[RE-wrenches] Generator and Enphase

[Mark Frye]

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: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] 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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