Campervan Fuse Keeps Blowing: Causes and Fixes

A fuse that blows once might be a one-off event — a brief overload, a spike, a connection that temporarily shorted. A fuse that keeps blowing is telling you something is wrong, and fitting a higher-rated fuse to stop it blowing is the most dangerous thing you can do. The fuse protects the cable. Remove that protection and the cable can overheat and start a fire.

This guide works through every cause of a repeatedly blowing fuse, in order of likelihood, with diagnostic steps and fixes.

Step 1: Confirm the Fuse Rating Is Correct

Before assuming there is a fault, verify the fuse is the right rating for the cable and circuit. An undersized fuse blows under normal load — this is not a fault, it is a mismatch.

Rule: The fuse rating must be lower than the cable's maximum current capacity, and high enough to allow the intended load to run.

If your 15A fuse is protecting a 2.5mm² cable feeding a fridge that draws 10-15A, the fuse should not blow under normal use. If the same fuse is on a 1.5mm² cable, it may be near its limit.

Check both cable gauge and actual appliance current draw before concluding there is a fault. See our fuse sizing guide for correct cable and fuse pairing.

Step 2: Calculate the Actual Load

Tools needed: Clamp meter or knowledge of appliance specs

Measure or calculate the actual current draw on the circuit:

• Compressor fridge: Typically 5-8A running, 10-15A startup surge
• Water pump: 8-15A when running
• Inverter: 10-200A+ depending on load and inverter size
• DC-DC charger: Up to rated output (20-50A)
• USB charging sockets: 1-5A depending on connected devices

If you have multiple appliances on one circuit and their combined draw exceeds the fuse rating, the fuse blows correctly. The fix is to either:

1. Reduce the number of appliances on that circuit
2. Use a higher-rated fuse and heavier cable if the current level genuinely needs to increase

For appliances with startup surges (compressor fridges, pumps, motors), the startup current can be 2-3 times the running current. A 10A fuse on a fridge that draws 15A at startup will blow on every startup. Fit a 20A fuse on appropriately-rated cable.

Step 3: Check for a Partial Short Circuit

A partial short is a connection where the positive conductor is touching or nearly touching something it should not — the van chassis, an exposed screw, or the negative conductor. It is "partial" because it allows some current to flow normally but bleeds additional current through the fault path, pushing the total over the fuse rating.

Symptoms of a partial short:

• Fuse blows after a few minutes of use rather than immediately
• Appliance works but the fuse is warm after use
• Fuse blows more readily under vibration (connections moving)

How to find it:

1. Disconnect the appliance from the circuit
2. Fit a new fuse of the correct rating
3. Leave the circuit energised (without the appliance connected) for several minutes
4. If the fuse blows with nothing connected, the short is in the cable run itself
5. If it holds, reconnect the appliance — if it blows when the appliance is connected, the fault is in the appliance or its wiring

Common locations for partial shorts:

• Cable entry glands at roof penetrations (cable chafing on the gland edge)
• Panel penetrations where cable passes through a drilled hole without a grommet
• Behind appliance mounting points where a screw has pierced cable insulation
• In fuse box connectors where a blade fuse has partially dislodged and arced

See our cable routing guide for the correct use of grommets and conduit that prevents this type of damage.

Step 4: Check for an Appliance Drawing Too Much Current

A faulty appliance can draw significantly more current than normal. This is common with:

• Failing DC motor: A water pump with a worn impeller or blocked outlet works much harder, drawing 2-3× normal current
• Sticking relay or solenoid: A 12V relay that is not releasing properly passes current continuously
• Short circuit inside the appliance: An internal fault in a fridge, heater fan, or similar

Test: Disconnect the appliance and connect a multimeter (set to DC amps, in series) or clamp meter around the positive cable. Run the appliance and measure the actual current draw. Compare to the appliance's rated current.

A pump drawing 25A on a 15A-rated circuit has an internal fault. A fridge drawing 20A startup current (normally 12A) may have a failing compressor.

Step 5: Check the Inverter Circuit

The inverter is the most common source of repeated fuse blowing in campervan systems. Reasons:

Overloaded AC output: If the total AC load exceeds the inverter's rated capacity, the inverter draws more DC current and can blow its fuse. Check total AC wattage being drawn.

Undersized DC cable: An inverter DC cable that is too thin for the current has significant resistance. Under load, the voltage drop across the cable means the inverter draws more current to maintain output, pushing over the fuse rating. See our inverter wiring guide for correct cable sizing.

Fuse sized too close to the limit: If you have a 200A fuse on a 2000W inverter (which draws up to 195A at full load), there is almost no headroom. The fuse will blow at full inverter load. Upsize the cable and fuse together — a 2000W inverter needs a 250A ANL fuse and appropriate cable.

Startup surge: Some high-draw appliances (air compressors, power tools, refrigerators) have startup surges 3-5× their running current. If the surge current exceeds the inverter's surge rating, the inverter trips its internal protection and may draw a fault current that blows the external fuse.

Step 6: Inspect Fuse Holder Condition

A degraded fuse holder causes problems that look like fuse faults:

• Corroded fuse contacts: Resistance at the contacts generates heat under load. The heat can blow a fuse even when current is within rating.
• Loose blade fuse connection: A blade fuse that does not sit firmly in its holder arcs and heats up. Replace the fuse holder.
• ANL fuse holder with damaged terminals: Check the bolt-down terminals for discolouration (burn marks) or pitting. This indicates arcing — replace the fuse holder.

Fuse holders are cheap (£1-10). If a holder shows any sign of heat damage, replace it rather than just the fuse.

Step 7: Check the Main Battery Fuse

If the main battery fuse keeps blowing, every circuit is affected simultaneously. This indicates a total system fault:

• Short circuit on the main positive bus: Check the bus bar for any contact with chassis metalwork
• Inverter internal short: A failed inverter can develop an internal short that presents a near-dead-short to the battery
• Loose connection arcing at main fuse holder: Check the ANL fuse holder for heat damage

A main battery fuse that blows means a very high current event. Do not reconnect the battery until you have found and fixed the fault — replace the fuse, reconnect briefly to locate the fault point (watch for any heating, sparks, or smoke), then disconnect immediately.

Summary: Diagnostic Decision Tree

FAQ

Is it safe to use a fuse of a slightly different rating in an emergency?

Only if the replacement fuse is lower rated than the blown one, never higher. Using a 10A fuse on a 15A circuit as a temporary measure until you get the right fuse is safe. Using a 20A fuse on a 15A circuit removes cable protection and is not safe under any circumstances.

My fuse blows only when I start the engine — why?

When the engine starts, the alternator can cause a brief voltage spike on the 12V system. If a DC-DC charger activates at the same moment as other high-draw items, the combined surge can exceed the fuse rating. Check the simultaneous startup current of all charging sources. It may also indicate the ignition signal for the DC-DC charger is not connected and the charger is being triggered by the voltage spike.

Can a fuse blow from overheating rather than overcurrent?

Yes, if the fuse or holder is in an extremely hot environment. Fuses are rated at 25°C — in a high-temperature location (engine bay, against an exhaust heat source), a fuse can blow at lower than its rated current. Relocate fuses away from heat sources.