Narrowboat 12v system keeps tripping 60A fuse — is my wiring undersized?

Running a fairly basic 12v setup on my narrowboat — 200Ah of Fogstar Drift LiFePO4, a Victron SmartSolar 100/30 MPPT, and a 1000W pure sine inverter for the odd kettle boil. The 60A fuse between the battery and the busbars keeps blowing when I run the inverter at anything over about 700W. Wiring is 16mm² cable throughout, runs are roughly 1.5m each way.

From what I can work out, 1000W at 12v is theoretically pulling ~83A, so the 60A fuse was always going to be marginal. But previous owner fitted everything and it "worked fine" apparently — so either they never pushed it hard or something's changed.

Should I be moving up to 100mm² cable and a 100A fuse for the inverter run specifically, or is 16mm² actually okay for short runs and I just need to uprate the fuse? Also wondering if the Fogstar BMS has any role here — could it be cutting out and causing a voltage spike that's killing the fuse rather than overcurrent?

Has anyone done a similar inverter install on a narrowboat where space and run length are tight?

EmmaPowell74 | 847 posts

@VivaroProject — worth checking the wire gauge between your battery and inverter first. A 1000W inverter at 12v is pulling ~83A at full load, so that 60A fuse is actually undersized for the inverter alone, never mind everything else on the system. You'll want at least an 80-100A fuse there, but only if your cabling supports it — most people run 16mm² minimum for that run, ideally 25mm².

What's the cable length and size between battery and inverter? If someone's used 10mm² it'll be getting warm under load and could be why it's tripping — the fuse is doing its job protecting undersized cable.

Also worth checking your battery BMS spec — Fogstar Drift 200Ah should handle the current fine but double-check the continuous discharge rating.

ColBurns63 | 1,203 posts

@VivaroProject — a 1000W inverter at 12v is pulling around 83A at full load, so that 60A fuse was never going to cope with a kettle. It'll trip every time. You'll want to upsize to at least a 100A fuse, ideally 125A to give yourself some headroom. As @EmmaPowell74 says, the cable needs attention too — for that kind of current you're looking at 16mm² minimum between battery and inverter, though 25mm² would be far more comfortable and keeps voltage drop sensible over any decent run. Keep the cable runs as short as possible on the boat — every extra metre hurts you at these current levels. What's the actual cable length you're working with?

BrianLewis79 | 412 posts

@ColBurns63 has hit the nail on the head there — the maths simply doesn't work with a 60A fuse protecting an inverter that size. You'll want at least a 100A fuse on that circuit, ideally 125A to give yourself a bit of headroom for inrush current when the inverter fires up. The fuse should be rated to protect the wire, not the load, so make sure your cabling can handle it too — for a 1000W inverter at 12v you're looking at 16mm² cable as an absolute minimum, and I'd honestly go 25mm² especially on a boat where voltage drop and heat buildup matter more than people realise. What length is the run between your battery and inverter?