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halfwalker avatar image
halfwalker asked

Choosing between one large inverter or two smaller for US 120-0-120 split phase

We're getting ready to install a new Victron setup at home. The old Trace (before Xantrex even !) finally died, and upgrading is in the winds ...

Being in the US, we need 120-0-120 split phase of course. Can anyone show the pros and cons between the two options here ?

* One large 230V Quattro to take in 240V from grid, 240V from gen (neutral not connected), spit out 240V to 100A autotransformer to sub-panel for two L1/L2 120V supplies.

* Two smaller 120V Quattro to take in 120V each from the two grid L1/L2 legs, and L1/L2 from gen, spit out two L1/L2 120V legs to sub-panel.

system designAutotransformersplit phase
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6 Answers
cookinwitdiesel avatar image
cookinwitdiesel answered ·

For a system with a fixed 240v input I like the single large unit with autotransformer approach. It is just simpler to manage, configure, install, scale out. It should also be cheaper I would imagine.

In my RV I have scenarios where I need to manage both 120v and 240v inputs so it is not as simple, but for a house with a 240v input and that wont change, the single unit seems better. You can throw in a 48/10000 or 48/15000 Quattro and have a very nice setup!

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Jason Bolduc avatar image
Jason Bolduc answered ·

I have used option #1 with the autotransformer to create split-phase here in Canada.

Since that time there have been zero (0!) Issues with the setup. I run 2 x Multiplus 5000va 230v in parallel and have frequently run total loads over 7000w for hours without issue.

As my family is off-grid it also feels good knowing I have some redundancy if one of the inverters decide to fail mid-winter etc.




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halfwalker avatar image
halfwalker answered ·

Ah, I like that, a third option for redundancy ...

* Two smaller 230V Quattro inverters to take in 240V from the grid, 240V from the gen (neutral not connected), spit out 240V to 100A autotransformer to sub-panel for two L1/L2 120V supplies.

For example, two 48/5000 at $2330ea for the power equivalent of one 48/10000 at $3778, but more expensive. Is that redundancy worth the extra $1000 though ...

Rough cost comparison (very rough)

  • Option 1 : $4457 One large 48/10000 ($3778) plus 100A Autotransformer ($679) no redundancy
  • Option 2 : $4660 Two smaller 48/5000 ($2330ea) no redundancy
  • Option 3 : $5339 Two smaller 48/5000 ($2330ea) plus 100A Autotransformer ($679) redundant

I don't think there's any benefit to Option 2 ... more expensive, more wiring etc. So it's really a choice if how much is redundancy worth ? Here it's about $900, probably a bit more though with the extra wiring/breakers/etc. more space. And while redundancy is nice, there IS also more to go wrong.

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halfwalker avatar image
halfwalker answered ·

Ah - something I hadn't considered. In the US we have 240V split-phase - the center/neutral is bonded to ground. So we CAN'T take in 240V from the grid to the Quattro, since that would ground one of the legs. Bad things, right ?

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halfwalker avatar image halfwalker commented ·

Hrm - I found this doc

https://www.victronenergy.com/upload/documents/Manual%20-%20Quattro%205k%20120-240V%20-%20rev%2002%20-%20EN.pdf

Which specifically shows taking in 120/240V US split-phase into a 230V Quattro, and spitting out 240V to an Autotransformer for 120/240V split-phase output.

So I guess you CAN use a larger European 230V Quattro. From what I can tell, that's because the Neutral-to-GND relay in the Quattro is disabled, using the Neutral-to-GND relay in the Autotransformer to connect the output neutral to GND.

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halfwalker avatar image
halfwalker answered ·

Is anyone here using a Quattro 230V model with an Autotransformer as described about ? That is, taking in 240V via US 120/240 split phase, feeding to Autotransformer and outputting regular 120-0-120 split phase ?

Before committing to a rather large expense of non-returnable items, I would like to have some confirmation .....

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solardude avatar image solardude commented ·

Yes, I have installed several 230V units with autotransformers. Works great.

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halfwalker avatar image halfwalker solardude commented ·

Are those residential installations ? How are you handling the Neutral from the Autotransformer going to the sub-panel ?

I'm envisioning a setup as follows :

  • Main panel : 100A breaker to Quattro AC1-in
  • Main panel : 100A breaker to sub-panel 100A breaker with interlock
  • Quattro AC1-out to Autotransformer
  • Autotransformer AC-out 120-0-120 hot-neutral-hot to sub-panel 100A breaker with interlock

The interlocked breakers are a back-feed setup for the sub-panel. Only ONE breaker can be on, the other must be off. So the sub-panel is fed by EITHER the 100A from the main panel, OR the AC-out from the Autotransformer.

In the US, there can be only neutral/ground bonding point, it must in the main service panel. So the GND relay in the Quattro should be disabled, and the GND relay in the Autotransformer should NOT be connected to the Quattro.

Does this all make sense ? Is it correct for a US residential install ?

I updated the image to make it a bit clearer (I hope)

https://www.autodraw.com/share/GAHSPWACM7TV

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Kevin Windrem avatar image Kevin Windrem halfwalker commented ·

This won't work. Consider pass-thru. The two hots going into the Quatro end up at the autotransformer which makes a new neutral but it may not be at the same voltage relative to the two hot legs as the one in the main panel so you can't connect them together.

You need to switch neutrals as part of your back-feed interlock in the sub panel.

I assume your back-feed interlock is for redundancy in case the Quatro dies. If so, you could do this with a switch that bypasses the Quatro and autotransformer, switching the two hots plus neutral into the sub panel.

You could also use an isolation transformer instead of an auto transformer. Then the neutral created by the transformer is floating relative to the source hot legs and CAN be connected to the main panel's neutral in the sub panel.

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solardude avatar image solardude Kevin Windrem commented ·

Your drawing looks ok other than the circuits from main panel to sub panel.

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halfwalker avatar image halfwalker solardude commented ·

Keep in mind that the two hots from the main panel and the two hots from the Autotransformer can NEVER both be connected in the sub-panel. That uses an interlock-breaker setup, such that only one OR the other can ever connect.

So the sub-panel would either be powered via the main panel, OR powered by the Autotransformer. Similar to this

The two hots from the Autotransformer will never connect with the two hots from the main panel.

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audreez avatar image
audreez answered ·

im assuming option#3 is 2x 120v quattros run in split phase config? if so the redundancy comes from autotransformers ability to step up 120 to 240? what happens if 1 inverter kick the bucket without autotransformer, will it still run 1 leg/side of subpanel or second one stop functioning if one dies? sorry for highjacking jsut trying to figure out a best scenario for a grid tied cabin with crappy grid (1 big 230v vs 2x120v in split phase)


thanks!

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