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

Wed Mar 30 13:38:38 PDT 2011

Right, so going totally on the fly at the white board, let's say we have an
inverter with a 45A AC output, it see's the short and pours it's little
heart out at 45A for 0.1 sec before shutting out, delivering 4.5 A-sec of
energy into the fault.  Meanwhile we take out a clearing time curve for the
generic 60A breaker that the inverter would be backfeed at the other end of
the feeder. It will deliver 10 times it's rating or 600A for 0.5 secs before
clearing. That's 300 A-sec of energy, 66 times more than the inverter.

So can we say that when looking at sizing the AC side of these inverters we
need more to concern ourselves with continuous overload conditions and not
so much with short-circuit/ground fault conditions?

Mark Frye 
Berkeley Solar Electric Systems 
303 Redbud Way 
Nevada City,  CA 95959 
(530) 401-8024 
 <http://www.berkeleysolar.com/> www.berkeleysolar.com  

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Exeltech
Sent: Wednesday, March 30, 2011 12:04 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] sizing a sub-panel used tocombinemultipleinverter
outputs

Per your question:

> 1 - What does a UL1741 inverter do when it is connected
> to a line that is experiencing a short-circuit or ground fault?

Presuming a hard short, the voltage in that circuit would be essentially
zero, and a UL1741 compliant inverter would cease producing power in less
than 0.1 second.

Dan
Sr. Engineer
Exeltech

--- On Wed, 3/30/11, Mark Frye <markf at berkeleysolar.com> wrote:

From: Mark Frye <markf at berkeleysolar.com>
Subject: Re: [RE-wrenches] sizing a sub-panel used tocombine
multipleinverter outputs
To: "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>
Date: Wednesday, March 30, 2011, 12:37 PM

That is a really good question Brian.

Mostly when thinking about these sizing issues I think about overload
conditions and not so much about short-circuit and ground fault conditions.

So two questions come up:

1 - What does a UL1741 inverter do when it is connected to a line that is
experiencing a short-circuit or ground fault?

2 - What does the feeder breaker do when it is connected to a line that is
experiencing a short-circuit or ground fault?

As far as number two, thanks to all the great work we do establishing low
impedence fault paths back to circuirt protectors, the breaker is going to
trip at it's rating and protect the wire at the same rating. But that
doesn't mean that the conductor won't see more than rated amps during the
fault. It will see alot more than for a very short time. During this very
short time frame, the actual currents flowing into the fault from the
utility side of the equation far far exceed the currents contributed by the
power limited inverter side.

Mark Frye 
Berkeley Solar Electric Systems 
303 Redbud Way 
Nevada City,  CA 95959 
(530) 401-8024 
 <http://www.berkeleysolar.com/> www.berkeleysolar.com  

  _____  

From: re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Brian
Teitelbaum
Sent: Wednesday, March 30, 2011 10:02 AM
To: RE-wrenches
Subject: Re: [RE-wrenches] sizing a sub-panel used tocombine
multipleinverter outputs

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 
 <http://www.berkeleysolar.com/> 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 
 <http://www.berkeleysolar.com/> 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 
 <http://www.berkeleysolar.com/> 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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