Need advice: Solar AC Connection

10 years ago

The solar system we are planning will have 56 Amp AC output from four enphase branches. All branches are combined in a solar-only subpanel (rated 125 Amp). Branch OCPD are: 3 x 20 Amp , 1 x 15 Amp. Actual max total current output is 56 Amp.

Main electrical panel is a 400Amp panel, but it has two 200Amp disconnects. One of them directly goes to the home subpanel, the other one goes to an internal busbar rated 200 Amp. From there, it goes to the garage subpanel (225 Amp busbar, 200 main breaker).

This is the main panel:

http://www.homedepot.com/p/Square-D-Homeline-400-Amp-30-Space-40-Circuit-Main-Breaker-Outdoor-Combo-Surface-Mount-CSED-SU3040D400CB/100140344

This is the garage subpanel:

http://www.homedepot.com/p/Square-D-QO-225-Amp-54-Space-54-Circuit-Indoor-Main-Lug-Plug-On-Neutral-Load-Center-with-Ground-Bar-QO154L225PG/204246796

This is the solar-only subpanel we will use:

http://www.homedepot.com/p/Square-D-QO-125-Amp-12-Space-24-Circuit-Outdoor-Main-Lug-Load-Center-with-Ground-Bar-QO11224L125GRB/100192088

I have two options as to where I can connect the solar-only panel output:

(Option-1) To the garage subpanel:

225 Amp busbar, 200 Amp main breaker => 225 * 1.2 - 200 = 70 Amp

So, maximum 70 Amp can be backfed into this subpanel.

56 Amp * 1.25 = 70 Amp (looks OK)

This 70 Amp will go back to the main panel through the 200 Amp busbar of that main panel. I think this is OK, since there are 200 Amp breakers on the way before it can reach that busbar. So, main panel busbar cannot get more than 200 Amp. Is this a correct way of thinking?

(option-2) To the main panel:

I cannot backfeed main panel busbar, since it is rated only 200 Amp. But the only breaker on that busbar is the 200 Amp quad breaker feeding the garage subpanel. (I had to use that option, because I could not find a feed-thru lug).

Another option is to do a line-side tap using two of these:

http://www.amazon.com/NSI-Industries-IPCS4020-Insulation-Connector/dp/B00CUNJGU0/ref=sr_1_1?s=industrial&ie=UTF8&qid=1458285879&sr=1-1&keywords=Tap+Insulation+Piercing+Connector

I would rather use Option-1. Do you see any problem with that?

What would you recommend?

Guts of the main panel looked like this before adding connections to garage and house subpanels. I can add its current picture if needed.

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Comments (2)

Ron Natalie
10 years ago
You've lost me. Your system output is only 56A. Why can't you backfeed the main panel bus bar? What is that piece of math you are using supposed to mean?
ylmzm
Original Author
10 years ago
Sorry, I think I have given too many disorganized information.

NEC Code requires that the OCPD be rated for no less than 1.25 times the Continuous Current of the circuit. (Article 215.3)

56A X 1.25 = 70A

NEC 690.64(B)(2), or 705.12(D)(2) mandates that: "Where two sources, one a utility and the other an inverter, are located at opposite ends of a busbar that contains loads, the sum of 125 percent of the inverter(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar. "

200 Amp * 1.20 = 240 Amp

So, maximum PV current you can have on this main panel busbar is:

(Busbar Rating (A) x 1.2) - Main Breaker Rating (A) = Max PV (A)

240 - 200 = 40 Amp

When the busbar is rated at 225 Amp with 200 Amp main breaker, max PV we can have is:

225 * 1.2 - 200 = 270 - 200 = 70 Amp
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