Campervan Cable Sizing Calculator: Wire Size Guide for 12V Electrical Systems
Undersized cable is the most common mistake in DIY campervan electrical systems. It causes voltage drop (poor charging, dim lights), overheating (fire risk), and mysterious equipment failures that are actually just the equipment not getting enough voltage.
I have seen the consequences of undersized cable twice. A friend's DC-DC charger was connected with 6mm² cable over a 6m run. The charger output 2A instead of 30A because the voltage drop was so severe. Another builder's 1,000W inverter was wired with 10mm² cable — it melted the insulation on the first full-power test.
This guide covers everything you need to size cable correctly for a 12V campervan system.
Why Cable Size Matters
Cable resistance causes voltage drop. In a 12V system, every 0.1V drop is significant. The alternator charges at 14.4V. If you lose 0.5V in the cable, the battery sees 13.9V — not enough to fully charge a LiFePO4 battery.
| Cable Length (both positive + negative total) | 6mm² Cable Drop at 50A | 16mm² Cable Drop at 50A | 25mm² Cable Drop at 50A |
|---|---|---|---|
| 2m | 0.14V (1.0%) | 0.06V (0.4%) | 0.04V (0.3%) |
| 5m | 0.36V (2.5%) | 0.14V (1.0%) | 0.09V (0.6%) |
| 10m | 0.72V (5.0%) | 0.28V (1.9%) | 0.18V (1.2%) |
Target: Keep voltage drop below 3% (0.36V at 12V, 0.43V at 14.4V). For critical circuits (DC-DC charger, solar, inverter), aim for 1–2%.
Wire Size Chart
Calculate the total cable length (positive + negative run) and the maximum current. Find the minimum wire size in this table.
| Current (A) | 1m | 2m | 3m | 4m | 5m | 6m | 8m | 10m |
|---|---|---|---|---|---|---|---|---|
| 5A | 1mm² | 1mm² | 1.5mm² | 1.5mm² | 2.5mm² | 2.5mm² | 4mm² | 4mm² |
| 10A | 1.5mm² | 2.5mm² | 2.5mm² | 4mm² | 4mm² | 6mm² | 6mm² | 10mm² |
| 15A | 2.5mm² | 2.5mm² | 4mm² | 6mm² | 6mm² | 10mm² | 10mm² | 10mm² |
| 20A | 4mm² | 4mm² | 6mm² | 6mm² | 10mm² | 10mm² | 16mm² | 16mm² |
| 30A | 4mm² | 6mm² | 10mm² | 10mm² | 16mm² | 16mm² | 16mm² | 25mm² |
| 40A | 6mm² | 10mm² | 10mm² | 16mm² | 16mm² | 25mm² | 25mm² | 25mm² |
| 50A | 10mm² | 10mm² | 16mm² | 16mm² | 25mm² | 25mm² | 35mm² | 35mm² |
| 70A | 10mm² | 16mm² | 25mm² | 25mm² | 25mm² | 35mm² | 35mm² | 50mm² |
| 100A | 16mm² | 25mm² | 25mm² | 35mm² | 35mm² | 50mm² | 50mm² | 70mm² |
| 150A | 25mm² | 35mm² | 50mm² | 50mm² | 70mm² | 70mm² | 95mm² | 95mm² |
Assumptions: 12V system, 3% max voltage drop, copper cable with 70°C insulation.
Common Circuit Sizes
| Circuit | Typical Current | Typical Length | Recommended Cable |
|---|---|---|---|
| LED lights (×4) | 2A | 5m | 1.5mm² |
| 12V socket | 5A | 3m | 2.5mm² |
| Water pump | 5A | 3m | 2.5mm² |
| Fridge | 5A | 5m | 4mm² |
| Diesel heater | 10A | 3m | 2.5mm² |
| USB sockets | 10A | 3m | 4mm² |
| DC-DC charger (30A) | 30A | 3m | 10mm² |
| DC-DC charger (50A) | 50A | 3m | 16mm² |
| Solar (from roof, 20A) | 20A | 5m | 6mm² |
| Battery to fusebox | 30A | 1m | 6mm² |
| Inverter (800W) | 67A | 1m | 16mm² |
| Inverter (1,000W) | 83A | 1m | 25mm² |
| Battery bank link | 100A | 0.5m | 25mm² |
Fuse Sizing
Every circuit needs a fuse within 30cm of the power source (battery or fusebox). The fuse protects the cable, not the appliance.
| Cable Size | Max Fuse Rating | Typical Application |
|---|---|---|
| 1.5mm² | 15A | Lights, small loads |
| 2.5mm² | 20A | Sockets, small circuits |
| 4mm² | 30A | Fridge, USB, small sub-circuits |
| 6mm² | 40A | Solar input, main feed |
| 10mm² | 60A | DC-DC charger input |
| 16mm² | 80A | Inverter, battery main |
| 25mm² | 120A | Large inverter, battery bank |
| 35mm² | 150A | Very large inverter |
Rule: Fuse rating should be 125% of the expected maximum load or 100% of the cable rating, whichever is smaller.
Cable Types
| Cable Type | Use | Pros | Cons |
|---|---|---|---|
| Single-core (thinwall) | Fixed wiring in conduit | Cheap, easy to route | Less flexible, harder to terminate |
| Multi-strand | General 12V wiring | Flexible, easy to work with | More expensive |
| Tinned marine cable | Damp environments, roof | Corrosion-resistant | Expensive, overkill for most vans |
| Solar cable (PV) | Roof to charge controller | UV-resistant, double insulated | Rigid, hard to bend tightly |
| Welding cable | Battery to inverter | Very flexible, high strand count | Expensive, overkill |
Recommendation for most van wiring: Multi-strand copper cable with 70°C PVC insulation. Tinned cable for exposed areas (roof, underfloor).
Connectors
The cable is only as good as its connection. Poor connections create resistance, heat, and voltage drop.
| Connector Type | Use | Reliability |
|---|---|---|
| Crimp ring terminal | Battery, inverter, DC-DC charger terminals | Excellent (with proper crimp tool) |
| MC4 connector | Solar panels | Excellent (weatherproof) |
| Butt splice | Joining two cables | Good (with heat shrink) |
| Screw terminal (Wago) | Junction boxes | Good (for low-current circuits) |
| T-tap / Scotch lock | Avoid | Poor — creates unreliable, high-resistance joints |
The right crimp tool matters. A hammer crimper crimps battery lugs acceptably. For small terminals (1.5–6mm²), use a ratchet crimper (£20–40). A combination crimp tool from Halfords does not create reliable gas-tight connections — the dies are wrong for automotive terminals.
Installation Best Practices
- Cable runs: Keep runs as short as possible. Route cables along existing van wiring paths. Avoid running cable where it can chafe on metal edges — use grommets through bulkheads.
- Cable clipping: Secure cable every 30cm with P-clips or cable ties. Do not use metal cable clips (they cut through insulation over time).
- Colour coding: Red = 12V positive (permanent or switched). Black = 12V negative (ground). Brown = 240V live. Blue = 240V neutral. Green/yellow = 240V earth.
- Fuse placement: Every positive cable leaving the battery needs a fuse within 30cm. Every positive cable leaving the fusebox needs a fuse.
- Ground connections: Clean the chassis connection point to bare metal. Use a star washer between the ring terminal and the chassis. Apply vaseline or dielectric grease to prevent corrosion. Tighten to 10–12 Nm.
FAQ
Q: What happens if I use undersized cable? A: The cable heats up, voltage drops, and the equipment at the end of the cable receives less voltage than it needs. A DC-DC charger with an 0.5V drop on the input will output 10–20% less current. An inverter with low input voltage will alarm and shut down. In extreme cases, the cable insulation melts and causes a fire.
Q: Can I use automotive wire from Halfords for campervan wiring? A: Yes, for low-current circuits (lights, sockets). For high-current circuits (inverter, DC-DC charger), use proper welding cable or tinned marine cable — the strand count is higher, making the cable more flexible and less prone to fatigue failure.
Q: How do I calculate voltage drop for a specific cable? A: Voltage drop = (2 × length in metres × current in amps × cable resistance in ohms per metre) ÷ 1,000. For a shortcut, use the chart above.
Q: Is it better to oversize cable? A: Yes, within reason. Oversized cable runs cooler, has less voltage drop, and allows for future upgrades. The downside is cost and physical size (25mm² cable is stiff and hard to route around tight corners).
Q: Can I share a ground cable between multiple circuits? A: Yes, if the ground cable is sized for the total current of all circuits sharing it. A common ground busbar is standard practice. Each circuit still needs its own positive fuse.
