Campervan Battery Cable Size: Current and Voltage Drop

Battery cable must support the design current under its installed conditions and keep voltage drop acceptable over the complete circuit.

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.

Worked planning check

At 100A, an illustrative total circuit resistance of 0.004 ohms creates 0.4V drop and 40W of heat. That includes cable and connections. A short battery link still needs a fault-current and protection assessment; low voltage drop alone does not prove safety.

The figures or dimensions in this example are explicit planning assumptions, not a product test or an installation specification. Replace them with your equipment data and actual measurements.

Planning questions

Can I choose a fuse just from the appliance current?

No. Check cable ampacity under the installed conditions, equipment instructions, terminal limits, DC voltage and interrupt capacity. Protection must coordinate with the complete circuit.

Does a wiring diagram prove the installation is safe?

No. It is a planning record. Actual routing, termination, protection and commissioning results must agree with the design and applicable requirements.

When should I involve an electrician?

Before work that exceeds your competence, particularly mains design, source transfer, bonding, high-current faults and final inspection or testing. Agree the scope and evidence of verification in advance.

Use the free electrical planner to organise your daily loads and charging assumptions.