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

[Brian Teitelbaum]

Wouldn't a fault in that conductor between the main and the sub have a potential for up to 160A of current? I would think that at noon on a sunny day, the inverter system could produce 80A from one direction (albeit really only the max amperage outputs of the inverters) and 80A of grid current from the main into a partial fault.

Brian Teitelbaum
AEE 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 9:49 AM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] sizing a sub-panel used to combine multipleinverter outputs

Al,

I would say that yes the meter is a load and this goes to show the relative inadequacy of the vocabulary we are using in the discussion. In the example we have been using with the 80A breaker, wire and sub panel, you could have up to 80A of loads installed in the sub and still not create an over current condition in any of that equipment.

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 Al Frishman
Sent: Wednesday, March 30, 2011 9:00 AM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] sizing a sub-panel used to combine multipleinverter outputs
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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