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

Philip Boutelle philboutelle at gmail.com
Wed Mar 30 09:04:02 PDT 2011


To echo the wording in Kent's response, John Wiles has always advised that
it doesn't matter if there are currently loads connected to the
panel/conductors; if the panel/conductors are capable of having additional
loads connected to it, you need to apply 609.64(B)(2). I have had luck with
AHJ approval by adding signage indicating that no loads can be added to this
panel/fpr combining PV inverter output only, and in one case installed a
panelboard dead-front that limited the number of breakers to six (for our
six combining inverters).

If you are installing  breaker for a meter, it sounds like the panel is
capable of having additional loads connected to it.

For discussion on code specific combiner panel requirements, or the lack
thereof, I'll quote from this list last year (response is from Bill Brooks):

**************


Sent: Tuesday, January 12, 2010 10:57 AM

To: RE-wrenches

Subject: [RE-wrenches] Solar accumulation panel


*******************

Team wrenches - I feel like I've seen a provision in the Code for
installing a sub-panel to aggregate PV inverter outputs for the
purpose of simplifying the actual grid connection (such as for a
line-side tap) but I can't find it in 690.  I also seem to remember
specific verbiage that needs to be on the panel along the lines of "PV
circuits only.  Do not add load breakers." to ensure compliance with
690.64(B).  Does anyone know where I can find this info in the NEC?

Andrew Truitt

Standard Solar Inc.

********************

Response:

 It does not exist. We tried like crazy to get it into the 2011 NEC,
but to no avail. At the last second the proposal was put on hold until
the 2014 NEC. Major bummer. For now it can only be done with AHJ
approval. Many allow it but it is not in the NEC. Alternatively you
must use the sum of supply breakers no greater than 120%. Much more
restrictive.

Bill.

********************

-Phil



On Wed, Mar 30, 2011 at 8:36 AM, Mark Frye <markf at berkeleysolar.com> wrote:

>  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
> *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 *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
> 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
>
>
>
>
>  ------------------------------
>
> *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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