Bilge pump keeps tripping the fuse on my narrowboat – 12v wiring issue?

Not a narrowboat owner but had almost the exact same issue in my motorhome with a water pump doing the same thing.

Nine times out of ten it's one of these:

• Wire gauge too thin for the run length — voltage drop causes the pump to work harder and draw more current
• Fuse rated too close to the pump's running amps — leaving no headroom for startup surge
• Partial blockage in the pump or outlet making it labour

Worth checking what gauge wire you've run from the battery to the pump and how long that run is. If it's more than a couple of metres and you've used anything thinner than 2.5mm² I'd upgrade it straight away.

Also, what fuse are you running? Bilge pumps can pull 2-3x their rated amps on startup. If your pump is rated at 5A and you've got a 5A fuse in there, that's your problem right there.

A proper inline blade fuse holder with the right rating sorted mine — picked up a Victron SmartShunt recently too which actually lets me see exactly what's pulling what, dead handy for diagnosing this stuff.

What pump is it and what's the fuse rating you're currently on? That'd help narrow it down.

@Clive1999 left us on a cliffhanger there — the wire gauge equivalent of a Netflix series finale.

@CotswoldCruiser ha, yes — @Clive1999 presumably typed that from a boat with no shore power.

To actually finish the thought: wire gauge is almost certainly the culprit. For a bilge pump on a typical narrowboat run of 3–4 metres, you want minimum 2.5mm², ideally 4mm² if the pump is pulling 10A+ at startup. Undersized wire creates resistance, which creates voltage drop, which makes the motor draw more current trying to compensate, which pops the fuse.

Also worth checking: is the fuse rated for the wire or the pump? Common mistake is oversizing the fuse to stop it tripping — that's how you get a wiring fire, not a fix. Size the fuse to protect the cable, not the appliance.

What's your pump model and fuse rating currently?

@Clive1999 presumably hit send mid-word because his pump tripped his fuse too.

To actually complete the list that's been abandoned twice now:

• Wire gauge too thin for the run length (voltage drop causes the motor to draw more current to compensate)
• Corroded connections adding resistance — same effect
• Pump motor failing internally, drawing locked-rotor current
• Fuse rated correctly for the wire but not the actual pump draw under load

On narrowboats specifically, the damp environment absolutely destroys crimp connections unless they're properly heat-shrunk adhesive-lined. Seen 14mm² cable effectively behaving like 2.5mm² because every connection along the run had gone high-resistance.

@FenlandSolar — worth asking OP what fuse rating they're running and the actual run length before we go further down the rabbit hole.

@Clive1999 started a list, lost power, and somehow that's still more useful than half the completed posts on here.

For what it's worth from someone who's had a static van, a garden office, and a very angry bilge-equivalent (it was a sump pump, long story) all on the same Victron system — the third culprit nobody mentions is a partially seized pump motor drawing way above its rated current on startup, thin wire or not. Shove a clamp meter on it while it kicks in and watch the inrush spike. If it's pulling 3x rated amps before it even gets going, the pump's on its way out and no rewire will save it. Fogstar won't save you either, mate.