anthem avatar image

Dual MultiPlus - split phase or parallel

Alright, question on setup. I'm replacing a pair of old Heart Interface inverter units. They are wired as such - coming in off of USA 50A/125/250 service. Each inverter gets one leg of 50am 125V. They each feed a panel. So technically they arent in parallel as the combination isnt fed to the same service panel - though I could bind the two old outputs to both Victron units to combine the two inverted loads to be parallel fed by the two Victron units.

OR just keep them separate - one inverter feeds P1, one inverter feeds P2. But for this - shouuld this be configured as split phase eventhough the outputs dont need to be phase corrected on output ? They are essentially dual inverters that are being run.

They are both being drawn off the same battery bank. And the generator feed comes out feeding them separate as well.

So should they be in split phase (is my guess), parallel, or I have read that they could be completely indepedent. But if they are independent, the CGGX wont report on them or show my entire system correctly ? right ?

Trying to determine the best configuration so that my CGGX can report on the two units and get an accurate reading.

MultiPlus Quattro Inverter Charger
2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

2 Answers
ben avatar image
ben answered ·

The main reason to choose split-phase is if you have 240V appliances. That's unlikely if you have two physically separate panels.

You can do parallel, and you get the benefits of redundancy (if one fails) and the extra capacity to handle a given set of loads that would have exceeded a single inverter's capability. You do have to wire them correctly, and what you wrote at the top doesn't sound quite right to me. (Draw a picture if you're not sure and post here, though.)

If you do two separate inverters, they will work fine, but you are right: Venus/CCGX can't display them neatly. (There is a sort of hack that will give you a way to see some of the second inverter's state in a menu, but it doesn't really show you things right in the GUI.) Your option then is to use two Venus devices and have two screens.

You can probably guess which one I'd pick!

2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Unfortunately, as is parallel mode implemented now, you do not get redundancy.

Which is a shame, and I believe Victron could do better.

That's a good point: the redundancy is not automatic. But, after a few settings changes, this RV owner could be back online. Good enough for that application (but not for yours).

The problem I see is that not only you do not get (automatic) redundancy a.k.a. failover/hot-standby and what we all know from HA computing, you actually get less availability from doing the parallel hustle.

As all devices have to run to have a functioning system, the probabilities of failure add up. It's actually very comparable to a RAID0 hard disk array. One disk fails: data gone.

Here: One Quattro/Multi fails -> outage.

In that respect - if availability is your concern - you are actually better off using e.g. one 10kVA device instead of 2 x 5kVA. Assuming they all have the same MTBF.

Which by the way would be a nice move from Victron to publish. The MTBF.

There's a 5y warranty, and if we assume the things live 10y on average (I don't know - just guessing, that's why an authoritative number from Victron based on the RMAs they DO have would be nice), then that would be a benevolent MTBF of 90k hours.

Compare that to 1.2M hours of a modern consumer HDD - and there you do not entrust it with critical data in a RAID0.

The analogy is something more like, if I have a RAID0 and one drive goes down, the other drive is also useless because all my data is kaput.

Whereas if one inverter goes down, with a small effort I can still use the other one.

In the field, where parts and 5kVA inverters may be hard to come by, that's an important difference.

I still agree that it might be possible in firmware to have at least the slave fail while keeping the master online.

Ok, JBOD then (JBOI - Just a bunch of Inverters) ;-)

Define small effort:

180kVA 3p system made out of 12 x 15kVA Quattros.

Phase 1: 4 x 15kVA

Phase 2: 4 x 15kVA

Phase 3: 4 x 15kVA

Now any one of the Quattros goes down in a blaze of glory. (we remember this is 12-times more likely to happen than if you had just one 15kVA Quattro, if we assume a 90k hours MTBF, it will happen after 8 months operation)

=> The whole system comes to a halt.

Please - how small do you think the effort is to convert it "quick in the field" to a 9 x 15kVA config?

And all of that just because one phase has 45kVA and not 60kVA. Pfff...

JBOI, lol.

I hear you regarding a huge install. Does anyone offer modular inverters that can sync waveform and allow arbitrary units to drop out in real time?

It's not something that's very important for small systems like ours, but it sounds like something there is a market for.

I dont think you get independent units working in split phase either. it looks like if one inverter is offline for whatever reason in split phase - then the other unit goes off line as well. So its essentially a requirement to have all units operational to have this working.

Granted, if in that situation - if you had ve.configure handy, you could break the split phase or parallel to bring one back on line. . .. .

Also, if in split phase - apparently the feed in to the other unit, must be in that phase. If you feed from a 30amp service, which is a dogbone type of bonding of l1 to l2 - (essentially not out of phase with one another),. then the other unit will not come on line.

So lots of issues and unfortunately a compromise decision has to be made with either choice.

Split phase is completely reasonable that all units have to work. THat's a technical necessity

In parallel or 3p mode it's not.

I.e. I can switch off single phases in a house installation - so what? Probably low hanging fruits in Victron firmware would be to accept failure of non-masters in 3p and parallel configurations.

IMHO mid-term it will be inevitable for Victron to get rid of the master-slave architecture and adapt a more elaborated voting/heartbeat scheme.

There is a lot of companies out there trying to distinguish themselves with unique features. Just saying...

very true. assuming split phase is used to get 120/240. But if you are only using it to power two inverters that are taking feeds from alternating phases of a power supply - namely 50amp 125/250 service - to just be operational, then the 240 isnt relevant. . .

Here is a question - if you configured it in parallel and it was feeding two independent panels. Can you feed one inverter from L1 and the other inverter from L2 and their outputs are NOT feeding the same output (different panels). Would that work ? Would the GX units be able to determine total output even if they werent balanced in output ?

[Feeding parallel from different phases]

It's an educated guess, but I am pretty sure that wouldn't work. The Quattros/Multis "sync" via VE.Bus and would actually see they are out of sync.

You could - also educated guess, but also pretty sure - feed with two parallel Multis (which are fed by the same Lx) two independent AC-Out branches.

On the other hand that could get nasty as soon as you'd have on one of these branches energy coming IN and on the other not. Maybe that could cause some trouble.

And of course both should still be connected to the same battery.

and that brings back to my original dilemma/question.. .

I have two inverters off same battery bank. I have shore power that feeds with two legs (180 deg phase). Each feeds a separate panel. If I split phase - no big deal. as essentially I'm feeding two separate panels off one leg of power each. But the incoming power must be out of phase with each other or only one leg of power is used.. .

If I parallel - does the incoming power to the inverters need to be in phase ? Because ideally, you want L1 in inverter 1 and L2 in inverter 2. These will be out of phase with each other. And then the follow -up - does the output need to actually be in "parallel" - as in both inverters feeding both legs evenly.

I have to admit, I'm starting to get lost with some nomenclature here.

Evidently you have two cases:


You configure the Inverters "Split Phase 180 degrees" for your shore power. This seems to be the right solution if your shore power has that specs. I do not get the "But the incoming power must be out of phase with each other or only one leg of power is used."


Yes, I strongly believe (! never tried that experiment) that if you want to drive two inverters in parallel and feed them different phases it won't work.

I base my guess on the fact that a 3p config will complain if the quattros/multis aren't given the "right" phase. If they could shift it to compensate, they wouldn't complain and then with that same tech two parallel inverters could accept inputs out of phase.


I guess someone from Victron would need to enlighten us if that is so, and maybe even if it could work in principle (enter the firmware gurus) or not.

I wrote an explanation of the options below.

Basically, when any multipluses are configured as a unit, they operate atomically. But there is an exception in split-phase mode when both are fed the same (single) phase.

IMO this is irrelevant for USA RV users, because the one time when split phase is available, each single phase is still 6kW, and 6kW is much more than most RVs can use on a sustained basis. Hence, using only one phase is okay.

But there are other tradeoffs that are also important, so I noted them below.

ben avatar image
ben answered ·

Here are the ways you can use two MultiPluses in USA RV parks.

[1] You can run them both completely independently, and then you need two CCGX, etc., if you want the full stack of monitoring and visualization.

[2] You can run them in a parallel "stack" and unify your load centers. The advantage of this is that you get the full power distribution across the whole set of loads. The downside is you cannot run 240V appliances, they run or fail as an atomic unit, and your minimum shore current in assist mode is twice the specified minimum for a single unit. You must wire both units to the same leg when you are on a 50A supply, so you will have a maximum of 6kW shore power available on 240V shore supplies.

[3] You can run them in a split-phase pair. There are two advantages of this approach compared to stacking. First, your two units are wired to each phase when on a 240V supply, so you have access to the full 12kW at the pole, subject to the internal limits of your MultiPluses. (They won't be able to charge that much, for example.) Second, if you have a single phase supply (120V 30A, 20A, 15A), you can feed it to one side and the other side will reject incoming and stay in inverter mode. You will have seamless load support on both legs, but you will be going through the battery for the second leg. A consequence of this setup is that if you have a large 120V generator, you may not be able to fully utilize it, because it can only feed one of your two units.

Which you should choose is pretty situational. Some examples:

  • If you mostly have loads that total less than 6kW (hint: you do, if you live in a regular RV), you could choose [2] for the max power delivery flexibility.
  • If you have 240V loads, you must choose [3].
  • If you regularly use very small shore supplies, like say a 100-foot extension cord connected to your friend's 15A exterior outlet, you may need to chose [1] or [3] so that you can set a low enough shore limit.
  • If you value having a quick failover in the event of a unit that dies, [2] gives you the cleanest path to bring your entire set of loads back online.
  • If you have a large 120V generator relative to your selected MultiPlus size, you cannot choose [3] if you wish to use the generator's full capacity.

If you're willing to add components, you can achieve the benefits of [2] while using [3] by adding an autotransformer on the output side of the inverters.

It's not too hard to toggle between these three modes, as long as your two inverters are the exact same model and hardware revision level. You could experiment with one approach and change down the road if you don't like the consequences.

2 |3000 characters needed characters left characters exceeded

Up to 8 attachments (including images) can be used with a maximum of 190.8 MiB each and 286.6 MiB total.

Ben. Great answer and what I have come to realize. Parallel limits you to one leg of input on your 2 legged 50 amp service. Phase limits you to one inverter if you have to use 30amp service (which may not be a bad thing since you only have 30amp anyhow).

I’m going to check the autoformer as well and see what that does.

Ok. Trying to wrap my head around the autotransformer. So it will feed both inverter outputs (configured in split phase ) to the AT and that will output ( up to 100amp potentially) in 120 (or 240 capability) in one common phase. So if I had two separate ac panels busses to power - I would power both off the AT output.

Thinking things through - for 50amp service this would be fine. No redundancy but you didn’t have that before anyhow. One set of gx monitors still. This seems like a good solution that fixes the parallel problem being out of phase ?

For 30amp/single phase you would still have a problem as inverter 2 would stay offline and have the same problem. And the AT would fail or am I missing something ?

In mode [3] above, placing an AT after the inverters means you still feed split-phase to your panel like usual. But, the transformer will transfer up to 28A/120V continuous (32A for a few minutes) from one leg to the other on demand. It acts as a balancer, redistributing power as needed.

That means you could have your inverter's base power PLUS up to 3360VA from your other inverter feeding a single leg.

You essentially get the power-sharing benefits of mode [2] but in split-phase mode [3].

There are other ways to use ATs as well, but that one is probably the most interesting for your scenario.

And presume that’s for the 32amp model. Whereas the 100amp model operates the same way just at the 100 amp level ?

Thank you for the explanation. So essentially the one leg issue is still present but the AT takes the power from the line fed inverter and battery inverted inverter to form its power input. At some point you run out of power from the battery side of the pull on the AT output exceeds what the line fed inverter can put in to the battery bank to feed the out of phase inverter.

Also, I think I need to turn switch as a group off to handle just one leg. Do you know where that setting is ? I cant seem to find it in VE.configure

The coil has the same capacity in both units, so both can sink or balance 32/28A continuous. That 100/32 number refers to the rating for the pass-through current (power transferred at 240V).

The scenario I'm suggesting you could consider using the AT is the one where you're not on one leg, you're on split-phase. (Well, you're on one leg when you find a 30A outlet.) Independent of the input side, the AT balances a pair of inverter outputs across two phases. That allows inverter 2 to support inverter 1 if loads get heavy on leg 1.

I don't run a split-phase inverter pair, so I'm not sure exactly where the setting is for getting it to work, sorry. You can pose a new question for that here and someone will probably be able to help you.

I do see a benefit in running the AT. . it gives you a balanced output off the inverters. Not sure that is that helpful is neither inverted leg will exceed that capability of any one legs worth of power. I do have 3 Cruisair AC units, but those are not currently run off inverter, but off direct shore power/generator power. Debating whether to run one unit off of the dual inverter setup, and perhaps this will also help in balancing that configuration out.

Quite a few significant tradeoffs in each direction. The AT almost brings it back to how you want it to be.

Just curious - do you run parallel or split phase on your setup ?

Yes, if your loads on one leg will never exceed your inverter's capacity, you don't really need the rebalancing. (Although having it would give you some options in a failure scenario.)

I run a 240V inverter and make split-phase using an autotransformer -- also on the output side similar to what we've been discussing. I went that route because there weren't a pair of 120V inverters big enough, at the time.