[RE-wrenches] sizing a sub-panel used to combine multiple inverter outputs

Jason Szumlanski Jason at fafcosolar.com
Wed Mar 30 11:07:36 PDT 2011


Here is a 690.64(B)(2) opinion from Wiles that may help. 

http://www.fafcosolar.com/download/409/690%2064%28B%29%282%29TECH-1.pdf

 

It's still my contention that the conductors need to be sized for 150A
in this example, unless an exception is made by the AHJ. If David
Brearley's post about Wiles' clarification is accurate, them the 80A
main breaker in the subpanel can be used for calculations, but the
conductor still needs to be sized for 80A + 80A =160A / 1.2 = 133.3A.

 

Jason

 

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Kent
Osterberg
Sent: Wednesday, March 30, 2011 12:57 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] sizing a sub-panel used to combine multiple
inverter outputs

 

Al,

How about a 15-amp single-pole breaker for power line carrier
communications equipment to talk to the inverters?
Technically, I think either way they are loads and that changes
everything. But in my opinion they are minor and should be allowed
without considering the sum of all sources.

Kent Osterberg
Blue Mountain Solar


Al Frishman wrote: 

Is a monitoring system installed in the Load Center to measure the
cumulative kWh's of the inverter's considered a load?  

The type of monitoring device I am talking about has CT's that go around
the conductors and the Voltage ref is taken by attaching conductors to
each phase, the Neutral and the ground bar.    The digital Meter has a
3-pole 5A breaker in it so the conductors in the load center can be
connected to lugs on the bus bar directly.

 

Al Frishman
AeonSolar

(917) 699-6641 - cell
(888) 460-2867
www.aeonsolar.com <http://www.aeonsolar.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 8:18 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] sizing a sub-panel used to
combinemultipleinverter 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 
www.berkeleysolar.com <http://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 
www.berkeleysolar.com <http://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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