Sigh... After talking with Generac today I went ahead and ordered the ~$1,200 worth of 400A fused disconnect to put upstream of their 400A ATS to protect it from the 480A of combined main breaker/PV back feed breaker that I want to install. Of course their literature says that with a 600A circuit breaker installed on the line side, the switch will withstand a hard short on the load side just fine. But let us not even think about letting 480A run through it for a few hours ever other blue moon. (Well that's what I get for saying we should look more at overload than short circuit conditions in these PV systems.) Still it's not like the my customer has been plagued for the last 13 years with nuisance tripping of the existing 400A main breaker because they tend to plug in one too many hair dryers....but...future load expansion and uncontrolled recpticals loads...they can be a problem, so better safe than sorry. So now I see, I have to do what I have to do if I want to be even half as good an electrican as my NABCEP certification says I am. Mark Frye Berkeley Solar Electric Systems 303 Redbud Way Nevada City, CA 95959 (530) 401-8024 <http://www.berkeleysolar.com/> www.berkeleysolar.com
_____ From: [email protected] [mailto:[email protected]] On Behalf Of Mark Frye Sent: Sunday, March 27, 2011 8:38 AM To: 'RE-wrenches' Subject: Re: [RE-wrenches] Generator and Enphase Larry, There are alot of ways to skin that cat. Here is what I am doing on my job. I have a 400A main disconnect with a feeder running directly to a 400 ATS then to a 400A main lug dedicated load center under the switch. All conductors are double - 250 mcm. I am going to tap the feeder on the line side, inside the ATS cabinet, using a ten foot feeder tap of #3 running to a new 100A breaker enclosure with an 80A breaker. From there I will go to a new 150A main lug load center with 95A worth of PV backfeed breakers. I really am enjoying this one because it brings up so many interesting Code quandaries. In my mind I am extending the premise wiring and interconnecting at the new load center which is on the line side of the ATS. Some folks might take issue with this, arguing that the "PV" circuit begins at the tap in the ATS and as such is not at a dedicated OCPD. Some folks have argued that feeder tap rules themselve do not apply to PV circuits. In any case, the new 150A load center is not dedicated solely to backfeeding the PV, there are other loads connected there. This is why I see it as an extension of the premise wiring and as the point of interconnect of the PV circuit. Interestingly enough, in order to follow NEC 2008, I will have to place the PV backfeed breakers at the bottom of the new 150A load center. But here is where it gets interesting. Taking the ATS out of the picture for a moment, the existing 400A main lug load center is now feed with through the main lug with a total of 480A of supply breakers. This complies with NEC 2008 vis-a-vis the 120% allowance for conductors and busbars. In this case it does sort of make it seem silly to go through the trouble of putting the backfeed breakers on the bottom of the 150A load center. Especially when thinking back, the whole idea of putting the breaker at the bottom of the load center was that you could then have PV backfeed equal to the full rating of load center busbar. Funny how things work out. Anyway, the real question comes up when we bring the ATS back into the picture. The 400A ATS will see the same 480A of supply breakers as the conductors and busbar of the 400A load center. Here is where I am taking leap of faith that the AHJ will agree with me that the same 120% allowance given to the conductors and load center will be given to ATS itself. Mark Frye Berkeley Solar Electric Systems 303 Redbud Way Nevada City, CA 95959 (530) 401-8024 <http://www.berkeleysolar.com/> www.berkeleysolar.com
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