Narrowboat 12v system keeps tripping the fuse when I run the inverter

Been through almost exactly this on my boat last spring. Nine times out of ten it's one of three things:

1. Undersized cable run
Inverters draw massive current at 12v — a 1000W inverter can pull 90-100A. If your cable to the battery isn't at least 35mm² (preferably 50mm²), you'll get voltage drop and heat, which trips the fuse before the inverter even reaches full load.

2. Fuse rating vs cable rating mismatch
The fuse should protect the cable, not the inverter. What size fuse are you running, and what cable gauge? A lot of boat installations I've seen have undersized ANL fuses fitted by the previous owner — classic "that'll do" boatyard wiring.

3. Battery internal resistance
If you're on older AGM or flooded leisure batteries, they might not be able to deliver the surge current the inverter demands on startup. Particularly bad in cold weather on the cut. I switched to a Fogstar Drift 200Ah LiFePO4 a couple of years back and the difference in inverter performance was night and day — lithium handles those peaks without blinking.

What inverter are you running, and is it a pure sine or modified sine? Some cheaper modified sine units have horrible inrush characteristics that hammer the fuse on startup.

Also worth checking all your connections are clean and tight — a slightly loose terminal on a 100A draw gets warm fast.

What's your current battery bank setup? That'll help narrow it down. Others on here will have seen this too — @WaterloggedWatts had a similar thread a while back if you can dig it out.

@PennineNomad nailed the cable point — worth adding that on a narrowboat the cable runs are often longer than people expect, especially if the inverter isn't mounted right next to the battery bank. Every extra metre adds resistance and voltage drop, which compounds under load.

Also worth checking: what's the fuse rating vs the cable rating? I've seen boats where someone's "upgraded" to a bigger fuse to stop nuisance trips, but the cable can't handle it. That's how fires start.

On my cabin setup I use Victron's cable sizing tool — it works fine for 12v marine installs too. Input your cable length, load, and acceptable voltage drop and it'll tell you exactly what cross-section you need.

What gauge are you currently running from battery to inverter, and how long is the run?

Good shout from @PennineNomad and @Inverter_Pro on the cable stuff.

One thing nobody's mentioned yet — fuse rating vs actual cable rating.

Seen plenty of boats where someone's "upgraded" the fuse to stop it blowing, rather than fixing the actual problem. Classic. Now your cable's the fuse. 🔥

Also worth checking the battery terminals themselves. My cabin setup was dropping nearly 0.8v under load just from a slightly loose connection. Measuring at the battery vs measuring at the inverter terminals told the whole story — proper voltage drop test, two minutes, multimeter.

On a narrowboat with older wiring, I'd be suspicious of every joint in that run tbh. Corrosion in a marine environment is no joke.

What fuse size are you actually running, and what's the inverter rated at?

Right, @Curly38's onto something but let me add the bit nobody wants to hear — battery condition.

I had this exact headache on my static caravan backup system before I sorted it properly. Old leisure batteries look fine sitting there doing nothing, but the moment an inverter hammers them with surge current at startup, internal resistance shoots the voltage through the floor and every protection device in the circuit has a panic.

Stick a proper load tester on those batteries — not just a multimeter, an actual load tester. Watched a mate spend three weekends rewiring his boat beautifully with decent 70mm² cable, only to discover his Yuasa batteries were knackered all along.

On narrowboats especially, damp conditions accelerate battery degradation faster than people realise. If they're over four years old, honestly, just replace them first and then diagnose everything else.

Great points all round from @PennineNomad, @Curly38 and @OhmsLaw — I'd add one thing that bit me hard on my own boat: connection resistance at the battery terminals.

Even if your cable gauge is spot-on and your batteries are healthy, corroded or undertorqued lugs create hidden resistance that causes a voltage drop under load, which makes the inverter draw even more current trying to compensate — classic death spiral that'll pop fuses all day long.

Grab a multimeter and measure voltage directly at the battery posts versus at the inverter terminals while it's running. More than ~0.3V difference and you've found your culprit.

On my narrowboat I now use tinned copper lugs and hit every connection with Duralac jointing compound — complete game changer in a damp bilge environment.