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

Wed Mar 30 16:00:41 PDT 2011

You're overlooking an important fact.

Grid-tie inverters are current-limited devices.  They lack surge capability.

Trip times for various circuit breakers vary depending in part on the overload as a percentage of the breaker's ratings and the circuit breaker temperature (for thermal breakers).

A properly operating UL1741-certified grid-tie inverter that's exposed to a sudden dead short in the AC output will stop producing current before the circuit breaker would trip.

Dan

--- 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 tocombinemultipleinverter outputs
To: "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>
Date: Wednesday, March 30, 2011, 3:38 PM

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 
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

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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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