Complete guide: Wiring a 12V campervan system

Right, here's what I've learned wiring my garden office setup, which transfers directly to campervans:

Battery placement first. Keep your 12V leisure battery as close as possible to your fusebox and inverter. Every metre of cable = voltage drop. I'm using 6mm² cable for my 100Ah setup and it's spot on.

Fuse everything immediately. Seriously. After the battery positive terminal, fuse within 50cm. I use ANL fuses—Victron kit integrates nicely with them. Prevents fires if something shorts. Each circuit then needs its own breaker or fuse rated to the wire gauge.

Colour your wires. Red = positive, black = negative, yellow = solar input. Saves your sanity later when troubleshooting. Label both ends of every cable.

Solar to controller first. Connect panels before batteries if you're adding solar. I ran my Renogy setup this way and it's dead simple. Then controller to battery. Never reverse this order.

Load circuits last. Lights, fridge, USB sockets—wire these in after your main battery circuit is solid. Group high-draw stuff (fridge, heater) separately from low-draw (lighting, comms).

Use a decent busbar. Saves cramming multiple cables onto battery terminals. Victron's Cyrix models are pricey but bulletproof.

Test everything with a multimeter before you seal it up. Voltage drop under load is your enemy—I learned that the hard way.

What gauge cable are you planning? That changes everything.

Good shout on the battery placement, @QuietTrekker. I learned this the hard way with my boat setup — running cable runs longer than necessary is a false economy. Every metre of cable adds resistance, and you're losing voltage before it even reaches your devices.

What I'd add: cable gauge matters more than people realise. I've seen folk use standard automotive cable and wonder why their 12V system feels sluggish. Invest in proper marine-grade cable, sized for your actual load. A decent crimper and heat-shrink makes all the difference too.

The garden office taught me to think about future expansion from day one. I wish I'd run conduit when I first installed everything — adding circuits later meant ripping things apart. Worth the extra faffing about upfront.

The cable run issue is crucial and often overlooked. I've got about 4 metres from my battery bank to the main fusebox on the boat, and even that required 95mm² cable to keep voltage drop under 3% at full load.

Worth noting: use proper marine-grade tinned copper cable, not standard automotive stuff. The corrosion in a damp environment will degrade regular PVC sheathing. I sourced mine from Fogstar and haven't looked back.

Also consider your fusebox placement as a secondary distribution point rather than the primary one. My setup runs heavy cables (starter circuits) directly from battery to engine/alternator, then uses a smaller busbar for auxiliary loads. Means you're not running massive cables the full length of the van or boat unnecessarily.

Cable glands and proper strain relief matter too—movement and vibration will crack connections over time.

Cable gauge is where most people slip up on longer runs. At 4 metres @ForestBoater, you'll want to calculate your voltage drop — aim for under 3% on your main runs.

Quick rule of thumb: for every metre over 2, you're essentially losing efficiency. I've got about 3.5m in my static caravan setup and had to jump from 6mm² to 10mm² just to keep things sensible when running the EV charger circuit.

Worth factoring in your maximum expected current draw too. If you're pulling 100A for leisure loads, those thinner cables will get warm fast — and warm cables are a fire risk in an enclosed space.

What's your total amp load looking like? That'll determine whether 4 metres is actually workable or if you need to rethink the battery placement.

The voltage drop calculation is dead straightforward and worth doing properly. Use this rule of thumb: for every metre of cable run, you lose roughly 3% voltage per 100 amps at 12V if you're using undersized cable.

In my static caravan setup, I've got about 3.5 metres from battery to distribution board. I'm running 70mm² dual-core and it keeps drop under 2% even when the inverter's pulling heavy load. Cost me a bit more upfront but saves headaches later — undersized cable generates heat, kills efficiency, and can be a fire risk if you're running high-draw kit like fridges or inverters.

Easier calculation: use an online voltage drop calculator (there's several decent free ones) and plug in your cable length, amp draw, and wire gauge. Beats doing it longhand.

The fusebox distance doesn't matter as much as people think — what matters is the battery-to-first-battery-disconnect distance. That's your critical run.

Spot on about placement, @QuietTrekker. I learned this the hard way with my cabin setup — had my battery bank tucked away in a storage cupboard initially, thinking it was tidy. Nightmare for voltage drop on a 6-metre run to the main panel.

What nobody mentions though is the thermal side. Your leisure battery generates heat, especially under heavy load. I've got mine in a dedicated external box now with ventilation gaps. Stops it cooking itself during summer when the solar's hammering in.

Also worth considering a split-charge relay or similar if you're running a camper engine alongside leisure circuits. Keeps your starter battery separate from your 12V system draws — saved me a stranded van situation last year.

The math's important, but don't overthink it. Victron's online voltage drop calculator does the heavy lifting if spreadsheets aren't your thing.

Placement makes a massive difference, yeah. I moved my battery bank closer to the fusebox in my tiny house setup and the difference was noticeable — less voltage drop, fewer headaches.

One thing worth mentioning: if you can't get the battery close enough, go thicker cable than the calculators suggest. Sounds wasteful but it's cheaper than replacing corroded connections or dealing with dodgy voltage later. I used 25mm² for a 3.5m run when 16mm² would've technically worked, and I don't regret it.

Also consider your future expansion. If you're planning to add more panels or upgrade your battery bank later, oversizing the initial wiring saves you rewiring everything. Learned that lesson the hard way with my solar setup.

Absolutely right on the placement side, but I'd add that once you've got the physical distance sorted, you need to properly spec your cable gauge. I made this mistake on my narrowboat initially — thought I could get away with undersized cables because "it's only 12V."

The voltage drop compounds quickly over longer runs. Use a proper calculator (there are decent ones online) based on your actual circuit amperage and cable length. For a leisure battery to fusebox run, I wouldn't go below 6mm² even for shorter distances, depending on your peak loads.

Also worth mentioning: use tinned marine-grade cable throughout. Freshwater systems corrode ordinary copper faster than you'd expect, and on a boat the environment's even harsher. Victron make some excellent pre-terminated cables if you want to avoid the hassle of crimping.

The fusebox location matters too — ideally between battery and your main loads, but accessible for maintenance.

The cable gauge thing's crucial too — most people undersize it thinking 12V is forgiving. It's not. I ran 4mm² initially in my garden office and got voltage drop over just 3 metres. Swapped to 10mm² and the difference was night and day, especially when the kettle's on and the solar's charging simultaneously.

Also worth mentioning the fuse placement. Your main fuse needs to sit within 45cm of the battery positive terminal — not optional, that's fire safety. I use a Victron Cyrix-ct as my split charge relay, but you need proper protection either way.

Cable routing matters as well. Keep it away from sharp edges and moving parts. Heat shrink everything that's exposed. Learned that one when a shepherd's hut project nearly went sideways because I was lazy about it.

What leisure battery are you running?

Brilliant thread this. Just a quick question though — @DefenderLife, when you say you undersized to 4mm², what distance were you running that over? I'm asking because I'm setting up a static caravan battery bank and I'm genuinely uncertain whether the cable sizing calculators account for the voltage drop differently on a 48V system versus splitting it across multiple 12V banks.

Also, has anyone actually measured the temperature difference in their cables after sorting the gauge? I want to avoid fire risk obviously, but I'm wondering if it's one of those things that sounds dramatic or if undersizing genuinely causes noticeable heat. Might help me justify the cost to the other half.

The distance thing's key yeah. I learned this the hard way in my van conversion — had about 3 metres from battery to fusebox and 4mm² wasn't cutting it, voltage drop was noticeable under load. Bumped it up to 6mm² and sorted.

Honestly though, just use the calculators online rather than guessing. Victron's got a decent one. Saves buying cable twice.

Also worth mentioning — keep your runs as direct as possible. I routed mine alongside the chassis initially thinking it was tidy, added another metre of unnecessary cable. Move it back and suddenly everything runs cleaner. Battery voltage stays more stable when you're running kettles or high-draw stuff.

Cheers for the thread — this is exactly what I'm wrestling with at the moment in my shepherd's hut setup. Got a 200Ah LiFePO₄ sitting about 2.5 metres from the fusebox and I'm second-guessing myself on cable gauge.

So if I'm reading this right, the distance compounds the problem? I've spec'd out 6mm² cable thinking that'd be safe, but @DefenderLife sounds like even that might be tight depending on your amp draw. What sort of loads are we talking about before undersizing becomes an actual issue?

Also — and this might be daft — but should the leisure battery position influence where you mount the fusebox, or is it more about working backwards from wherever the battery logically sits? I've got a bit of flexibility in my layout still, so happy to move things around if it makes the wiring sensible.

Using Victron gear for monitoring, so I can at least see if there's voltage drop across the run, but I'd rather get it right first time than have to rip it all out.

The battery placement bit really resonates with me — I've got three different setups and the lesson was the same each time. When I first wired up my narrowboat, I positioned the 200Ah lithium bank about 4 metres from the main fusebox purely because it "looked neater". Absolute nightmare. Voltage drop was killing my solar charge controller efficiency, and the cables were constantly warm to touch.

Moved it all in the end, ran the cables directly, and the difference was night and day. That's when I realised the aesthetic doesn't matter — function first, hide the mess later if you must.

@RussScott, for a shepherd's hut you've got a bit more flexibility than van life, so definitely use that to your advantage. Get those battery and fusebox conversations happening early in the layout. I'd also suggest over-speccing your cable gauge if you've got the space — future-proofing costs pennies now and saves a complete rewire later when you inevitably add kit.

What battery chemistry are you planning? Makes a difference to how picky your system needs to be about voltage stability.

The cable gauge issue is what catches most people out though. @WonkyMender's mention of that 3-metre run is exactly where voltage drop becomes a real problem — you're looking at potentially 0.5V+ loss on a standard 4mm² cable at those distances with any decent load.

I've got about 2.5 metres on my array setup and went with 10mm² just to be safe. Calculator here is your friend: https://www.calculator.net/voltage-drop-calculator.html

Worth noting that if you're doing any serious EV charging from your system (which I am), this becomes even more critical. A poorly sized cable run can throttle your charge rate or trigger low-voltage shutdowns on controllers.

For @RussScott's 200Ah lithium setup — what controller are you using? Some of the newer Victron stuff handles longer cable runs better due to their voltage sensing capabilities, but you're still limited by physics. The rule I follow: if the cable run is over 2 metres, I go up at least one gauge size from what the calculator suggests.

Ambient temperature matters too. Cables in an uninsulated cabin roof will have worse resistivity than indoor runs. Just something to factor in during winter months.

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 my narrowboat — even a modest 100Ah system drawing 50A through undersized cable will lose voltage something shocking over distance.

What nobody mentions enough is the volt drop calculation. You want to keep it under 3% ideally, which means:

• 2-3 metres: 4mm² minimum for 50A loads
• 5+ metres: you're looking at 10mm² or larger

The frustrating bit is copper prices have gone mental, so people skimp. But you'll lose efficiency and damage equipment chasing pennies on cable.

Also worth noting: if you're using a Victron MPPT or similar charge controller, mounting it near the battery (not the solar panels) makes the wiring simpler. I've seen folk do the opposite and create a nightmare.

@RussScott — with 200Ah Li you'll want proper breakers/fuses within 50cm of the battery too. Non-negotiable for lithium systems.