel and couldn't figure out why my 12V lights were dim even with a full charge.

Simple maths: work out your cable run (there and back), check the current draw, use a voltage drop calculator. Even 0.5V loss sounds tiny until you're running a fridge or inverter. Went from undersized 4mm² to 10mm² and the difference was night and day.

Cable length matters more than people think. If you've got the space, route it sensibly rather than coiling it behind the fusebox "for neatness." Straight runs, proper sizing, and you won't regret it.

Also — and this sounds obvious — but actually label everything before you zip-tie it all up. Future you will be grateful.

Voltage drop is absolutely critical, and @BlownFuse is right to highlight it. I'd add that it's not just about distance — cable gauge matters enormously. Most people underestimate how much cross-sectional area you need.

I've got a 300Ah LiFePO4 setup in my converted horsebox, battery sat about 4 metres from the fusebox. Initial cable run was 35mm² — seemed massive — but calculations showed I'd lose ~0.8V on heavy loads (inverter startup, EV trickle charging simultaneously). Upgraded to 50mm² and sorted it properly.

The formula's straightforward: voltage drop = (2 × length × current) / (conductivity × area). For 12V systems, you're aiming for <3% drop maximum, ideally <2%. At 4 metres with 100A draw, that 35mm² was borderline.

One thing worth mentioning — use proper marine-grade tinned copper cable. The cheap stuff oxidises and your effective conductivity drops over time. Victron's documentation on this is solid if you want the technical specifics