[RE-wrenches] sizing a sub-panel used tocombinemultipleinverter outputs

[Mark Frye]

Jason,
 
I think, going back to the diagram in the article, the feeder breaker is
rated at 80A, the feeder conductors are rated at 80A, and the subpanel is
rated at 80A, assuming that there are no loads connected in the subpanel.
Even if there were up to 100A of loads in the subpanel, you would not exceed
the allowance for the feeder or the sub-panel. Still, if you do have loads
in the subpanel, you have to up sizes the sub-panel, but not the feeder
conductor.
 
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 Jason
Szumlanski
Sent: Wednesday, March 30, 2011 5:18 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] sizing a sub-panel used tocombinemultipleinverter
outputs



Another key is to remember that this discussion also applies to the
conductor between the main panel and subpanel. In a large PV system, this
could result in a pretty large wire between the two panels, and a
significant cost that is often overlooked. In some cases it makes sense to
locate the subpanel close to the main panel and run multiple sets of smaller
wires from the inverters to the subpanel.

 

And because the calculation is based on the first OC protection connected to
the inverters, adding a main breaker (theoretically 80A in this example) in
the subpanel doesn't change things. Even though this wire would be
theoretically protected by an 80A breaker at each end, you can't size the
wire for 160A / 1.2 = 133.3A. You have to size for 180A/1.2 = 150A. (not
that it makes much of a difference in this example, but it still must be
considered)

 

At least that's how I understand it.

 

Jason Szumlanski

Fafco Solar 

 

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Mark Frye
Sent: Wednesday, March 30, 2011 1:33 AM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] sizing a sub-panel used to
combinemultipleinverter outputs

 

Opps!

 

My bad, I was thinking of a single phase system, not the three phase system
shown in the article.

 

For the three phase system Kent is correct in counting 180A of supply per
bar.

 

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: Tuesday, March 29, 2011 10:17 PM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] sizing a sub-panel used to combine
multipleinverter outputs

I think Kent and I agree. For the case where the subpanel is not dedicated a
PV sub-panel he is calculating for 2 - 50A breakers and I calculated for 3 -
50A breakers.


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 Kent
Osterberg
Sent: Tuesday, March 29, 2011 9:26 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] sizing a sub-panel used to combine multiple
inverter outputs

Per 705.12(D) the sub-panel could be any distribution equipment on the
premises. So the question becomes: is the sub-panel capable of supplying
branch circuits or feeder loads? If yes, then the sum of the breakers
(potentially) feeding the bus is 180 amps so a 150-amp rating is required
and the inverters would have to feed the opposite end of the bus bars. If
no, the code is not clear on the requirement, but obviously the 80-amp
breaker in the main panel limits the maximum current flowing through the
sub-panel. 

Kent Osterberg
Blue Mountain Solar



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