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Dual Multiplus 12/3000 in Split Phase Running on Single Phase

Hi folks,

I'm new to the forum but have been looking for an answer. I have two Multiplus 12/3000 inverters with a Venus GX (and additional Victron solar items as well) in a motorhome. These were all purchased in last 90 days or so, so fairly new hardware. The motorhome is wired in split phase and everything works 100% of the time when i'm on a 50amp standard USA RV service. My setup also cuts over to my QD1000 generator flawlessly and everything works fine.

My issue is when i'm plugged into a 30amp single phase plug. When I am, I understand normal operation "should" be that my master inverter will invert and charge the battery and my slave inverter will simply invert. This is a Victron limitation on single phase power as i understand it which is fine. My question is. When i'm plugged into a single phase 30amp plug, neither the master nor slave inverter will recognize the AC input. The input shows in the Venus panel as there on both legs (single phase on both legs because of the 50a to 30a dogbone adapter that pairs the single phase 120v to both legs) but the units will not initiate a charge, its basically like i never plugged anything in at all. See attached screenshot. Any advice on what setting may be causing this to occur? I'm fine with only one inverter charging, i just need to understand why thats not happening..

Any helps or advise would be greatly appreciated.

MultiPlus Quattro Inverter Chargersplit phase
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2 Answers
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epm96 answered ·

Vincent this is exactly what i needed to know (assuming its correct lol!) - I'll test this and report back!

Another question: I'm curious if just modifying the dogbone adapter so it doesn't combine the L1 & L2 but rather just uses L1. Then would it be possible to just leave "switch as group" enabled. Then it would only ever see either 240v correctly phased or just a single 120 on L1? Would i still need to disable the switch as group in this situation?

Also to answer your question on the ATS - I have a SurgeGuard ATS that favors the generator if it senses it, then shore power so if I start the generator after about 30 seconds it cuts over to the generator which would be providing the standard 240 in the 180 degree correct phases on L1 & L2. Not sure if that would cause anything to freak out if it was previously running on just a 120 input from L1.

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The inverters will only "switch as group" when they see the input scheme you have configured them with. In your case, they are looking for 120/240 split phase, each phase 180 apart. If they don't see that, they won't switch. If switch as group is disabled, then each one just looks for 120V.


Let's say you are running in this mode with 1 inverter passing through and the other inverting. Then when the gen starts, the ATS will sense it and switch over. The generator will take all the load on L1 immediately, while the second inverter is validating it's input on L2. The generator makes a dirty waveform because it's completely off balance. Smaller generators may even stall.


Your generator is big so it might be ok. It just has to survive the 5-10 seconds while the second multi validates the waveform on L2.

Makes total sense. Im heading back this weekend to a place i have 30a again to test all situations under load and will let you know what I find out! Thanks for the help!

Did 30 amp shore work with the setting change

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vincent answered ·

IF your 30A 120V adapter (single phase) is jumping to both L1 and L2, then yeah that's not going to work.


Your two inverters are setup in split-phase which means they qualify the input making sure that L1 is 180 degrees out of phase with L2. If they are not - it will reject the input.


You can fix it by disabling "Switch as Group" in the VE system configurator. This tells each inverter to qualify its own input and switch on its own. Then it "should" behave how you want, with one inverter going into pass-through and charge, and the other will stay in "invert" to make up the other phase.


You may run into problems with your generator though with "switch as group" disabled. I have had these issues with a Honda 6500 and with an Onan 7500. What will happen is that in some load conditions, one of the inverters will switch its load to the generator while the other does not. This causes a big load imbalance on the generator, causing it to throw an error. Sudden out of phase loading is a death sentence for generators.


Whether you have this issue or not will depend on how your ATS is wired. It's worth trying, just make sure you test all conditions.


The best way to solve this properly is with Victron's auto-voltage isolation transformer on the input. They only have it in 3600W, but from my understanding, it will automatically detect the voltage on the input (120 or 240) and switch the primary winding to suit. Then you would always feed 120V or 240V to your inverter system (the output does not switch voltages, you need to choose one, only the primary follows the input voltage). The downside of course is you would be limited to 3600W total on your shore input. You'd need to ask Victron if you can parallel them (in theory, should be ok as long as they are matched). An additional challenge is now you need to pick 240V only on the output, or 120V only. So you'd have to set the inverters to 120V (you can still have L1 and L2, they'll just be in phase), and I know for a fact that your generator can be set to 120V only as well. But if you have any 240V loads at all, they will not work. If you want to get around that problem, then you also need an autotransformer at the output. Autotransformers are a fantastic idea at the output of a system like this, because it lets you access all the power no matter what phase existing loads are on, they automatically balance it (inherent in their design)


Another alternative, if single-phase-input is an edge case for you, is to add a second shore inlet for single phase, and connect it to a big charger (or a couple) like a Centaur 12/100.


If I've said anything wrong please someone from Victron correct me!

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