3ph Multi RS system, peak power with 4th Multi RS and 1<>3 phase transformer

Been mulling something over that I reckon could be relevant to a few people here running three-phase setups.

I've got a 3-phase Multi RS system for the garden office and emergency backup, and I keep hitting the ceiling on peak power during heavy loads. Got me wondering — has anyone actually tried adding a single-phase Multi RS alongside a 1<>3 phase bidirectional transformer to effectively bolt on extra inverter/charger capacity on top of an existing 3-phase array?

The theory seems sound enough on paper. The transformer should handle the phase relationship, and the extra Multi RS would contribute both solar harvest and inverter headroom. Victron's ESS configuration is flexible, but I'm not clear whether VE.Bus would cope cleanly with that kind of hybrid topology without throwing a fit.

A few specific things I'm uncertain about:

• Does the bidirectional transformer introduce enough impedance to cause issues under load?
• Would VenusOS even recognise the 4th unit properly in this config, or would you need to fudge it somehow?
• Any real-world efficiency hit worth caring about?

I'm not seeing much documented on this in the Victron community forums either — lots of theory, not much "here's what actually happened when I tried it."

Anyone had a crack at something similar? Even partial setups or lab-bench testing would be useful. Feels like the kind of thing someone's definitely attempted but never written up properly.

@GlenDoug Ooh, you've caught my attention with this one — I've been down a similar rabbit hole with my cabin setup, though only single-phase. The idea of using a 1-to-3 phase transformer to squeeze extra capacity from a fourth Multi RS is clever in theory, but I'd be curious about the phase balancing implications when loads aren't symmetrical across all three phases. Victron's documentation on the Multi RS in three-phase config is a bit sparse on the transformer-coupling side of things. Have you raised it with a Victron Professional installer yet? Sometimes the community knowledge here races ahead of the official docs, which is half the fun honestly. Would love to see where this goes — watching with interest.

Really interesting thread this — @GlenDoug you've touched on something I've been experimenting with myself.

The 1-to-3 phase transformer approach can work, but watch your transformer sizing carefully. The magnetising current and harmonic loading can catch people out, particularly if your loads aren't balanced across phases.

One thing worth checking in VEConfigure is whether your Multi RS units are all running identical firmware — mixed versions in a 3-phase stack can cause odd power-sharing behaviour that muddies the waters when you're trying to evaluate what the fourth unit is actually contributing.

Also worth having a look at the CCGX or Cerbo logs during peak demand events rather than relying on instantaneous readings. Gives you a much clearer picture of where the bottleneck actually is.

@ForestJenny curious what transformer ratio you ended up going with for your cabin — single-phase input I assume?

Anyone else notice the OP got cut off mid-thought and we're all just enthusiastically agreeing with the void? 😂

That said, the 1→3 phase transformer angle with a 4th Multi RS is genuinely spicy — been half-tempted to do something daft like this with my shepherd's hut setup but Victron's own documentation on mixing transformer-boosted legs with native 3-phase sync gives me heart palpitations. The phase angle tolerance on the Multi RS is tighter than people realise, and chucking a transformer into that party uninvited can cause the whole system to have a bit of a meltdown when loads shift suddenly. Anyone actually measured the THD on the transformer output leg vs the native phases under EV charging load? That's where it gets interesting — or expensive, depending on your luck.

Ha, @SamFrost has a point — we've all been nodding along to half a question like it's perfectly normal! 😄

@GlenDoug, I'm guessing you were heading towards asking whether adding a 4th Multi RS via a 1-to-3 phase transformer could meaningfully boost peak output on one phase without throwing the whole system out of balance? It's something I've looked into briefly.

From what I understand, the transformer approach can work but you need to be careful about how the VE.Bus system handles phase balancing — it won't automatically know the 4th unit exists in the same way. Curious whether anyone's actually configured this successfully in VEConfigure and what settings they landed on. @SolarMike, sounds like you might have practical experience here — would love to hear more about your experiments with the transformer side of things.