eg4 6000xp critical load panel shared neutral?

[mattbrad2]

I'm probably over thinking this, but I'm planning on having an eg4 6000xp as a grid assisted inverter to a critical load sub panel. I'll have about 8 circuits in this panel that will be completely separate from the main panel (both hots, ground and neutral). Originally I was just going to have solar panels feed a bank of batteries and this sub panel, while simultaneously having grid power the rest of the house at the main panel. I wasn't going to have anything connected to the grid input, but someone brought up the idea of having a 'grid assisted' mode where you would keep a 50A circuit to grid input, and let the grid could take over in fringe cases where we had a string of cloudy days and the batteries wouldn't be sufficient to carry the sub panel load. My question is how to wire up neutral in this scenario? With 4 wires to grid input and a shared neutral to the loads, you have a scenario where the inverters will be supplying ~25 - 30A from the batteries/solar to a separate load panel (with neutral shared with grid), while simultaneously having the grid power the main panel and using the same shared neutral. When you draw it out on paper it makes sense, but you're basically creating an unbalanced load to the main neutral wire going back to the main transformer. Neutral would effectively be the same return path for two completely separate outputs. This is slightly different than a multiwire branch circuit where your neutral going *back* to the main panel is potentially carrying an unbalanced load, and I know the transformer neutral is sized appropriately for these fringe scenarios - but it seems like I'm missing something obvious here. The alternative is having a triple pole double throw transfer switch where your switching both hots AND neutral to the subpanel. This seems a bit overkill if its in fact completely safe to just let both neutrals meet back at the main panel, where of course you'll have your only N-G-Bond.

 

[timselectric]

It's not a shared neutral. Both systems are not using it.
It's referred to as a common neutral. And is what you want.
You have already figured out why. (One N/G bond)

 

[mattbrad2]

But both systems would be using it. The grid would be running most of the house, while the inverter would be powering a subpanel (from batteries/solar). So the unbalanced load on the neutral back at the main panel wouldn't be an issue? Eg. the grid will be powering 60A, while solar/battery is powering 20A, but common neutral will be able to handle both power sources simultaneously?

 

[timselectric]

But both systems would be using it. The grid would be running most of the house, while the inverter would be powering a subpanel (from batteries/solar). So the unbalanced load on the neutral back at the main panel wouldn't be an issue? Eg. the grid will be powering 60A, while solar/battery is powering 20A, but common neutral will be able to handle both power sources simultaneously?

It's not handling both simultaneously.
Part of it is handling the grid loads and Part of it is handling the inverter loads.
But no Part of it is handling both.
Current flows in a circuit (circular) from the source, through the load, and back to its source.
Unless you have done some incorrect wiring, the two paths won't overlap each other.

 

[mattbrad2]

Ok, that makes sense. I was thinking that the subpanel would still go out the mains neutral (back to the grid transformer), but in fact it would go back to the inverter. I guess because of the way the eg4 ties both AC-in and AC-out neutrals together, had me thinking neutral would always flow back out to the mains. When its in bypass mode, it would work that way but when running off battery/solar, neutrals electrical path would return back to the inverter.

 

[timselectric]

Ok, that makes sense. I was thinking that the subpanel would still go out the mains neutral (back to the grid transformer), but in fact it would go back to the inverter. I guess because of the way the eg4 ties both AC-in and AC-out neutrals together, had me thinking neutral would always flow back out to the mains. When its in bypass mode, it would work that way but when running off battery/solar, neutrals electrical path would return back to the inverter.

Yup
You've got it.