2 x Multiplus ii 8Kva Not working in parallel

Interesting timing on this thread — I've been going back and forth on whether to run a parallel pair of Multiplus II 5kVAs in my shepherds hut conversion rather than just stepping up to a single larger unit.

Can I ask those who've done parallel Victron setups — is the MK3-USB interface and VE.Configure absolutely mandatory before they'll play nicely together? I've read conflicting things. Some say you can get them "talking" via VE.Bus without touching the config, others say they'll just sit there doing nothing or worse, fighting each other.

Also, a few specific questions:

• External transfer switch — did you use a dedicated ATS contactor or did you find a specific brand that plays well with the Victron ecosystem? Struggling to find clear UK supplier recommendations.
• Ground relay wiring — is there a gotcha here when you're on a site with an existing earth rod rather than a floating system?
• Phase pairing — do both units need to be identical firmware versions, or is there some tolerance there?

My setup will be running off a 48V Fogstar Drift LiFePO4 bank with a mix of shore power and solar input, so the parallel inverter capacity would mainly be needed for occasional high-draw tools during the build phase.

Has anyone done this successfully on a narrowboat or off-grid building where grid code compliance isn't really a factor? Wondering if that simplifies things at all.

@DerekMoore89 had the exact same dilemma for my shepherd's hut build. Ended up sticking with a single Multiplus II 3kVA and honestly it's plenty for hut use — kettle, induction hob occasionally, laptop, lights. Parallel setups add complexity and you really need to get the cabling identical lengths/gauges or you'll have headaches like this thread shows.

Unless you're running something seriously power-hungry, what's actually driving the 5kVA requirement? Might be worth listing your loads — could be a single unit does the job fine and saves you the parallel faff entirely.

@DerekMoore89 parallel setup sounds appealing on paper but the config faff is real — had a nightmare getting two units talking properly on my boat before I even got to the actual fault-finding stage. VE.Bus comms cable needs to be spot on, and if your RCD setup isn't right it'll just keep tripping.

For a shepherd's hut honestly a single unit is probably the saner choice. What's your peak load actually going to be? Most huts I've seen barely justify a 3kVA let alone parallel 5kVAs 🙄

The redundancy argument is the only thing that'd sway me toward parallel — but then you're also doubling your points of failure tbh.

@DerekMoore89 worth considering the N+1 redundancy angle too — with parallel units, if one develops a fault you're not completely dark. That said, for a shepherd's hut the load profile rarely justifies it unless you're running something seriously hungry (EV charging, perhaps?).

The config faff @BayLisa mentions is real, but once VE.Configure has the parallel assistant loaded correctly and both units are on the same firmware revision, it's actually quite stable. The critical bit people miss: the UTP cable linking them must be the Victron-supplied one — not a random Cat5 from the parts drawer. Learned that the hard way on my static caravan setup.

Single larger unit honestly makes more sense for a hut unless your inverter budget has already committed to two 5kVAs sitting in boxes somewhere.

@DerekMoore89 running parallel Multiplus IIs in my shepherd's hut — can confirm the config isn't as bad as people make out once you've got the MK3-USB cable and VE.Configure sorted. The real gotcha is making sure your cable lengths between units are matched precisely, otherwise you get sync issues that'll drive you mad.

That said, for a shepherd's hut specifically, I'd question whether you actually need that much grunt. What loads are you running? Mine tops out at maybe 1.5kW peak and a single 5kVA feels almost overkill already.

If it's future-proofing you're after, the parallel route makes sense. But if you're not genuinely maxing out a single unit, you're adding complexity for not much real-world gain.