battery back-up transfer switch sizing

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electro7

Senior Member
Location
Northern CA, US
Occupation
Electrician, Solar and Electrical Contractor
Hi,
I am installing a 10.6kw interactive with battery back-up PV system including 2 Sunny Boy 5000's and 2 Sunny Island 6048's. The customer already has a 15kw Generac Guardian generator installed with a 100amp ATS that feeds his house sub-panel. Am I thinking along the right lines in that I can use the existing 100amp ATS for the 2 Sunny Islands? They are 56amps at 120volts each. The ATS is 240volts. I would replace the house feed with the Sunny Island feeds. Thanks for your help.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Your question isn't super clear. Do you have in mind to insert the Sunny Islands between the ATS and the house subpanel? So that the AC2 on the Sunny Islands is connected to either the grid or the generator?

If so, the answer is yes, you can do that, although you must also wire the proper relays for generator control by the Sunny Island(s).

(If you have in mind something else, such as putting the Sunny Islands on the supply side of the ATS, then I think you'd be on the wrong track.)

I'd contact SMA tech support if you have any questions about whether you're doing it right.
 

electro7

Senior Member
Location
Northern CA, US
Occupation
Electrician, Solar and Electrical Contractor
Yes, that is what I was thinking. The AC2 connected to either the grid or generator and re-wiring circuits for the protected loads panel. And I did read the technical note about the relay that needs to be added. So two Sunny Islands connected to a 100amp ATS is fine size wise, right?
Thanks
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
So two Sunny Islands connected to a 100amp ATS is fine size wise, right?
Thanks

For the Sunny Islands, yes. They are divided between the two legs so it's only 56A on each leg (requires 70A conductors).

Your equipment and conductors also need to be sized for the Sunny Boy 5000s, with the 120% rule and all that. I'm guessing you'll have 60A of backfeed breakers from those Sunny Boys, and an AHJ might claim that your ATS is part of conductor that must be rated for at least 5/6ths of that 60A plus the utility or generator feed. So you might have to limit the utility and generator feeds to the ATS to 60A breakers.
 

electro7

Senior Member
Location
Northern CA, US
Occupation
Electrician, Solar and Electrical Contractor
Okay, I was wondering about that. Thanks a bunch for your help! Do I need to add an OCPD inbetween the Sunny Islands and the transfer switch? or is it fine without?
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Okay, I was wondering about that. Thanks a bunch for your help! Do I need to add an OCPD inbetween the Sunny Islands and the transfer switch? or is it fine without?

If you have OCPD on both the utility and generator feeds to the ATS then I would say no. If you don't, then yes. Not totally sure of code requirements on this.

It might be a good idea to at least put a disconnect between the ATS and the Sunny Islands to allow for servicing one or the other or both.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
If you have OCPD on both the utility and generator feeds to the ATS then I would say no. If you don't, then yes. Not totally sure of code requirements on this.

It might be a good idea to at least put a disconnect between the ATS and the Sunny Islands to allow for servicing one or the other or both.
Many folks that use Sunny Islands install bypass switches around them for that reason.
 
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