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Manually switching panels output between grid-tied inverters or off-grid inverters

S

solarhombre

New Member
Joined
May 19, 2023
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113
Location
Florida
Hi there.

I have 11.6kWh Tesla T400H panels + 7.6kWh Tesla inverter installed. Recently I added 2xEG4 6500 and 6xEG4-LLv2 rack batteries.

In order to have solar power in case of a grid power outage, I would like to be able to manually switch the solar panels output from feeding the Tesla inverter to feeding the EG4 6500s.

This is the idea I've come up with (see pic below):
* Move the wires coming down from the solar panels from the Tesla inverter to a new metallic electrical box.
* The two wires from each of the three strings go to a breaker (500V, 16A for two of the strings, 32A for one paralleled string).
* The output from the above breakers goes to:
1) newly installed surge protector devices (SPD), 600Vdc, 40kA, which works fine for the Tesla inverter.
2) newly installed changeover switches, one per string. The manual changeover switches (MTS) I found are 400Vdc and 63A.
* In each MTS:
-Position 1 sends to power to the Tesla inverter (default)
-Position 2 sends the power to the EG4 inverters (emergency)

I have to run about 70ft of EMT conduit from where the PV wires currently connect to the Tesla inverter to the garage, where the EG4 6500s are. Once in the garage, the wires go through 3 breakers and SPDs. Maybe these SPDs here are redundant, but there's >70ft of wire run that could cause trouble. These SPDs are 500Vdc, 40kA because the EG4 max input voltage is 500V.

Other than being able to replenish the batteries during a power outage, I might also be able to squeeze some more power from the same panels when the Tesla inverter is clipping (>5h/d between April and August at this latitude!)

Am I missing anything? Specially if related to safety/code compliance. Any way of achieving the same goals and a simpler design? Would you improve anything?

Looking forward to your comments/advice/chides :)
TIA.

SharedPanels.png
 
solarhombre said:
Am I missing anything? Specially if related to safety/code compliance.
Click to expand...
I take it you have a permitted through your local AOJ the Tesla installation, probably done by the installer.

How do you plan on getting this permitted And then inspected.
======
In a much smaller, not-permited install, I wanted to put in a DC switch to switch panels to different places, but cost of DC switches drove me away from this.

Would you mind including a link ot this switch.
 
chrisski said:
I take it you have a permitted through your local AOJ the Tesla installation, probably done by the installer.

How do you plan on getting this permitted And then inspected.
======
In a much smaller, not-permited install, I wanted to put in a DC switch to switch panels to different places, but cost of DC switches drove me away from this.

Would you mind including a link ot this switch.
Click to expand...
The Tesla install permitting was handled by them. For the EG4 6500+batteries install I managed to do all the permitting and part of the work myself (Florida has a special owner-builder thing. Btw, a Tesla installer quoted $2800 for permitting to add 2 powerwalls, and I paid just $91!!!).

I do not plan to ask for a permit for the new wiring, but maybe I should give them a call and check whether that's ok. In any case, a friend of mine who is a licensed electrician will review everything before I start.

The changeover switches I am looking at are https://www.ebay.com/itm/266225617295, and i think that's the weakest link in the plan as it is because the VOC for the 2x8 panels string is ~360V.
 
solarhombre said:
The Tesla install permitting was handled by them. For the EG4 6500+batteries install I managed to do all the permitting and part of the work myself (Florida has a special owner-builder thing. Btw, a Tesla installer quoted $2800 for permitting to add 2 powerwalls, and I paid just $91!!!).

I do not plan to ask for a permit for the new wiring, but maybe I should give them a call and check whether that's ok. In any case, a friend of mine who is a licensed electrician will review everything before I start.

The changeover switches I am looking at are https://www.ebay.com/itm/266225617295, and i think that's the weakest link in the plan as it is because the VOC for the 2x8 panels string is ~360V.
Click to expand...
Another option for the changeover switch that i am considering is https://a.co/d/7IFq0np (20A version). There's also a 40A version with 3 poles that I would use for the paralleled string. These are big, but should fit in the 12"x12"x6" box I would install.
 
solarhombre said:
Another option for the changeover switch that i am considering is https://a.co/d/7IFq0np (20A version). There's also a 40A version with 3 poles that I would use for the paralleled string. These are big, but should fit in the 12"x12"x6" box I would install.
Click to expand...
Doesn't look DC rated. Assuming switching loaded panels doesn't risk damage to gear, I would consider DC panel breakers with a lock-in/out plate. Then again, some of you have codes and inspections to deal with.
 
Agree with sunsurfer.
Those are AC only switches.
Not rated for DC.
DC breakers with interlock is the most cost effective option.
 
Just use 2 DC breakers They do not need an interlock so long as you understand to turn one off and the other on. If the worse happened where both get turned on is you might have a conflict between the Teslas SCC and the EG4 for a bit which is unlikely to harm anything. It is not like you are paralleling a AC circuit.

BTW. Avoid switching while under load.

I use the same arrangement to feed either my #2 AIO from the second array or a SCC. See image.
P1010007.JPG
 
Last edited:
Mattb4 said:
Just use 2 DC breakers They do not need an interlock so long as you understand to turn one off and the other on. If the worse happened where both get turned on is you might have a conflict between the Teslas SCC and the EG4 for a bit which is unlikely to harm anything. It is not like you are paralleling a AC circuit.

BTW. Avoid switching while under load.

I use the same arrangement to feed either my #2 AIO from the second array or a SCC. See image.
View attachment 151659
Click to expand...
This is a very good and affordable option. I have a few of those same breakers around and i could even throw in a breaker lock like https://a.co/d/cn0rAtY to lower the odds of someone turning both breakers on for a string without anything hinting at a potential problem.

When you say switching under load, what scenario(s) are you referring to? Does that apply to EG4s and Tesla inverters or just EG4? SBU vs SUB vs USB?

Thanks!
 
DIYrich said:
Use the rapid shutdown button to turn off production. Then switch where the pane
Click to expand...

timselectric said:
While the array is producing. And current is flowing from the panels.
Click to expand...
Ack. So for a given string that i want to switch, I must turn off the breaker for that string that connects to SPD and the 2 breakers that go to each inverter. Then turn on the breaker that goes to the inverter that will receive the current, and then turn on the SPD breaker again, right? Or should I turn off all strings and not just the one I am flipping?
 
Hedges said:
I might use a switch like this to interrupt DC current flow:


... Then inside the box swap MC connectors from one inverter to the other.
Box cover is interlocked; switch must be turned off to open cover, preventing inadvertent disconnect of MC connectors under load.
The 3PST switch could open three strings.
Click to expand...
Expensive and only 30A would not work for me. The breaker going from the Tesla inverter to the main panel is 40A.
 
solarhombre said:
Ack. So for a given string that i want to switch, I must turn off the breaker for that string that connects to SPD and the 2 breakers that go to each inverter. Then turn on the breaker that goes to the inverter that will receive the current, and then turn on the SPD breaker again, right? Or should I turn off all strings and not just the one I am flipping?
Click to expand...
You just have to stop the flow of current before switching over. If current is flowing while you switch over. High voltage DC arcing can occur inside the switch. Damaging the switch and possibly causing a fire.
 
solarhombre said:
Ack. So for a given string that i want to switch, I must turn off the breaker for that string that connects to SPD and the 2 breakers that go to each inverter. Then turn on the breaker that goes to the inverter that will receive the current, and then turn on the SPD breaker again, right? Or should I turn off all strings and not just the one I am flipping?
Click to expand...
Turning off the breaker moves the spark to the breaker, shortening the life of the breaker. Better to stop production.
 
solarhombre said:
Expensive and only 30A would not work for me. The breaker going from the Tesla inverter to the main panel is 40A.
Click to expand...

You can find them on eBay for cheaper.

40A breaker on AC side? If you can stop DC current by opening an AC breaker, that is better than opening DC. Or an AC disconnect (many standard duty ones available, less expensive than this heavy-duty Square-D one.)

What is the current per string or per MPPT on DC side? That's what this 3-pole, 30A, 600V AC/DC disconnect would be for.
 
DIYrich said:
Turning off the breaker moves the spark to the breaker, shortening the life of the breaker. Better to stop production.
Click to expand...
I see, DC is not very forgiving :).

For the Tesla inverter, the disconnect is actually the 40A AC breaker in the main panel, and as @Hedges said opening that breaker will be better than opening a DC breaker. In case of a grid outage the inverter will be off which makes things easier.

When switching from EG4 that would mean I have grid power and I can turn off the inverters and then the breakers to be on the safe side. Or stay in bypass mode until it's dark outside ?

Thanks!
 
Hedges said:
You can find them on eBay for cheaper.

40A breaker on AC side? If you can stop DC current by opening an AC breaker, that is better than opening DC. Or an AC disconnect (many standard duty ones available, less expensive than this heavy-duty Square-D one.)

What is the current per string or per MPPT on DC side? That's what this 3-pole, 30A, 600V AC/DC disconnect would be for.
Click to expand...
Yup, I'll open that breaker then. Thanks!

Max current per string is ~10.7A for 2 strings and ~21.4A for the third one. Mmm, one more option then.
 
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