How to properly ground an off-grid inverter

Right, I've been putting this off for far too long and figured I'd better ask before I fry something expensive.

Got a Victron Multiplus II 48/5000 in my narrowboat setup, and I'm dead set on getting the grounding sorted properly before the season really kicks in. Currently I've got a dodgy arrangement with some copper tape running to what I think is a decent earth point on the hull, but I'm not confident it's actually doing anything.

The thing is, most guides I've found are written for house installations with proper earthing rods and all that. Bit different when you're sat on the canal with a steel hull and limited space. I've got my battery bank (LiFePO4) and charger sorted, but the inverter earthing is the bit that's got me scratching my head.

Should I be looking at a dedicated earth stake driven into the bank when I'm moored up? Or can I genuinely rely on the hull connection if I beef it up properly? And what resistance should I be aiming for with a multimeter?

Also wondering if anyone else running similar setups on boats has actually managed this without turning their vessel into a floating electrical hazard. I do plan on adding EV charging capability eventually, so I'd rather get the fundamentals right now rather than rewire the whole thing later.

Cheers in advance — clearly I'm out of my depth here!

Has anyone specified whether you're going TN-S or TT earthing yet? That'll be your first decision point on a boat, especially if you're moving between moorings with dodgy shore power.

I've got a Victron setup in my garden office and went TT with a local earth rod — proper job with the RPD protection and all that. But for a narrowboat you might be better served by the hull bonding route if you're regularly plugged in at marinas.

The Multiplus manual's actually pretty clear on the DC side (busbar to negative, that's straightforward), but the AC earthing can get fiddly depending on your RCD requirements. Are you running any shore power backup or fully off-grid? That changes things quite a bit.

What's your current setup — batteries, solar, the lot?

On a boat you're basically forced into TT since there's bugger all chance of getting a decent earth rod to the actual earth, so Victron's bonding guidance becomes your best mate here. Make sure your DC negative and AC earth are properly bonded together at one point only — the Multiplus II manual's dead clear on this, and it's the bit most people mess up because they think "more earths = safer" (spoiler: it doesn't).

Use proper marine-grade cable too, not some dodgy automotive stuff. Your hull can technically act as your earth plane if it's conductive enough, but I've seen folk tie themselves in knots trying to verify that. Easier to run a dedicated earth busbar and call it a day. What's your hull material — steel or fibreglass?

The Multiplus II actually handles this quite elegantly—your PE and N are already bonded internally, so you're basically sorted for TT which is what you're stuck with on a boat anyway. Just whack an earth rod down through the hull into the water (assuming freshwater, obviously), run it back to your DC negative and AC earth on the inverter, job done. I've got mine wired similarly on my setup and the Victron diagnostics are happy as Larry. Only gotcha is making sure your earth connection doesn't corrode to buggery in saltwater—use proper marine-grade connectors or you'll be back here in six months wondering why your RCDs won't stop tripping.

On a boat you're in a tight spot though—TT's your only realistic option like @BatteryAlan says, but the grounding itself is the tricky bit. Can't exactly bash an earth rod into the canal bed reliably.

What I did with my shepherd's hut setup was a proper copper plate bonded to the hull/frame, then a heavy gauge cable run to the inverter earth. Victron's documentation actually recommends this for off-grid installations where you haven't got mains earth to reference against.

The key thing @SolarKeith's hinting at—make sure your DC negative and AC earth are properly bonded together at one point only (usually at the inverter itself), otherwise you'll get floating grounds causing all sorts of grief with RCD nuisance trips.

Fair warning though: if you're in a metal boat, corrosion on your bonding connections is a nightmare. Check it regularly. I use marine-grade tinned cable and regular inspections on mine.

What's your boat construction—steel, fibreglass? That'll change the approach a bit.

On a narrowboat you're basically doing TT whether you like it or not—the water's your "earth" and it's rubbish at the job, so that copper plate you bolt to the hull becomes your best mate.

The Multiplus II will work fine with proper TT, but here's the thing nobody mentions: your earthing conductor needs to be massive because you're relying on a tiny contact patch through corrosion and barnacles. I went 16mm² and still worry about it.

What @BatteryAlan and @SolarKeith haven't mentioned is the RCD side—you'll want a Type A RCD (not Type AC) because modern inverters generate DC leakage. Victron's documentation actually spells this out, which is refreshing.

Bolt that plate to bare hull (scrape the paint off), use proper marine-grade cable lugs, and don't cheap out thinking "it's only a boat." The water will corrode your setup, so plan for maintenance every couple of years.

The water conductor issue @T5Project mentions is spot on—you'll get awful earth fault loop impedance figures relying on hull contact alone. I'd strongly recommend installing a proper earth rod setup independent of the boat's structure.

On my narrowboat, I went TT with a dedicated 1.2m copper-bonded rod driven into the towpath when moored (yes, you're technically trespassing for 30 seconds, but Canal & River Trust won't prosecute). The Multiplus II's RCD sensing works brilliantly once you've got a decent earth established—mine trips reliably on test.

Key points:

• Earth rod resistance target: Below 100Ω ideally, though on clay you might hit 150-200Ω
• Cable sizing: Use 6mm² minimum from rod to your DC negative/PE bus
• RCD setting: The Multiplus II's internal RCD is fine, but pair it with a Type A 30mA breaker on your AC output for belt-and-braces safety
• Permanent mooring considerations: If you're semi-static, drive the rod

The water conductor issue @T5Project mentions is the real headache here. I went through this exact nightmare on my own narrowboat setup two years back—initially tried relying on the hull as earth return and got readings that made my sparky weep.

What actually worked: I installed a proper external earth rod driven into the canal bed when moored, but crucially, I also fitted a PME (protective multiple earthing) compatible setup with a dedicated earth stake at my home mooring. Between that and a quality RCD, the Multiplus II behaves itself.

The water will corrode any sacrificial connections, so don't cheap out on the hardware—stainless steel bolts, proper cable lugs. Your fault loop impedance will be marginal at best relying purely on water, which means your RCD needs to be absolutely spot-on sensitive.

If you're constantly moving moorings, honestly, this becomes impractical. That's when you're looking at installing a proper TT earthing system wherever you permanently base the boat. Worth having a qualified electrician who's done canal boats before—they're not your average installation.

Narrowboat here too—been wrestling with this. The water's genuinely unreliable, especially in winter when hull paint degrades the conductivity. I've ended up running a dedicated earth rod to the towpath whenever moored long-term. Victron's tech support suggested this as well. Have you considered a portable setup you can deploy?