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

Wed Mar 30 12:04:01 PDT 2011

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

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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 
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 
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 

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  

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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