240v "visibility" and qualification in 2xMPII US split phase?

So this got me thinking about my narrowboat setup — running a pair of Victron Multiplus-II 5000s in a sort of stacked config and the whole 240v visibility thing is a bit murky when you've got shore power feeding through the inverters first.

On a boat you obviously don't have split-phase like the Americans do, but the principle feels similar — shore power comes in, passes through the inverters, then hits your AC panel. The question is: what does your downstream equipment actually "see" at that point, and does it matter for earthing, RCD behaviour, etc.?

I've been going back and forth on whether the Multiplus treats pass-through differently to inverter output in terms of neutral-earth bonding. On my boat the marina shore power is properly earthed but once it's gone through the Victron stack I'm never 100% sure what's happening with the neutral reference.

Also — and this is the bit that keeps me up at night — does any of this cross into Part P territory for those of you running this in a garden office or tiny house? I know boats are slightly different regs-wise but I'd imagine a fixed dwelling setup raises more questions about who needs to sign this off.

Anyone running dual Multiplus-IIs on shore input in a UK context? Particularly curious about:

• How you've handled the earthing/neutral bonding
• Whether you used an RCBO arrangement downstream
• If anyone got a spark or an approved inspector involved

Would love to know how others have approached this — feels like one of those areas where the Victron docs don't quite cover the real-world wiring questions. 🔌

@PanelKate the narrowboat angle makes this more interesting — I ran into exactly this rabbit hole when I was wiring the shepherd's hut off a similar stacked arrangement.

The key thing nobody mentions clearly: in a true US split-phase config, what Victron calls "240v" is actually the phase-to-phase relationship, and the MPII's internal monitoring reports it differently depending on firmware version. Check your VRM portal — under Advanced metrics — and you'll see L1-L2 voltage listed separately from individual leg readings.

On the qualification side, if you're UK-registered (which on a narrowboat you almost certainly are under BSS), the 240v AC work falls under Part P equivalent marine regs. Shore power complicates it further because you're bridging two regulatory worlds.

Worth raising a ticket with Victron's UK technical support rather than relying on forum guesswork for the compliance bit.

Slightly different angle on this — in a static caravan context I had a similar head-scratcher when trying to get the Multiplus-II to "see" the full 240v picture for things like my EV charger threshold decisions.

The key thing I'd flag for @PanelKate's stacked config: are you using a shared neutral or split neutral arrangement? That changes what the inverters actually report back via VRM considerably. In my setup, until I got the Venus GX configured correctly with proper AC input sensing on both legs, the ESS assistant was making some genuinely odd decisions about when to qualify grid presence.

Worth checking:

• Is AC input 1 properly sensing on both units?
• Are your VE.Bus cables correctly daisy-chained rather than star-wired?

The Victron configurator documentation on parallel/stacked is decent but doesn't always make the "visibility" logic explicit.

@PanelKate the split phase rabbit hole is real 😅

Worth noting that in a UK context you're essentially manufacturing a centre-tapped 240v system which doesn't naturally exist here — so the "visibility" question gets weird fast. The MPII's in parallel mode will show you individual AC output stats in VRM but the combined 240v picture needs a bit of mental arithmetic or a decent energy meter bridging both legs.

For the garden office feed I ended up just chucking a Victron Energy Meter on the output and letting VRM do the maths. Saved a lot of head-scratching.

Also — are you actually needing true split phase or just stacked for capacity? Because the qualification requirements differ and that might simplify things considerably 🤔

@PanelKate this actually came up when I was sorting my garden office build — I had a single MPII and kept scratching my head why Cerbo GX wasn't "seeing" the full picture cleanly until I dug into the ESS assistant config.

The 240v visibility question in a stacked pair really hinges on how you've set the master/slave relationship up in VE.Configure. If your phase compensation isn't dialled in correctly, each unit's essentially reporting its own little world rather than the combined output.

For qualification purposes in a UK install, bear in mind your DNO notification obligations shift once you're synthesising voltages that don't come directly from the grid — worth having a conversation with a G98/G99 aware sparky before assuming it's all straightforward.

What does your current ESS assistant config look like?