Narrowboat 12v system keeps tripping after fitting the new inverter

Sounds like a cable sizing issue more than anything else. Inverters are notorious for pulling massive surge currents at startup — even a modest 1000W unit can spike well beyond what the steady-state draw would suggest.

Few things worth checking:

• What gauge are your battery-to-inverter cables? Most narrowboat installs I've seen use undersized cable because people go off the continuous rating rather than peak. You want to be generous here — 70mm² minimum for anything over 1000W, ideally 95mm²
• Where's the fuse/breaker sitting in the circuit? If it's right at the inverter rather than at the battery, you're potentially protecting the wrong thing
• What inverter did you go for? Modified sine wave units can be particularly spiky at startup

My garden office setup had similar gremlins when I first wired in a Victron Phoenix — kept nuking the breaker every time the compressor fridge kicked in. Turned out my cable run was just long enough to cause enough voltage drop that the inverter was compensating hard on startup.

On a narrowboat the cable runs are usually shorter, which is a point in your favour, but the marine environment adds its own complications — any corrosion on terminals will add resistance and make the problem worse.

Worth getting a clamp meter on those cables if you can and logging the actual startup surge. Might tell you exactly where the weak point is.

What's the battery bank — lithium or lead acid? That changes things quite a bit in terms of how the BMS might be interacting with sudden load spikes.

@HamishMitchell yeah the cable run length matters massively too — boats always seem to have these awkward long runs from the battery bank to the inverter location. Even if the wire looks chunky enough, voltage drop under surge can cause the BMS to trip before the breaker even blinks.

What gauge is the existing cable and how far is it running? On my setup I've got 70mm² for anything serious. Anything under 35mm² on a 1000W+ inverter and you're asking for trouble, especially if the connections aren't properly crimped — corroded ring terminals are a silent killer on narrowboats with all that condensation.

Also worth checking: is it the inverter itself tripping, the fuse, or the Victron BMS if they've got one? Makes a big difference in diagnosing where the weak point is.

@HamishMitchell is absolutely right on the surge current point — worth adding that the breaker rating itself needs to account for this too, not just the cable. A lot of people size the breaker to the inverter's continuous rating and then wonder why it trips under load.

What breaker are you currently running, and what's the total cable run length from your battery bank? On my static setup I ended up going 2/0 AWG with a 200A fuse right at the battery terminals, which made a significant difference.

Also worth checking whether your battery itself can actually deliver the surge — if you're on older lead-acid, internal resistance may be causing a voltage sag that triggers the inverter's low-voltage cutoff, which can look like a tripping issue but isn't.

Welcome to the forum by the way — brilliant first thread, exactly the kind of technical detail that makes for a proper diagnosis.

@Nessa and @HamishMitchell — both good points, but can I ask a related question specific to boats?

On my narrowboat I'm running a Victron MultiPlus and I've always been a bit unsure whether the breaker should sit as close to the battery bank as physically possible, or whether there's any tolerance for it being a metre or two down the cable run. Seems like every installation guide says "as close as possible" but the battery box placement on narrowboats makes that genuinely awkward sometimes.

Also — is anyone running a Class T fuse rather than a breaker for the main protection? I've seen a few marine installs doing this and wondered if it's considered better practice for surge-heavy setups. The Victron documentation seems a bit vague on whether a breaker alone is sufficient for inverter-side protection on a 12v system.

@MaxFrost go on then, ask away — but while we wait, I'll just say that on my van conversion I learned the hard way that fusing as close to the battery as physically possible is the real game-changer, because a long unfused cable run is basically just a very expensive fuse made of your boat.

For narrowboats specifically, a Victron Smart Fuse or a proper ANL fuse holder right at the terminal sorts a lot of these mystery trips — the breaker downstream can then do its actual job rather than compensating for dodgy protection upstream.

Also worth checking: are you on a leisure battery bank or lithium? Lithium (shoutout to Fogstar Drift cells) will dump current far faster than AGM, which means your old breaker rating might now be hilariously inadequate. 🔥

@MaxFrost don't keep us in suspense — though I'd wager my Victron MultiPlus that whatever you're about to ask, the answer is "bigger cable, shorter run, proper fuse right at the battery."