Which Fuse for Which Cable? Campervan Fuse Sizing Guide
Every positive cable in your campervan must be fused. Not most cables — every single one. The fuse protects the cable, not the appliance. If a cable shorts against the van body and the fuse does not blow, the cable overheats, melts its insulation, and starts a fire. This is the number one cause of campervan electrical fires.
Getting fuse sizing right is not complicated, but it requires understanding one key principle: the fuse rating must be lower than the cable's current capacity. The fuse blows before the cable overheats. In this guide we cover every fuse in a typical campervan build and how to size each one.
For the complete wiring picture, see our campervan fuse box wiring guide and wire gauge calculator.
Size your whole system
Our free calculator recommends cable gauges and fuse ratings for every circuit based on your actual loads.
The Golden Rule: Fuse the Cable, Not the Load
A fuse is sized to protect the cable it is connected to, not the appliance at the other end.
If your cable can carry 30A safely, your fuse must be 30A or less. If you fit a 60A fuse on a cable rated for 30A, and the cable shorts, the fuse will not blow — the cable will carry 60A until it burns through its insulation.
The appliance's own overcurrent protection (if it has any) is a separate matter. Your fuse sizing is about protecting the wiring.
Cable Current Ratings
Before sizing fuses, you need to know your cable ratings. Here are typical values for automotive-rated single-core cable in a campervan installation:
| Cable Size | Max Current (open air) | Max Current (bundled) | Typical Use |
|---|---|---|---|
| 1.5mm² | 17A | 13A | LED lighting, small accessories |
| 2.5mm² | 24A | 18A | USB sockets, water pump, fans |
| 4mm² | 32A | 25A | Fridge, larger accessories |
| 6mm² | 41A | 32A | DC-DC charger output, inverter small |
| 10mm² | 57A | 44A | MPPT output, mid-size inverter |
| 16mm² | 76A | 59A | Main distribution, large inverter feed |
| 25mm² | 101A | 78A | Battery to bus bar |
| 35mm² | 125A | 96A | Heavy inverter feed |
| 50mm² | 151A | 117A | Main battery negative/positive |
"Bundled" means the cable runs alongside other cables and cannot dissipate heat as freely.
The Main Battery Fuse
What It Does
The main fuse sits as close to the battery positive terminal as possible — ideally within 30cm. It protects the entire downstream wiring system. If any cable downstream shorts, this fuse is the last line of defence.
How to Size It
The main fuse should be rated just above your maximum simultaneous load, but never higher than your main cable can carry.
For a typical mid-range build with a 200Ah lithium battery, 30A DC-DC charger, and 2000W inverter:
- Inverter at full load: 2000W / 12V = 167A
- DC-DC charger: 30A
- Other 12V loads: ~20A
- Peak simultaneous draw: ~220A
A 250A ANL or MIDI fuse on 50mm² cable is appropriate. Use an ANL fuse holder or MIDI fuse holder for currents above 80A — standard blade fuses are not rated for these levels.
Fuse within 30cm of the battery positive
The cable between battery positive and the main fuse is unfused. Keep it as short as possible — 30cm is the target, 50cm absolute maximum. Any longer and you have significant unfused cable that can start a fire if it chafes against the chassis.
Fuse Box / Blade Fuse Ratings
Your 12V fuse box distributes power to individual circuits. Each way gets its own blade fuse. Size each one to the cable on that circuit:
| Circuit | Cable Size | Max Fuse Rating | Typical Fuse Used |
|---|---|---|---|
| LED lighting | 1.5mm² | 16A | 10A |
| USB charging sockets | 2.5mm² | 23A | 15A |
| Water pump | 2.5mm² | 23A | 15A |
| 12V sockets | 2.5mm² | 23A | 20A |
| Compressor fridge | 4mm² | 30A | 20A |
| Diesel heater | 2.5mm² | 23A | 15A |
| Roof vent fan | 2.5mm² | 23A | 10A |
| Battery monitor shunt | 1.5mm² | 16A | 5A |
| Bluetooth relay/monitor | 1.5mm² | 16A | 5A |
Notice the fuse rating is always lower than the cable's maximum — often significantly lower. The fuse is sized to the cable's weakest point, which might be a connector, a run through a tight gland, or a section routed alongside a heat source.
Size down, not up
When in doubt, use a lower fuse rating. A 10A fuse on a 15A-rated circuit will blow under a fault before the cable overheats. A 20A fuse on the same circuit might not. The downside of undersizing is nuisance blowing — easy to fix. The downside of oversizing is fire.
Inline Fuses for Short Runs
Some components sit away from the main fuse box and are connected with relatively short cable runs directly from the battery or bus bar. Each of these needs its own inline fuse close to the power source:
DC-DC Charger Input
Fit a fuse on the input cable from the starter battery side (the vehicle's existing wiring usually has its own protection) and on the output side going to your leisure battery. Size to the charger's rated input current — typically 30-60A depending on model. Use a MIDI or blade fuse in a waterproof holder.
MPPT Charge Controller
Fuse the cable between battery positive and the MPPT's battery terminal. Size to 1.25x the MPPT's maximum output current. A 30A MPPT gets a 40A fuse on a suitable cable.
Solar Panel Input
Fuse between the solar panels and MPPT if the cable run is long or passes through areas where it could be damaged. Most MPPT controllers specify a maximum PV input fuse — check your manual.
Inverter
The inverter typically draws the most current of any device in your system. Use an ANL fuse holder as close to the battery as possible. Size the fuse to the maximum continuous current draw:
| Inverter Size | Max DC Current (12V) | Recommended Fuse |
|---|---|---|
| 600W | 60A | 80A ANL |
| 1000W | 100A | 125A ANL |
| 1500W | 150A | 175A ANL |
| 2000W | 200A | 250A ANL |
| 3000W | 300A | 350A ANL |
Fuse Types Explained
Blade Fuses (ATO/ATC)
Standard automotive blade fuses in mini, standard, and maxi sizes. Used in fuse boxes for loads up to 30-40A. Cheap, widely available, easy to replace. Not suitable for high-current applications.
MIDI Fuses
Bolt-down fuses in a sealed plastic body. Rated from 30A to 200A. Good for intermediate applications like DC-DC charger cables and medium-sized inverters.
ANL Fuses
Larger bolt-down fuses for main battery protection and large inverters. Available from 80A to 500A. Require a dedicated ANL fuse holder. These are the standard for serious campervan builds.
Automotive Breakers (CBs)
Some builders use resettable circuit breakers instead of fuses on the main battery connection. These are convenient but respond slightly slower than fuses under a dead short. Acceptable for most applications; not recommended as the sole protection on an inverter feed.
Fuse Placement Rules
- Main fuse: as close to battery positive as physically possible — within 30cm
- Inline fuses: within 30cm of the power source, not at the appliance end
- Fuse box: near the battery, not at the far end of a long cable run
- Accessible: you need to be able to reach every fuse without dismantling the van
Common Mistakes
Fitting the Fuse at the Appliance End
A fuse at the appliance end protects the appliance, not the cable. If the cable shorts halfway along its run, the fuse cannot see the fault. Fuses go at the source.
Oversizing "Just in Case"
"I'll fit a 30A fuse in case I want to add something later" — on a 1.5mm² cable. When that cable faults, the 30A fuse will not blow. The cable will. Fuse the cable you have, not the cable you might fit one day.
Using the Wrong Fuse Type
A standard 20A blade fuse in a holder rated for 15A will run hot and eventually fail — or worse, start a fire. Check the current rating of your fuse holders as well as the fuses themselves.
Not Fusing the Negative Side of the Inverter
The inverter negative cable carries the same current as the positive. While it is conventional to fuse the positive only, ensure your negative cable is also correctly sized — it must match the positive cable gauge.
FAQ
Can I use a higher-rated fuse temporarily if I don't have the right one?
No. A higher-rated fuse does not protect the cable. Fit the correct fuse before using the circuit. Keep a selection of spare blade fuses in every rating you use.
My fuse keeps blowing — should I fit a higher-rated fuse?
Never. A blowing fuse is a symptom of a problem — too high a load on the circuit, or a partial short. Investigate and fix the cause. Fitting a higher fuse is how electrical fires start.
Do I need to fuse the battery negative?
No. The negative is the return path and does not require a fuse. However, it must be correctly sized — the same gauge as the positive cable it mirrors.
What is the difference between a fuse and a circuit breaker?
A fuse is a single-use sacrificial element — once blown, it must be replaced. A circuit breaker is resettable. Fuses respond faster to dead shorts; breakers are more convenient for circuits that trip regularly (like an inverter with occasional overloads).