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

Wed Mar 30 11:25:04 PDT 2011

Agreed:

http://solarprofessional.com/article/?file=SP2_3_pg16_QandA_2&search=

You¹ll notice that none of the more permissive proposals that John mentions
here actually made it into NEC 2011.

On 3/30/11 1:07 PM, "Jason Szumlanski" <Jason at fafcosolar.com> wrote:

> 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
>  
> It¹s 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 Brearley¹s post
> about Wiles¹ 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
> kWh¹s of the inverter¹s considered a load?
> The type of monitoring device I am talking about has CT¹s 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 doesn¹t change things. Even though this wire would be
> theoretically protected by an 80A breaker at each end, you can¹t 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 that¹s how I understand itŠ
>  
> 
> 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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> David Brearley, Senior Technical Editor
> SolarPro magazine
> NABCEP Certified PV Installer 
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