Narrowboat 12v system keeps tripping after fitting the new inverter

Not a narrowboat owner myself but I've dealt with similar tripping issues on my motorhome when I upgraded my inverter, so hopefully some of this is useful.

First thing I'd check — what size fuse or breaker do you have between the battery bank and the inverter? A lot of people undersize this when fitting a new, larger inverter. Even if the inverter isn't pulling full load, the inrush current on startup can be massive and enough to trip a breaker that should theoretically be fine.

Also worth asking:

• What inverter did you fit? Pure sine or modified sine? Some motors and pumps on narrowboats react badly to modified sine and draw more current as a result.
• What's the cable run length and gauge? Long runs with undersized cable cause voltage drop, which makes the inverter work harder and draw more amps.
• Is your battery bank up to the job? If you're running AGM or older lead acid, they might not be delivering the peak current the inverter needs. What are you running — lithium?

On my motorhome I was running Fogstar lithium with a Victron MultiPlus and still had weird trips until I realised my BMS was set too conservatively for inverter loads. Might be worth checking your BMS settings if you've gone lithium.

What does the trip point appear to be — is it the breaker on the inverter itself, or upstream somewhere? That'll narrow it down quite a bit.

@Smithy98 good shout on the fuse side of things, but I'd push further — it's almost certainly the cable gauge that's the real villain here.

Fitted a 2000W Victron in my static caravan last spring and had identical phantom trips. Turned out my cabling was criminally undersized for the inverter's surge demand. Inverters don't just draw their rated load — they can pull 2-3x that on startup surge, lasting milliseconds but enough to pop breakers repeatedly.

Key things to check:

• Cable run length (longer = more voltage drop = harder surge)
• Connection quality at the battery terminals
• Whether the BMS on any lithium pack is cutting out rather than an actual fuse

What battery chemistry is the narrowboat running? That changes everything about how you diagnose this.

Great points from @Smithy98 and @FellKev already. I'd add that on narrowboats specifically, the cable runs from the battery bank to the inverter are often longer than people realise once you factor in routing around bulkheads and through bilge spaces. Even if the cable gauge looks adequate on paper, that extra length means more voltage drop under load, which can cause the inverter to pull higher current trying to compensate — and that's what's tripping things.

Worth the OP measuring the actual cable run rather than estimating. There's a decent voltage drop calculator on 12voltplanet.co.uk that accounts for run length. If you're seeing more than a 0.5v drop under full inverter load, your cables need upsizing regardless of what the inverter spec sheet suggests.

yeah @RogerRoberts85 the long cable runs on boats are a proper pain — had the same headache on my narrowboat actually

worth checking your battery terminals too, not just the cables. loose or corroded connections cause all sorts of weird tripping behaviour, especially when the inverter kicks in under load and suddenly demands a big current spike

also — what inverter did you fit? some cheaper units have really aggressive low voltage cutoffs that trip before your fuse even gets chance to blow. i had a nightmare with one before switching to a Victron. night and day difference in how it handles surge loads

if you've got a multimeter, check voltage at the battery and at the inverter terminals under load — if there's more than about 0.5v difference between the two you've found your culprit right there

Good points all round — one thing nobody's mentioned yet is whether the inverter's remote on/off is wired correctly. Some Victron units in particular will behave oddly if that line is floating rather than properly terminated.

Also worth asking @Smithy98 — what's the battery state of charge when it trips? If it's happening under load rather than at startup, low voltage cutoff on the inverter itself could be the culprit rather than a fuse or cable issue. Lithium and AGM have very different discharge curves, so if the battery bank is mixed or older, the inverter might be seeing voltage sag it doesn't like.

What inverter did you actually fit? That'd help narrow it down considerably.