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carlk avatar image
carlk asked

120v / 240v simultaneously with Multi+ Compact split phase config?

I am a home owner with a battery backup system I built for my home. The system provides a limited power source when grid power is not available. It uses a Victron MultiPlus Compact 12/2000/80-50 120v inverter/charger. It has a 400 Ah battery capacity and a small solar array. When used, it needs to be connected manually to the house system, I don’t use the automatic transfer function. It currently only provides a 120v output for the house circuits.

I would like to expand the system capability by adding another identical Victron inverter and configuring them as a split phase system to provide both 120v and 240v to the house breaker panel. I believe this is possible using a second MultiPlus Compact inverter. Appendix E in the inverter manual is my source. I’d like to verify that, in the split phase configuration, the outputs could be connected to a standard household breaker box and operate the same as the grid power. Each inverter 120v output would be connected to one of the ‘hot’ breaker legs and the common neutral would be connected to the breaker box neutral.

There is one 240v circuit for a water pump that I am trying to power (.37kW single phase).

Would this operate as I am expecting?

Thanks in advance for any help.

split phase
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ingo21 avatar image ingo21 commented ·

if the waterpump .37kw is the only 220v thing you want to power , you just could get a 110v to 220v transformer. or a 110v waterpump.

i am running a 110v and 220v system of a 24Vto110and220v inverter. too due to the fact where i live. i live on st martin island in the Caribbean an the french side is 220v and the dutch side is 110v so i have tools and other devices with both voltages... is not nice!

if i were you i would try to stay with one voltage


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carlk avatar image carlk ingo21 commented ·

Thanks for the comments. I’ve considered switching to a 110v pump but it might be too long of a run based on some things I’ve read. I’ll probably get a well company out to give me an assessment about both options for replacing it. It’s twenty two years old so I wouldn’t feel bad about replacing it even if it isn’t broke.

I still haven’t found the answer to my primary question about having both 120v and 240v simultaneously with the split phase configuration shown in appendix E of the MultiPlus Compact manual. I believe one of the benefits of doing that would be the increased available wattage to my breaker box.

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klim8skeptic avatar image klim8skeptic ♦ carlk commented ·
Is your grid connection split phase?
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carlk avatar image carlk klim8skeptic ♦ commented ·
The grid connection is the typical US household split phase. I’m trying to mimic the grid voltage and phase. I know I will be very limited in capacity and will also need to look at load balancing. I’m in the process of measuring the actual wattage of each item I expect to be powered when I use the backup system.
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2 Answers
Kevin Windrem avatar image
Kevin Windrem answered ·

You would need to configure the two Multi Compacts as a split phase system with (I assume 180 degrees between the two legs). They need to be interconnected with a VE.Bus cable.

The incoming grid needs to be split phase power (120 volts from each leg to neutral, 240 volts between the legs). The output will then be the same.

You get more power but each leg will be limited to the 2000 VA of the inverter for that leg. A 240 volt load connected across the two hot legs would draw the same current from both inverters so the power for that load would be split between the two inverters.

Depending on you house loads, two 2000 VA inverters may not be enough. The Multi Compact will pass 50 amps from input to output when grid is present but only about 16 amps when the grid goes away and batteries are supplying your loads. Maybe you have a second breaker box to feed critical loads and can manage the power drawn from the inverters that way.

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carlk avatar image carlk commented ·
I think you answered my main question and I can power the two 120v legs and have 240v all at the same time with two MultiPlus Compacts in the split phase configuration. Capacity and load balancing need to be assessed.


I won’t have the inverter hooked to the grid when grid power is available. It will be a manual switch over after the grid is down. Based on what you are saying about the VE Bus connection, I assume that is what will keep the two inverters 180 degrees apart. It won’t have the grid power as a reference at start-up.

I’m currently doing a detailed measurement and mapping of the circuits I intend to run with the battery system. I’m pretty sure I’ll have the total wattage I need but may need to swap some circuits to have a more balanced load on the 120 legs.

Thanks for your help.

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Kevin Windrem avatar image
Kevin Windrem answered ·

I think you need to give some thought to how you'll charge the batteries. The Multis will charge the batteries from the grid but only if their inputs are connected to the grid. Otherwise you'll need to look into an alternative charging mechanism.

You also might look into a secondary load distribution panel that has the loads you want to feed from the inverter. This could be permanently connected to the Multi outputs. They'd get grid power as long as it was present but would switch instantly to the inverter (and the batteries) when the grid disappears. The remainder of the house would be fed from the first breaker box and only receive grid power. That panel would also provide a 50 amp 120/24 to the Multis.

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carlk avatar image carlk commented ·
Good evening Kevin

There are two ways I charge the batteries. I can use the solar array or plug the inverter into an outlet when there is grid power and put the inverter in ‘charge only’ mode. Right now I’ll recharge from the grid after an outage and keep them topped with the array.

Because I hook the system up manually, after the power goes out, I’m at the main breaker box anyway. The only circuits I don’t want to potentially use are the air conditioners. So when I turn the main breakers off coming from the grid power, I also flip off the a/c breakers in the box.

It’s not elegant but for as often as the power goes out it’s not that inconvenient.

Thanks for the suggestions and making me reanalyze my thought process and options. This type of system is new to me. I’m a mechanical engineer with a significant amount of time spent on the electrical side. I like to analyze systems. Hopefully I’m not in a position to know just enough to be dangerous.

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