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Grid assist off-grid system design.

Lukiya

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Joined
Mar 31, 2023
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Location
Newark CA
I'm trying to build a split phase grid assist off-grid system, which will allow me to use grid power when I don't have enough battery power left.

Just in case my inverters broken, I want to switch to grid only (not through inverter line mode) as a backup.

Did some research, seems a manual transfer switch is a good option, but it cost too much, and I found several ones are all for single phase use, not sure whether they fit my split phase build or not.

I'm thinking use interlock kit to do the manual switch. But I can't figure out how to connect ground and neutral to my main panel.

Devices are 2 eg4 6500w inverters, will be in parallel split phase mode. Which will give me 110v/240v 13kw output. My whole tiny house peak load is about 14kw, I never used that much power at the same time.

Below is a rough design, missing some breakers & surge protectors, I'll add them later.
My question is how do I connect Grid G, Grid N, G1, N1, G2, N2? Or this design is totally wrong.

Any advice would be appreciated.

****** Update: For anyone who has the same question, look at below link:

https://diysolarforum.com/resources/grounding-and-bonding-for-eg4-6500-ex-inverters.322/
 
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2 possible solutions
1) Use an ATS.....
Progressive 50a @ 240v/120v Automatic Transfer Switches like this - https://www.amazon.com/gp/product/B003VAWNVK/ref=ppx_yo_dt_b_search_asin_title - will do L1, L2, N (US 240v/120v split phase) up to 50a per 120v leg or 50a @ 240v. That's 12,000w of inverter input.

Mine has grid -> grid input and inverter -> generator input. This means it defaults to grid but will switch to inverter power as long as it senses the inverter is providing power. If the inverter shuts-off, it automatically goes back to grid.

The only issue with this switching is I need a UPS to smooth the transfers for sensitive equipment.

2) Use an AIO like MPP Solar
Feed grid, battery, pv into the MPP Solar and then choose the priorities - battery vs grid. The advantage here is that it has the UPS built-in so it can seamlessly switch between grid and battery.
 
I'm thinking use interlock kit to do the manual switch. But I can't figure out how to connect ground and neutral to my main panel.
That is the easy part. Ground and Neutral would go from your inverters to the main panel and should not be switched. The only place Ground and Neutral are connected to the same place is in you main panel.
I chose to use an interlock on two pole breakers so it would switch both legs of split phase.
 
2 possible solutions
1) Use an ATS.....
Progressive 50a @ 240v/120v Automatic Transfer Switches like this - https://www.amazon.com/gp/product/B003VAWNVK/ref=ppx_yo_dt_b_search_asin_title - will do L1, L2, N (US 240v/120v split phase) up to 50a per 120v leg or 50a @ 240v. That's 12,000w of inverter input.

Mine has grid -> grid input and inverter -> generator input. This means it defaults to grid but will switch to inverter power as long as it senses the inverter is providing power. If the inverter shuts-off, it automatically goes back to grid.

The only issue with this switching is I need a UPS to smooth the transfers for sensitive equipment.

2) Use an AIO like MPP Solar
Feed grid, battery, pv into the MPP Solar and then choose the priorities - battery vs grid. The advantage here is that it has the UPS built-in so it can seamlessly switch between grid and battery.
Interesting, I'll do some research on these devices.
 
That is the easy part. Ground and Neutral would go from your inverters to the main panel and should not be switched. The only place Ground and Neutral are connected to the same place is in you main panel.
I chose to use an interlock on two pole breakers so it would switch both legs of split phase.
Like this?
 
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Like this?
The N and G connections look okay..I can't tell because I can't see the other electrical panel on the right. The interlock should show two sets of feeds, one set to loads panel and one set to inverters. Because you have two inverters I do not know the code and there may not be an interlock that can switch that many breakers. If code permits you may be able to connect both inverters to a double pole breaker.
 
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