8000 W Isolation Transformer on the Output of a 6 kVA UPS?

Been mulling this one over since I nearly did something similar on the motorhome build — anyone here actually run a Victron isolation transformer downstream of a UPS rather than straight off the inverter?

The French sparky crowd are apparently debating whether the 8000 W Victron unit plays nicely bolted onto the output of a 6 kVA UPS, and honestly it's got me curious about the practicalities for UK setups too.

My gut says the transformer doesn't care much where its 230 V comes from — grid, inverter, or UPS output — but I'd imagine the UPS's output waveform quality matters enormously here. A modified sine from a cheaper unit feeding into an isolation transformer sounds like a recipe for a very warm and very expensive paperweight.

A few things I'd want to know before attempting it:

• Does the inrush current on transformer energisation cause the UPS to have a wobble?
• Any derating needed given the UPS is already doing the heavy lifting?
• Would a Quattro-based system just make this whole arrangement redundant anyway?

Running a Fogstar 48 V bank through a Multiplus II on the van, so it's slightly different territory, but the underlying question about stacking kit in series feels relevant whenever someone wants galvanic isolation without ripping everything out.

Anyone actually wired something like this up on a static off-grid site or even a larger motorhome/boat install? Curious whether it's genuinely viable or just an unnecessarily complicated way to find out your RCD has feelings.

@AlexHobbs done something close to this on the shepherd's hut — not a UPS but a Victron Multiplus feeding an isolation transformer downstream. Worked fine electrically but the transformer inrush on startup caused the Multiplus to throw a fault until I tweaked the dynamic current limiter settings.

Worth checking your UPS output waveform too — some cheaper UPS units output modified sine which transformers hate, runs hot and hums badly. If yours is pure sine you're probably fine but still watch the kVA headroom. An 8kVA transformer on a 6kVA UPS is tight if you're anywhere near load capacity.

What's the actual use case? Sometimes there's a neater solution depending on why you need the isolation.

@AlexHobbs this is actually relevant to my garden office setup — I've been wondering whether to add an isolation transformer between my inverter/charger and the office supply for safety reasons (damp-ish brick outbuilding).

One thing I'm not clear on though: does running an isolation transformer downstream of a UPS affect the UPS's ability to detect overloads or faults accurately? My concern is that the transformer's inrush on startup might cause the UPS to trip or throttle back before the actual load even kicks in.

Has anyone measured what the inrush looks like on an 8 kVA isolation transformer? Wondering if it would even be an issue with a 6 kVA unit upstream — that's already quite close to the transformer's rating.

@AnneWatson curious what Victron kit you're running — did you notice any startup quirks with yours?

@AlexHobbs the VA rating mismatch is the obvious concern — you're asking a 6 kVA UPS to drive an 8 kVA transformer core before you've put a single load on it. Transformer magnetising current isn't massive but it's not nothing either, and some cheaper units don't love inductive loads on the output.

More importantly: why does this setup actually need galvanic isolation? If it's purely for fault protection on a floating off-grid system you may be solving a problem that doesn't exist.

I've got a Victron Multiplus-II feeding an isolation transformer on my setup but the transformer sits before the UPS in the chain, not after. Made more sense for my shore power input situation.

What's the actual use case here? The answer changes depending on whether you're on grid, hybrid, or fully off.

@SOC_Nerd makes a fair point on the VA mismatch but the real killer is the inrush on that transformer core at startup — your UPS is going to absolutely hate that. Most UPS units will trip on overcurrent before the transformer even gets going properly.

Did something vaguely daft like this on the van conversion — transformer wasn't 8 kVA but the inrush still caught me out. Ended up adding a softstart relay upstream which sorted it.

Worth checking if the UPS has any inrush tolerance spec in the manual (spoiler: most don't mention it and you find out the hard way 😅).

If the isolation is purely for safety rather than noise rejection, might be worth asking whether you actually need 8 kVA of transformer there or if something smaller does the job.