Hi all — I’m reposting this as its own topic because it got buried at the end of an existing thread and I never got a definitive answer. Original discussion for reference: MultiPlus-II 48/5000/70-95 120V not accepting wide range 45Hz to 65Hz — and the model-number breakdown that helped me understand the problem: Multiplus-II 120V System frequency

I would rather have a blunt “no, that will not work” than a maybe. I am about to spend real money on replacement units, so I need to know the actual behaviour before I order anything.

My system as it exists today

· 2 × MultiPlus-II 48/5000/70-95 120V, part number PMP482505110 (the 60Hz-only variant, as I now understand it)

· Configured as a 120/240V split-phase pair (L1 + L2) on VE.Bus, producing standard US 120/240V 60Hz when inverting

· “Switch as a group” is currently UNCHECKED

· 48V battery bank

· Shore power is fed to Victron 3600W isolation transformer (240/120 Auto model) which then feeds ONE of the Multiplus II units.

· Loads include one 240V dive compressor fed L1–L2. Everything else is 120V. L1 panel has only 50hz compatible equipment, L2 panel has equipment that requires 60hz

· This is a vessel, so shore power coming from the same cable is one of three things: 1) 240/120 60 hz split phase 2) 120V 60hz 3) 230V 50hz European style power

Behaviour I rely on today (please confirm I’ve described it correctly)

With “switch as group” unchecked and 120V / 60Hz fed to the AC input of one unit only:

· That unit closes its transfer relay, passes the 120V/60Hz through to its AC output, and goes into charger mode to charge the battery bank.

· The other unit stays in inverter mode and stays synchronised, so the pair still produces correct 120/240V 60Hz split-phase at the outputs.

That is what I observe now and it is important to me — it lets me use the standard output of my isolation transformer.

What I actually want to add

I want to also be able to plug into 230V at 50Hz shore power (I have built plug adapters for the same cabling) and at minimum charge the batteries, without damaging anything and without having to reprogram the system every time I move between a 60Hz country and a 50Hz country.

I understand from the linked threads that:

· PMP482505110 = MultiPlus-II 48/5000/70-95 120V (what I have) — 60Hz only (What my dealer installed)

· PMP482505120 = same, UL 1741

· PMP482505300 = MultiPlus-II 48/5000/70-95 120V 50/60Hz (What I think my dealer should have installed)

My specific questions

1. Inverter output after upgrading. If I replace both units with PMP482505300, can they still be configured as a 120/240V split-phase pair producing 60Hz output when inverting (i.e. does the 50/60Hz variant let me lock System Frequency to 60Hz for the US-style output)?

2. 60Hz shore power on one leg. With two PMP482505300 units in split phase and “switch as group” unchecked, does the behaviour I described above (one unit passes through + charges, the other keeps inverting, pair still delivers 120/240V 60Hz split phase) remain exactly the same? This is the behaviour I do not want to lose.

3. 50Hz shore power on one leg — this is the crux. With the same configuration, if I feed 240V / 50Hz into the Isolation transformer which changes it to 120V 50hz AC which is input to one unit only:

· (a) Will that unit actually accept and synchronise to 50Hz, close its relay, pass 50Hz through to its own AC output, and charge the batteries? Provide power assist?

· (b) What does the other unit do? My assumption was that it would keep inverting at 60Hz on its own leg — but I want to know whether that is actually possible. In a VE.Bus split-phase system, are the two units required to remain frequency- and phase-locked to a single system frequency? If so, does the whole system follow the incoming 50Hz, or does the system simply refuse the 50Hz input, or does it fault?

· (c) If the two legs genuinely do end up on different frequencies, am I right that there is then no valid 240V between L1 and L2, and that I must not run my 240V dive compressor in that state? Is there any risk to the units, the loads, or the neutral/ground bonding in that condition, beyond simply not using the 240V loads?

· (d) If instead the entire system follows the 50Hz input, what happens to my 120V 60Hz loads on the output — do they simply see 50Hz until I disconnect shore power?

4. Mixing models. Can I replace only one of the two units with a PMP482505300 and leave one PMP482505110 in place, and still get all of the above (including proper 120/240V 60Hz split phase when inverting, with the 50Hz only ever applied to the new 50/60Hz unit)? Or does Victron require identical part numbers / Comp_IDs / firmware for both units in a split-phase VE.Bus system? If mixing is not supported, please say so plainly — I would rather buy two units than build something unsupported.

5. Configuration. Are there any settings changes required to make the 50Hz input work on a PMP482505300 — “Accept wide input frequency range (45–65Hz)”, System Frequency, grid code, “switch as group”, UPS function, or anything in the Multiphase/Split-phase configuration in VE.Configure? Or is it plug-and-play once the correct hardware is in place? Screenshots of the relevant VE.Configure tabs would be hugely appreciated.

6. If the answer to #3 is “no”. If a split-phase 120V system fundamentally cannot take 50Hz on one leg while inverting 60Hz on the other, what is the recommended Victron approach for a US-style 120/240V split-phase vessel that needs to charge from 50Hz shore power? Specifically:

· Is the correct answer a separate standalone charger (e.g. a Skylla-IP44/IP65 or Phoenix charger fed from the 50Hz shore supply) feeding the DC bus, with the MultiPlus-IIs left permanently inverting at 60Hz and never seeing the 50Hz input at all?

· Or is there an autotransformer / isolation transformer arrangement that Victron considers correct here?

· (For completeness: Quattros will not physically fit in my installation, which is why the MultiPlus-II route was chosen. So “use a Quattro” unfortunately isn’t available to me.)

· Or can I reconfigure by unplugging one unit from VE bus and installing config files for each inverter to run as separate stand alone unit (yes I will shut off breaker to sive compressor in this case) - This is my least favorite option but might be the only way and I would accept that.

What would help most

A definitive answer from someone who has actually done this on 120V hardware, or from Victron directly, rather than an inference from the 230V models — the 230V and 120V units clearly behave differently here, and the manual is genuinely ambiguous on this point.

Tagging a few people who have already contributed useful information on exactly this topic, with apologies for the direct ping: @OGPS (you documented the three part numbers and the stocking levels), @tonic (you hit the same model change),

Thank you very much — I’m happy to run tests and post results back here if that helps anyone else in the same position.