Narrowboat 12v system keeps tripping after running the inverter — any ideas?

Been dealing with something almost identical on my narrowboat earlier this season, so watching this thread closely.

Mine turned out to be a combination of issues — undersized cable runs between the battery bank and the inverter, plus a fuse rating that was borderline even under normal load. The moment I ran anything heavier than a kettle, it would trip within a few minutes.

A few things worth checking:

• Cable sizing — what gauge are you running from batteries to inverter? Even a short run of undersized cable creates enough resistance to cause voltage drop and heat buildup
• Battery state of health — if you're on flooded lead-acid or older AGM, they may be struggling to deliver the surge current the inverter demands at startup
• Inverter brand/model — some cheaper units have poor surge tolerance and trip their own protection circuits before the main fuse even reacts

What inverter are you running and how large is your battery bank? I'm on a Victron MultiPlus with Fogstar lithium now, which sorted most of my grief, but I spent a frustrating summer on the old lead-acid setup first.

Also — is it a thermal trip or a fuse blowing? That distinction might narrow things down considerably.

Would be good to hear what others on here have found with narrowboat inverter setups specifically, since the cable routing challenges are quite different to a static off-grid install.

@BoatGemma sounds like your post got cut off mid-sentence — would love to hear the rest of that, especially the cable run details.

On the inverter tripping issue generally: nine times out of ten on narrowboats it's one of these:

• Undersized fusing — the fuse is protecting the cable, not the inverter's actual draw
• Voltage sag under load tripping the inverter's low-voltage cutoff
• Loose connections at the battery terminals that look fine but aren't

Worth checking your resting voltage versus voltage under load with a decent multimeter. If it's sagging below 11.5v when the inverter kicks in, your bank either isn't big enough or the cables are dropping too much voltage.

What's your cable gauge between batteries and inverter, and how long's the run? On my boat I went to 70mm² for anything over a metre to the inverter.

Hey @BoatGemma, yes looks like your post got cut off — gutted, I was really interested in what you were going to say about the cable runs! Please do finish that thought when you get a chance.

In the meantime, @BoatGemma's point about undersized cabling is worth taking seriously regardless. On narrowboats especially, people often underestimate how long the cable runs actually are once you factor in routing around bulkheads and engine bays. Voltage drop adds up quickly and can absolutely cause nuisance tripping under inverter load.

OP — can you tell us what gauge your battery-to-inverter cables are, and roughly how long the run is? Also worth checking whether your fuse or breaker is rated correctly for the inverter's maximum draw, not just its continuous rating. Sometimes the issue is as straightforward as that. 🙂

@BoatGemma yeah looks truncated alright, but while we wait — cable runs are almost always the culprit when inverters trip things. Ran into similar on my motorhome build. Had voltage drop across undersized cable that looked fine on paper but under actual inverter load (even a modest 1000w) the system just couldn't cope.

Few things worth checking regardless of what Gemma was going to say:

• Fuse rating vs cable rating — these need to match, not just "close enough"
• Connection quality at both ends, corroded lugs are sneaky
• Battery internal resistance if the pack is older

What inverter are you running and what's the cable length from batteries? That'll tell us a lot. Victron inverters at least give you proper fault codes to work from — if it's a no-name unit you're a bit blind.

@BoatGemma nothing worse than a cliffhanger on a forum post — my van's Victron nearly went the same way before I realised I'd basically wired the inverter with something resembling a shoelace.