Advice for PV system connection

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ylmzm

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Los Altos, CA
The solar PV 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 from all microinverters 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...mbo-Surface-Mount-CSED-SU3040D400CB/100140344


This is the garage subpanel:


http://www.homedepot.com/p/Square-D...-Center-with-Ground-Bar-QO154L225PG/204246796


This is the solar-only subpanel we will use:


http://www.homedepot.com/p/Square-D...nter-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

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. "


Using the 120% rule => 225 * 1.2 - 200 = 70 Amp


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

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


56 Amp * 1.25 = 70 Amp (looks OK)


This 70 Amp will go back to the main panel through the 200 Amp busbar of the 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.


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


http://www.amazon.com/NSI-Industrie...-1&keywords=Tap+Insulation+Piercing+Connector



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


What would you recommend?
 
ylmzm said:
The solar PV 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 from all microinverters 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...mbo-Surface-Mount-CSED-SU3040D400CB/100140344


This is the garage subpanel:


http://www.homedepot.com/p/Square-D...-Center-with-Ground-Bar-QO154L225PG/204246796


This is the solar-only subpanel we will use:


http://www.homedepot.com/p/Square-D...nter-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

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. "


Using the 120% rule => 225 * 1.2 - 200 = 70 Amp


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

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


56 Amp * 1.25 = 70 Amp (looks OK)


This 70 Amp will go back to the main panel through the 200 Amp busbar of the 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.


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


http://www.amazon.com/NSI-Industrie...-1&keywords=Tap+Insulation+Piercing+Connector



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


What would you recommend?
Click to expand...
The problem with Option 1 is that you must also apply the 120% rule at the main panel. And you cannot connect it to either 200A bus in that case, since 1.20 x 200 - 200 is only 40A.
Technically, even if there are no loads on the internal bus bar in the main, if there are spaces where you could add load breakers and since the existing feeder goes to both solar and loads, some inspectors will force you to apply the 120% rule there too.
It makes perfect sense that if you are backfeeding from PV into the main bus you will not be also drawing loads from elsewhere on that bus. But the language of the code does not, IMHO, explicitly recognize that.

If you try to connect at the main panel, that is even less acceptable since there will be a separate load on the bus, namely the subfeed to the other panel. If you are under [2014], then you could argue that you have two sources on the 200A bus but only one load, which is protected at the bus rating, and so the 100% load rule would apply. And you would only have to count the feed as 70A instead of the 75A sum of the breakers.
But under earlier (pre 2014) NEC you would have to compare the sub panel connection feed as 75A, the sum of the breaker values, rather than 1.25 x actual current.

So your available options will depend on what code cycle you are on.
You can get more headroom in both garage and main panels by downsizing the breaker feeding them.
If your AHJ and POCO allow supply side connection, that will work with any code cycle.
 
Thanks for the explanation. One of the 200 Amp breakers in the main panel (the top one in the picture below) does not have any busbar attached to it. It goes directly to home panel. Can the backfeed breaker be located in home panel in this case?

One more option: I can split enphase branches into two subpanels (home and garage), so each one gets less than 40 Amp. Is this a code-compliant method?


home-design.jpg
 
I think you mispoke in your first post, when you said 'from there.' I gather you are trying to say that the 200A breaker in the main that does not feed the internal busbar feeds the other 225A suboanel. In that case you're math is good and you are fine connecting to the garage subpanel. You've been unclear in your descriptions, but I've seen similar 400A panels and it would not make much sense if it was installed differently. You are lucky that the remote panel is rated 225A. I've never gotten lucky like that on such a service.

EDIT: Except that you'll have rearrange your enphase circuits so the breakers don't added up to more than 70A. In California we are still on the equivalent of the 2011 NEC which does not allow using the 56A output directly in the calculation. So you'll need to rearrange to 2×20+2×15, or 3×20+1×10
 
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Btw if this happens to be in Fremont then the feeder to the remote sub will also have to be rated 225A or more. If the feeder is only 200A that may screw you. I don't know of another bay area AHJ that uses that interpretation, but we don't go south of San Jose, and there's a few others I haven't pulled permits in.
 
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ylmzm said:
One more option: I can split enphase branches into two subpanels (home and garage), so each one gets less than 40 Amp. Is this a code-compliant method?
Click to expand...

Your AHJ may not like that; for residential PV systems some AHJ's will not allow multiple interconnection points.
 
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