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Use Main Panel as Critical Loads Panel?

If you mean while connected to the grid, yes. There are other subpanels on the property.
While house is powered from grid, IF the autotransformer is in parallel (L1, L2, N) to the grid, then the other subpanels will also be balanced by the AT.
If you’ve wired it so this cannot happen, then everything is fine.

If you mean while the grid is down, no. Even though I am using my main panel as critical loads panel, no other panels or loads will be fed by the AT while on backup.
sounds good

Note that I won't be using the AT to power any loads while the grid is up.
If L1 L2 from the AT is not connected while grid is up, everything is fine.
or even if just the AT neutral is disconnected, that works too.
 
While house is powered from grid, IF the autotransformer is in parallel (L1, L2, N) to the grid, then the other subpanels will also be balanced by the AT.
If you’ve wired it so this cannot happen, then everything is fine.
The way I think Growatt designed it, I think they assume that the (presumably lower impedance) utility transformer will carry the lion’s share of the balancing work. The utility transformer is wound to handle kilo-Amps worth of VA vs the 30A of these ATs (granted some of that headroom would be from better cooling)

I would think the neighbors are always strictly closer to the utility transformer than your AT, since the distance to your AT always adds on to their distance to the pole. So at least the wiring part of the impedance gives the utility transformer a better shot.

The other subpanels can be closer to the AT than to the utility transformer.
 
While house is powered from grid, IF the autotransformer is in parallel (L1, L2, N) to the grid, then the other subpanels will also be balanced by the AT.
If you’ve wired it so this cannot happen, then everything is fine.
Good to hear. The AT will not be in parallel with the the grid while powered from the grid.

Thanks!
 
I would think the neighbors are always strictly closer to the utility transformer than your AT, since the distance to your AT always adds on to their distance to the pole. So at least the wiring part of the impedance gives the utility transformer a better shot.
I'll have to look where their transformers are, but mine is on my property about 150 feet from the street (underground lines). Then another 300 feet from the transformer to my house/main panel. I think if I followed the wires from my utility transformer back to the street, through their transformers and then to their house, it would be about the same distance as my house is from my utility transformer. So there are two utility transformers between my house and my neighbors. Again, I will have to confirm this.
The other subpanels can be closer to the AT than to the utility transformer.
One of the subpanels is within 30 feet of the utility transformer. The other one is about 75 feet beyond the house (furthest away)
 
In that case their loads cannot affect your AT. Since those two utility transformers are both isolation transformers and don’t have any wire connection between their secondary neutrals.
Just to confirm, the transformers for my neighbors are at the street just outside my property. So I am redundantly isolated from the neighbors. Electrically speaking, of course.
 

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