New Victron 3 phase Multiplus-II 6k5 setup with 50.67 kWh storage (Gelderland, Netherlands)

Been following a few threads about 3-phase Victron setups lately and it's got me thinking seriously about whether I should go down that route eventually.

Currently running a single-phase Multiplus-II 48/5000 with a Fogstar Drift 48V 100Ah battery — so nowhere near the kind of storage capacity being discussed in some of these larger European installs. My setup powers a static caravan and handles occasional EV top-ups, but I'm increasingly hitting the limits when I want to charge the car overnight and run everything else simultaneously.

A few questions for anyone who's done a multi-unit Victron build:

• Is 3-phase genuinely worth it for a UK off-grid residential setup, or is it mainly relevant when you've got 3-phase loads (workshop machinery, etc.)? Most UK homes and static caravans are single-phase.
• DC cable sizing — I've seen 70mm² mentioned for the 6k5 units. My current 5000 is on 50mm² and I already find the cable runs a bit of a faff. Does anyone have a good UK supplier for decent tinned copper in those heavier gauges?
• Battery bank scaling — before jumping to 50+ kWh, has anyone found a sweet spot for off-grid EV charging without going completely overboard on upfront cost? Fogstar seems the obvious go-to for UK pricing but curious if anyone's compared them against Pylontech for larger banks.

Would be genuinely useful to hear from people who've gone through the upgrade journey rather than just built big from day one.

Reply by Dizzy75:

@TelHall interesting timing on this thread! One thing worth considering before you commit to three-phase is whether your actual loads genuinely need it. A lot of folks upgrade and then realise the majority of their critical stuff is single-phase anyway. That said, if you've got workshop machinery, heat pumps or EV charging that's already three-phase, the case becomes much stronger pretty quickly.

What's your current battery bank situation? The reason I ask is that scaling to three-phase essentially means you're running three inverters drawing from the same bank, so your C-rate requirements change significantly. Worth stress-testing your existing setup first to see where the bottlenecks actually are before spending the money.

What size Multiplus-II are you on currently - the 3k or 5k?

worth flagging that this thread is technically in the wrong subforum? 3-phase inverter setups probably belong in Inverters & Battery Storage not Solar Panels & Controllers 🙃

anyway — @TelHall going 3-phase from single-phase isn't a small jump, cost-wise or complexity-wise. unless you've actually got 3-phase loads that need balancing (like a big workshop or something) it's often overkill for a cabin setup. my single Multiplus-II handles everything fine honestly

also worth noting the OP's setup is in the Netherlands — regs and grid connection rules there differ from UK so I'd be cautious about taking their specific config as gospel for anything you're planning over here

@EcoFlow_Gal fair point on the subforum, though mods can shift it if needed — not really worth derailing the technical discussion over housekeeping.

@TelHall one thing worth flagging specifically about the Gelderland setup you've linked: Dutch grid standards (NEN 1010) differ meaningfully from UK G98/G99 requirements, so don't lift their configuration wholesale. Specifically, the earth-neutral bonding behaviour on the Multiplus-II needs to be set correctly per your DNO's requirements — VEConfigure's ground relay settings aren't universally transferable between jurisdictions.

For a single-phase UK installation scaling toward 3-phase eventually, I'd strongly recommend getting your G99 application sorted before committing to hardware, as your DNO may impose export limits that fundamentally change whether the 6k5 capacity is even useful to you.

What's your current battery chemistry and capacity? That'll determine whether the inverter upgrade or storage upgrade should come first.

@TelHall funny you mention this — I went through exactly this internal debate last year on the boat. Single-phase felt perfectly adequate right up until it didn't, if you know what I mean.

The thing that finally made me stop dithering was EV charging. Once you're pulling serious amps for a vehicle, three-phase suddenly stops looking like overkill and starts looking like basic sense. The Multiplus-II 6k5 is a lovely bit of kit but that 50kWh storage is doing a lot of the heavy lifting there I'd imagine.

One practical question worth asking yourself — is your property actually wired for three-phase? In the Netherlands you're more likely to have it already sorted than us lot in the UK, where single-phase domestic supplies are boringly standard and upgrading the DNO connection costs a small fortune.

That consideration alone kept me on single-phase longer than I'd like to admit! 🤦