Campervan Battery Cable Size: How to Choose the Right Wire
Undersized cables are one of the most common wiring mistakes in campervan builds. A cable that is too small for the current it carries heats up, wastes energy, and can start a fire. Here is how to size cables correctly for a UK van conversion.
Why mm² matters
In the UK, cable size is measured in cross-sectional area (mm²), not American Wire Gauge (AWG). A larger mm² = more copper = more current-carrying capacity and less resistance.
Key relationship: Resistance per metre decreases as mm² increases. A 6mm² cable has about half the resistance of 4mm² cable per metre.
Current capacity by cable size
These ratings are for flexible copper conductor cable routed in free air (not bundled in conduit, which reduces ratings):
| Cable size | Maximum continuous current | Typical campervan use |
|---|---|---|
| 1.5mm² | 16A | LED lighting circuits |
| 2.5mm² | 25A | 12V accessories, USB sockets |
| 4mm² | 32A | Water pump, diesel heater |
| 6mm² | 40A | DC-DC charger (20–30A) |
| 10mm² | 57A | DC-DC charger (40A), MPPT output |
| 16mm² | 76A | Small inverter (up to 800W) |
| 25mm² | 101A | Medium inverter (1,000–1,200W) |
| 35mm² | 125A | Larger inverter (2,000W) |
| 50mm² | 160A | Large inverter (2,000W+) |
| 70mm² | 200A | MultiPlus 3,000W, main bus bar feed |
Always fuse the circuit at or below the cable's rated current. The fuse protects the cable, not the load.
Voltage drop: why cable length matters
A long cable has more resistance, which causes voltage to drop under load. Excessive voltage drop reduces the power available at the appliance and can cause electronics to malfunction.
Acceptable voltage drop: Under 3% for most 12V circuits. For sensitive electronics, keep drop under 2%.
Voltage drop formula:
Voltage drop (V) = Current (A) × Resistance per metre (Ω/m) × Cable length (m) × 2
(×2 because current flows both ways — positive and return)
Approximate resistance per metre by cable size:
| Cable size | Resistance per metre (Ω/m) |
|---|---|
| 4mm² | 0.0047 |
| 6mm² | 0.0032 |
| 10mm² | 0.0019 |
| 16mm² | 0.0012 |
| 25mm² | 0.00077 |
Example: 25A DC-DC charger, 3m cable run (6m total, positive + negative), 6mm² cable:
- 25A × 0.0032 × 6m = 0.48V drop
- At 12V, that is 4% — slightly high. Go up to 10mm² for this run.
Cable sizing by circuit type
Main battery cables (battery to bus bar)
The main positive and negative cables from your leisure battery to your distribution bus bars carry the total system current — everything that flows in or out. Size for your largest likely simultaneous load.
With a large inverter (1,000W+): Size for the inverter's peak current. A 1,000W inverter at 12V draws ~90A at peak. Use 25mm² minimum.
Without a large inverter: Size for your MPPT output current plus all 12V loads combined. Typically 70–100A → 16–25mm².
Inverter cables (battery/bus bar to inverter)
Size specifically for the inverter rating — this is often the largest cable in the van.
| Inverter | Continuous DC current | Cable size |
|---|---|---|
| 500W | ~45A | 10mm² |
| 1,000W | ~90A | 25mm² |
| 2,000W | ~185A | 50mm² |
| 3,000W (MultiPlus) | ~280A | 70mm² |
Keep these cables as short as possible — every extra metre adds resistance and voltage drop at high current.
Solar MPPT output cable
Size for the MPPT's maximum output current (usually 15A, 30A, or 40A).
- 15A MPPT: 4mm²
- 30A MPPT: 6–10mm²
- 40A MPPT: 10mm²
12V accessory circuits (lighting, fridge, etc.)
Individual circuits with a dedicated fuse can use lighter cable because the fuse limits current to what the cable can handle.
- LED lighting circuit (max 10A): 1.5mm²
- Fridge (5–10A typical): 2.5mm² recommended for runs over 2m
- Water pump (5–8A): 2.5mm²
FAQ
Can I use standard automotive speaker or speaker cable for 12V circuits?
Not for high-current circuits. Speaker cable is rated for audio signal use, not sustained 12V power loads. Use proper automotive-rated cable or flexible welding cable for all power circuits.
Is automotive cable rated the same as electrical installation cable?
Similar but not identical. Automotive cable (to BS 6862) is rated for higher temperatures and more flexible than domestic installation cable. Use automotive or marine-grade flexible cable in a campervan, not domestic twin and earth.
My fridge came with thin cable. Should I replace it?
If the fridge cable is correct for the fridge's current draw and cable length, leave it. If the run is longer than the supplied cable or the supplied cable feels thin for the load, replace with the appropriate size.