How to choose fuses for your camper van: complete guide

A fuse is a deliberate weak point in your electrical circuit. When too much current flows through a cable, whether from a short circuit, a faulty device or an overloaded circuit, the fuse blows before the cable overheats. Without a fuse, the cable itself becomes the weakest point. Overloaded copper wire gets hot enough to melt its insulation, and once bare copper touches something flammable, you have a fire.

In a camper van this risk is amplified. Your cables run through walls, behind panels and under floors, places you cannot see or reach. A smouldering cable behind your wall cladding can go unnoticed until it is too late.

The golden rule of fuse selection: the fuse protects the cable, not the device. You size the fuse based on the maximum safe current the cable can carry, not on what the connected device draws. If you use a 2.5 mm² cable rated for 21A, the fuse should be 20A or less, regardless of whether the device only draws 5A.

Every positive cable leaving your battery or fuse box must be fused. No exceptions. This includes cables to lights, the fridge, USB outlets, the water pump and especially the high-current cable to your inverter.

There are several fuse types commonly used in camper van installations, each suited for different current ranges.

Blade fuses (also called ATO or ATC fuses) are the most common for low to medium current circuits. They come in mini, standard and maxi sizes, covering 1A to 80A. They are cheap, easy to replace and fit in standard fuse holders and fuse boxes. Use them for lighting, USB, fridge, water pump and other circuits up to about 40A.

ANL fuses (also called wafer fuses or strip fuses) handle high currents from 40A up to 500A or more. They are used for the main battery cable, inverter cable and battery-to-battery (B2B) charger connections. They require a dedicated ANL fuse holder bolted to the cable lugs.

Mega fuses are similar to ANL fuses and cover 100-500A. They are often used as the main battery fuse.

Circuit breakers are resettable alternatives to fuses. Instead of replacing a blown fuse, you simply flip the breaker back. They are convenient for circuits that occasionally trip, but they are more expensive and bulkier. Some builders use a main circuit breaker as a master disconnect.

NH fuses (Niederspannungs-Hochleistung, low-voltage high-performance) are industrial-grade fuses used in some high-end camper van builds for the main battery protection. They are very reliable but require specific fuse holders.

A fuse protects the cable, not the appliance. That single sentence decides where it goes: at the start of the cable, on the side the energy comes from, and as close to the source as possible.

At a battery that means within roughly 20 to 30 centimetres of the terminal. The piece of cable between battery and fuse is not protected; damage it and the battery delivers its full short circuit current, hundreds of amps with nothing to interrupt it.

Where a fuse belongs:

• On every battery positive cable, close to the battery
• At both ends of a cable linking two batteries or two sources, because energy can arrive from either side
• At the input of the fuse box, as the main fuse for the distribution circuit
• On the battery side of the inverter, the charge controller and the B2B charger

What needs no fuse is the negative cable. It should specifically not have one: if the negative is interrupted while the positive stays live, the current looks for another way through the chassis or through a signal wire.

The fuse protects the cable, not the device. That is the whole principle: the rating stays below what the cable can take, and above what the appliance normally draws.

The steps:

1. Determine the cross-section of the circuit's cable
2. Look up what that cable can take under the conditions it actually runs in
3. Pick a fuse below that, but comfortably above the appliance's normal draw

What a copper conductor can take depends on its cross-section, the temperature and the number of cables in the bundle. This is what is left for a cable running behind the panelling in a bundle of three, at 40 degrees:

• 1.0 mm²: around 15 A
• 1.5 mm²: around 19 A
• 2.5 mm²: around 23 A
• 4 mm²: around 31 A
• 6 mm²: around 42 A
• 10 mm²: around 54 A
• 16 mm²: around 77 A
• 25 mm²: around 104 A
• 35 mm²: around 131 A

Run that same cable loose and cool and about a quarter more goes through: 1.5 mm² reaches 25 A, 6 mm² reaches 55 A and 35 mm² reaches 170 A. That difference is exactly why you should not follow a fixed table blindly. The fuse calculator asks for the temperature and the number of cables in the bundle and works it out for your circuit.

An example: your fridge draws at most 6 A and you used 2.5 mm², in a bundle. That cable takes 23 A there. So you pick a 10 A fuse. Not 20 A, because then the fuse only acts once the cable is well past its comfortable load.

For appliances with an inrush current, such as a compressor fridge or a water pump, choose a slow-blow fuse or go one size up. Then check that the cable still carries that larger rating.

In an installation with several fuses in series, the smallest must always blow first. Otherwise a short in a reading light takes down your whole system, or worse, the small fuse blows while the cable ahead of it is already burning.

The resulting order: each load circuit has the smallest fuse, the cable to the fuse box has a larger one, and the battery fuse is the largest. Every link is at least as large as everything behind it, but never larger than its own cable can handle.

That is where the common mistake sits: people add up all the circuits and take that sum as the main fuse. That is only allowed if the cable to the fuse box can genuinely carry that sum. If it cannot, the cable sets the fuse rather than the arithmetic, and you either upsize the cable or split the circuits across two boxes.

Check the chain from the back forwards: load, circuit fuse, cable, main fuse, cable, battery fuse. At every step the cable has to be heavier than its fuse.

A fuse is a wire that burns through; a breaker is a switch that trips. They do the same job, but not equally well everywhere.

A fuse is cheap, small, reliable and has no moving parts. It is single use: blown means replaced, so you need spares in the sizes you use. For load circuits and for the heavy battery fuse this is the usual answer.

A breaker can be reset and doubles as a switch, which helps on a circuit you regularly want to isolate. The downsides are price, size, and the fact that a cheap breaker is in practice less accurate than a fuse costing the same.

A workable division: fuses for everything fitted and forgotten, breakers where you work regularly or want a separate off switch. On the 230 volt side the picture is different: there, miniature circuit breakers and a residual current device are the norm, and those two do not replace each other.

A well-organized fuse box is the heart of your 12V distribution system. It is where all your circuits come together, each individually fused and easy to troubleshoot.

Choose a fuse box with enough slots for all your circuits plus a few spares for future expansion. A 12-way blade fuse box is a good starting point for most camper vans. Mount it in an accessible location, not behind a panel you need to unscrew. Many builders install it under the bed or in a dedicated electrical cabinet.

Feed the fuse box from your battery through a main fuse (typically an ANL or Mega fuse). This main fuse protects the cable between battery and fuse box. Size it for the cable, not the total of all circuits.

Label every circuit clearly. Use a label maker or write on tape which fuse goes to which circuit: fridge, lights kitchen, lights bedroom, USB, water pump and so on. This saves hours of troubleshooting later.

Connect the negative (ground) cables to a common bus bar near the fuse box. Use a separate bus bar for negative connections rather than running all grounds back to the battery individually. This keeps your wiring tidy and makes fault-finding much easier.

Consider a combined fuse and relay box if you want switched circuits. Some fuse boxes come with integrated relays that let you control circuits from a switch panel, a clean solution for a professional-looking installation.