Campervan Electrical Cable Sizing Guide UK

A fridge that cuts out when the water pump starts, dim lights at the rear doors, or an inverter alarm under load are often cable-sizing problems rather than faulty appliances. This campervan electrical cable sizing guide sets out how to choose 12V cable properly, based on current, cable run and acceptable voltage drop - not guesswork or the diameter of a cable’s outer insulation.

Campervan electrics are usually low voltage, but they are not low risk. A 12V system can supply very high current. Undersized cable creates resistance, heat and voltage loss; if the circuit is not correctly fused, it can become a fire risk. Size the cable for the circuit it serves, protect it with a suitable fuse, and install it so vibration, sharp metal and moisture cannot damage it.

Start with the load, not the cable reel

Every cable choice begins with the appliance’s maximum current draw. This should be shown on the product label, instruction manual or data sheet. For a 12V appliance, calculate current with:

Current (amps) = power (watts) ÷ voltage (volts)

A 60W compressor fridge at 12V draws around 5A while the compressor is running. A 120W water pump can draw 10A, although its normal operating current may be lower. Use the manufacturer’s stated maximum or fuse recommendation where available, especially for pumps, heaters and motors that have a higher start-up current.

For an inverter, calculate from its potential output rather than what you expect to plug into it on a typical day. A 1,000W inverter can draw more than 80A from a 12V battery before allowing for inefficiency. At that level, cable selection, terminal quality, fuse type and cable length all matter significantly.

Do not size a cable from the combined capacity of your leisure battery. A 100Ah battery does not automatically require large cable throughout the van. The cable must suit the current of its individual circuit. The main battery-to-fuse-box feed, however, must safely carry the likely combined load of everything supplied downstream.

Cable size means conductor area in mm²

For UK campervan builds, cable is normally specified by copper conductor cross-sectional area in square millimetres, written as mm². The useful figure is 2.5mm², 6mm² or 16mm², for example. It is not the outside diameter, and it is not simply the thickness of one visible copper strand.

Use flexible, multi-strand automotive cable for most 12V DC installations. It copes better with vehicle vibration and movement than solid-core cable. Twin-core sheathed cable is tidy for many appliance runs, while separate red and black cables can be practical for larger battery and inverter connections. Keep polarity consistent throughout the build: red for positive and black for negative is the usual convention.

American Wire Gauge markings can cause confusion when buying online. AWG gets smaller as the conductor gets larger, while mm² gets larger as the conductor gets larger. Check the stated copper area rather than assuming the two systems directly match.

Calculate voltage drop using the full circuit length

A cable can be rated to carry an appliance’s current and still be too small for a long campervan run. This is because voltage drop increases with both current and distance. At the far end of a van, a compressor fridge, diesel heater or USB charging point may receive less voltage than the battery is supplying.

Measure the route the cable will actually take, including rises into furniture, runs around wheel arches and sections through conduits. Then double that figure. Current travels from the battery or distribution point to the appliance through the positive cable and returns through the negative cable, so both sides of the circuit count.

A practical target is to keep voltage drop at around 3% or less for sensitive 12V equipment and longer continuous loads. For a 12V system, 3% is only 0.36V. Lighting may tolerate a little more, but voltage-sensitive equipment, particularly fridges and heaters, benefits from a low-loss supply.

The simplified calculation is:

Voltage drop = current × total cable length × cable resistance per metre

Cable manufacturers publish resistance figures, so use the data for the exact cable where possible. Online calculators are useful as a check, but they only work if you enter the return length, system voltage and maximum current correctly. When a calculated size sits close to the limit, move up a cable size. The cost difference is small compared with replacing cable behind a finished interior.

Typical 12V cable applications

The table below is a sensible starting point for short to moderate runs in a campervan. It is not a substitute for checking voltage drop and current capacity on your own installation.

| Typical circuit | Common current range | Usual starting cable size |
|---|---:|---:|
| LED lighting, control panels and small USB outlets | Up to 5A | 1.0mm² to 1.5mm² |
| Water pump, 12V sockets and medium fridge runs | 5A to 15A | 2.5mm² to 4mm² |
| Larger fridge feeds, fans and diesel heater supplies | 10A to 20A | 4mm² to 6mm² |
| Main DC distribution feed | 20A to 50A | 6mm² to 16mm² |
| Battery charger, DC-DC charger or larger inverter | 40A and above | 16mm² and above |

Cable runs in a long-wheelbase van may need a larger size than the table suggests. A 15A circuit running two metres from a fuse box is very different from one running seven metres to the rear of the vehicle. Equally, a 1,500W inverter or high-output DC-DC charger should be specified from its installation manual, not selected from a generic chart.

Fuse the cable to protect the cable

A fuse is there primarily to protect the cable from carrying more current than it safely can. Fit it as close as practical to the positive power source, usually the leisure battery, before the cable travels through the van. Each branch circuit from a fused distribution board also needs its own correctly rated fuse.

Choose a fuse rating above the appliance’s normal maximum current but below the safe current rating of the cable. If a 10A appliance is supplied by appropriately rated 2.5mm² cable, a 15A fuse may be suitable depending on the appliance’s start-up demand and the cable specification. Do not fit a larger fuse just because a smaller one blows. Find out whether the issue is start-up current, a poor connection, a seized motor or a cable fault.

High-current circuits may need specialist fuses and holders. Inverter, battery-to-battery charger and main distribution feeds can require MIDI, MEGA or similar heavy-duty protection rather than a small blade fuse. Match every terminal, fuse holder, isolator and busbar to the circuit current as well as the cable size.

Account for heat, bundling and installation route

Current ratings quoted for cable are not fixed in every situation. A cable clipped in open air can shed heat more easily than one bundled tightly with several other loaded cables, enclosed in insulation or routed through a hot engine bay. Allow extra margin where cables pass through confined furniture voids or share conduit with other power circuits.

Keep 12V cables away from sharp edges, moving seat bases, exhaust heat and areas likely to collect water. Use grommets wherever cable passes through metal, and protect exposed runs with split conduit or suitable trunking. Support the cable at regular intervals so it cannot chafe against the vehicle body over time.

Avoid relying on the van body as the negative return for leisure circuits. Dedicated negative cables back to a negative busbar make faults easier to trace and reduce the chance of poor earth connections causing intermittent problems. The leisure electrical system may still need a correctly designed connection to vehicle chassis earth, but that is not a reason to use random bodywork points as every appliance return.

Separate 12V DC and 230V AC work

The principles above apply to low-voltage DC wiring. Mains wiring is a different installation with different cable types, protective devices, earthing arrangements and safety requirements. Use correctly rated 230V cable, keep it segregated from DC wiring, and have mains work designed, installed or inspected by a competent person.

Do not run 230V and 12V conductors together in the same unsuited conduit or enclosure. It makes future fault-finding harder and can create avoidable safety issues. Label both systems clearly, especially around chargers, consumer units and inverters where AC and DC equipment often sit close together.

Plan cable routes before fitting the furniture

Cable sizing is easiest when it is planned alongside insulation, windows, furniture and appliances. Mark the position of the leisure battery, fuse board, charger, fridge, water pump, lighting switches and any future solar or inverter equipment before panels go on. Leave service loops where practical, label both ends of every cable, and photograph hidden routes before lining and furniture conceal them.

For builders adding a bespoke interior, decide where access panels, battery compartments and cable channels will sit before finalising the furniture layout. A clean installation is not one with no visible cable at any cost; it is one where every cable is protected, supported, fused and still serviceable.

Choose cable generously where the route is long or the equipment is critical. A slightly larger conductor is cheap insurance against voltage drop, nuisance cut-outs and the unpleasant job of reopening a finished campervan wall.