[RE-wrenches] sizing a sub-panel used tocombinemultiple inverter outputs

Wed Mar 30 11:47:55 PDT 2011

I hope there are no potential loads between the meter and the
transformer, so no! Technically, if it is a supply side tap, the
evaluation would include the conductor from the meter to the main
distribution panel I believe, but if it is a load side connection it
does not matter because of the main breaker in the main distribution
panel. Honestly, I've never closely scrutinized the meter->MDP wiring
for supply side connections, but I am confident that a limitation never
came into play.

Jason

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of
Kristopher Schmid
Sent: Wednesday, March 30, 2011 2:28 PM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] sizing a sub-panel used tocombinemultiple
inverter outputs

By this argument, wouldn't it be necessary to check every conductor in
the backfeed path adding potential PV amps to their required ampacity
including the utility conductor from the transformer?

Kris

Legacy Solar, LLC
864 Clam Falls Trail
Frederic, WI 54837
715-653-4295
solman at legacysolar.com
www.legacysolar.com 

	-----Original Message-----
	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 1:08 PM
	To: RE-wrenches
	Subject: Re: [RE-wrenches] sizing a sub-panel used to
combinemultiple inverter outputs

	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
	(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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