Van Life Skills Series · Article 30
Wiring is the step most van builders get wrong — and the one that causes fires. Here's how to design a safe 12V system, size every component correctly, and wire it up in the right order, without an electrician.
Of all the van build steps, electrical wiring generates the most anxiety and the most forum threads. It deserves both — done wrong, it causes fires. Done right, it's the system you'll use every single day. The good news: van 12V wiring is not complicated once you understand three principles: everything needs a fuse, wire size must match the current, and grounds matter as much as positives.
This guide covers the wiring phase of a van build — after your insulation and before your interior. If you're starting with solar panels and batteries, read our solar setup guide first. This guide focuses on the wiring that connects everything together safely.
Prerequisites: This guide assumes you've already chosen your battery bank (we recommend LiFePO4/lithium for van life) and solar charge controller. You need those specs to size your wire and fuse correctly. If you haven't done that yet, start with the solar setup guide.
A complete van electrical system has six components that all wire together:
The wiring order: Solar panels → Charge controller → Battery → Fuse block → Everything else. The inverter connects directly to the battery (large wire, large fuse). The DC-DC charger connects between the van's starter battery and your house battery.
Draw your complete system diagram before purchasing any wire. Every component, every connection, every fuse. Estimate the distance from each component to the battery, then double it (wire runs both positive and negative). Most first-time builders underestimate wire length by 30–40%.
Free tools: Victron's wiring diagram tool, or hand-draw on paper. The goal is to know every wire length and current before you buy anything.
The battery goes in before any other wiring. Common locations: under the bed platform (most common), behind the driver's seat, or in a ventilated box. Lithium batteries can be mounted in any orientation and need no ventilation (unlike lead-acid). Secure them so they cannot move — battery mount failures at highway speed can cause catastrophic shorts.
Main fuse: Within 18 inches of the battery positive terminal, install an ANL fuse or MIDI fuse sized at 125% of your maximum continuous system current. For a 200Ah battery with a 2,000W inverter: peak draw is ~167A. A 200A ANL fuse is appropriate. Never skip this fuse.
The main cable from battery positive to fuse block (and inverter) must be sized for peak system current. Use 4/0 AWG marine-grade wire for systems up to 300A. Tinned copper marine wire costs more than automotive wire but won't corrode in the humid environments a van can see.
Grounding: Run a dedicated negative cable from the battery negative to a grounding bus bar mounted near the battery. Then run a separate negative cable from the grounding bus to the van chassis (a grounded bolt on the floor). All accessories ground to the bus bar, not directly to the chassis — this prevents ground loops that cause electrical noise in audio systems and false sensor readings.
Mount the charge controller close to the battery — within 3–5 feet if possible. The wire from panels to controller should be sized for the controller's maximum input current (typical MPPT controllers: 20–60A). Use 10 AWG for most 30–40A controllers; 8 AWG for 60A.
Run the panel wires through the van's ceiling before insulating. Most van builders use MC4 connectors at the roof penetration with a cable gland (waterproof seal) through the roof skin. The entry point must be sealed against water — a slow leak at a roof penetration will destroy your insulation and ceiling panels over time.
A DC-DC charger (also called a B2B charger or battery-to-battery charger) connects your van's starter battery to your house battery and charges the house battery from the alternator while the engine runs. This is the second most important charging source after solar.
Why not a simple isolator? Smart alternators in modern Sprinters and Transits (2014+) detect battery voltage and reduce output when they sense a charged battery — a simple isolator fools the alternator into thinking batteries are always low, potentially causing alternator failure. A DC-DC charger presents a constant load that the alternator handles correctly. Get a Victron Orion-TR Smart or Sterling B2B.
Wire the DC-DC charger: from the van's starter battery positive (fused), through the charger, to the house battery positive. Run a trigger wire from a switched 12V source (ignition-switched) to tell the charger to operate only when the engine is running.
The fuse block is your distribution center — each output feeds one circuit (lighting, USB ports, 12V outlets, fan, fridge, water pump). Size each fuse at 125% of the device's rated current. Size each wire at its circuit's expected current.
Standard branch circuit wire sizes for a van:
Use ring terminals or ferrules at every connection — never bare wire. Crimped connections are more reliable than soldered in vibration environments. Use a proper ratchet crimper, not pliers.
The inverter handles your 120V AC loads (laptop, monitor, small appliances). Size it at 1.5–2x your maximum simultaneous AC draw. Most van builders: 1,000–2,000W pure sine wave inverter.
Wire the inverter directly to the battery — not through the fuse block. The large current draw (a 2,000W inverter can pull 167A at peak) would overload most fuse blocks. Use large cable (4 AWG for 1,000W, 2/0 AWG for 2,000W) and an inline ANL fuse sized for the inverter's peak current.
Mount the inverter close to the battery — every foot of cable is voltage drop and money. Under the bed near the battery is ideal. Inverters generate heat; ensure airflow around the unit.
| Circuit | Typical Current | Recommended Wire | Fuse Size |
|---|---|---|---|
| Battery to main fuse | System max (100–300A) | 4/0 or 2/0 AWG | ANL 150–300A |
| Main fuse to fuse block | Same as above | 4 AWG minimum | ANL 60–100A |
| Solar panels to controller | 20–60A | 8–10 AWG | In-line 30–60A |
| Controller to battery | 20–60A | 8–10 AWG | In-line 30–60A |
| DC-DC charger to battery | 20–40A | 10 AWG | In-line 25–40A |
| Battery to inverter (1000W) | Up to 90A | 4 AWG | ANL 100A |
| Battery to inverter (2000W) | Up to 167A | 2/0 AWG | ANL 200A |
| Fridge circuit | 5–10A | 12 AWG | 15A |
| Lighting / USB per circuit | 2–5A | 16 AWG | 7.5–10A |
Undersized wire heats up under load. At best, this causes voltage drop that makes your devices run poorly. At worst, it melts insulation and causes fires. Always size wire for the maximum expected current, not the average current. Use a wire sizing calculator and round up to the next gauge.
A fuse protects the wire, not the device. Size the fuse for the wire's capacity, not the device's rating. A 14 AWG wire can safely carry 15A — so the fuse should be 15A even if the device only draws 8A. If you put a 30A fuse on a 14 AWG wire, the wire melts before the fuse blows.
Poor grounds are responsible for more electrical gremlins than anything else — flickering lights, random device resets, false low-battery readings, and inverter faults. Use large wire for your ground bus (same size as your positive main cable), sand the chassis contact point down to bare metal before connecting, and use star washers on all chassis ground connections to maintain contact through vibration.
Fire safety: Keep a 2.5 lb dry chemical or CO2 fire extinguisher mounted in the van — not in a storage box, but accessible from the driver's seat or immediately inside the side door. Lithium battery fires are rare but extremely dangerous. Know your exit. If your battery smells like burning plastic, ventilate immediately and get away from the vehicle.
This covers wiring only — not the battery, solar panels, controller, inverter, or DC-DC charger. For those components, see the solar setup guide which includes full build costs for two complete system sizes.
Wire gauge depends on the current and run length. Battery to fuse block main cable: 4/0 or 2/0 AWG. Solar controller to battery: 8–10 AWG. Fuse block branch circuits for lighting and USB: 14–16 AWG. Fridge circuit: 12 AWG. Battery to inverter (2,000W): 2/0 AWG. Never undersize — use a wire sizing calculator for every circuit.
Not required, but useful. A shore power inlet lets you charge from a campsite pedestal, a friend's garage, or any outlet when solar isn't enough — common in winter. Many van builds skip it and rely on solar plus a DC-DC charger from the alternator. If you'll camp at RV parks occasionally, add shore power while the walls are open.
Calculate your daily amp-hour (Ah) consumption first. Most builds consume 50–150Ah per day. Lithium batteries can be used to 80–90%, so 200Ah lithium gives 160–180Ah usable. A single 100Ah lithium handles minimal builds (lights, USB, fan). Two 100Ah (200Ah total) covers most builds including a 12V fridge. Add more for electric cooking or heavy laptop use.
Yes, with careful planning and proper fusing. Key safety rules: every circuit must be fused close to the power source, never undersize wire, always disconnect the battery negative before working, and have your wiring reviewed before first power-on. Many van lifers successfully self-build complete electrical systems — the Faroutride wiring guide and Victron's documentation are excellent resources.