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

Wed Mar 30 10:36:05 PDT 2011

 I concur with Jason and Brian's interpretation of 690.64(B)(2).  I think
one of the intents of the code is to ensure that if there is a fault in a
busbar or conductor and the maximum amperage is being delivered to that
fault from all OCPDs supplying it (solar + utility), the busbar or conductor
should be able to handle the sum of those fault currents.  I have
encountered numerous jurisdictions that interpret it this way, and it is
hard to argue with them from a strictly code perspective.

However, Jason Fisher once pointed out to me that if the fault occurs in a
feeder between the main service panel and a subpanel (solar accumulation
panel or otherwise) it is impossible for any part of that conductor to carry
more current than allowed by the largest OCPD feeding the conductor.  Its
the same principle behind the 690.64(B)(7) requirement to locate the solar
interconnection breaker at the opposite end of the busbar from the incoming
feeders.  This argument obviously doesn't apply to accumulation panel
busbars where you have more than 2 OCPDs supplying current, but I think it
is a valid argument for basing your feeder conductor sizing on the largest
OCPD protecting that conductor.

For a brighter energy future,

Andrew Truitt
Principal
Truitt Renewable Energy Consulting
NABCEP Certified PV Installer™ (ID# 032407-66)
(202) 486-7507
http://www.linkedin.com/pub/andrew-truitt/8/622/713

[image: 24 copy.jpg]

"Don't get me wrong: I love nuclear energy! It's just that I prefer fusion
to fission. And it just so happens that there's an enormous fusion reactor
safely banked a few million miles from us. It delivers more than we could
ever use in just about 8 minutes. And it's wireless!"

~William McDonough

On Wed, Mar 30, 2011 at 10:12 AM, Kent Osterberg <kent at coveoregon.com>wrote:

> Jason,
>
> Now you are opening up a can of worms.
>
> It's bad enough that 705.12(D) doesn't say anything about a panel that
> can't (or shouldn't, or won't) have anything connected but interactive
> inverters. But when that is the case, a bus rating of 100% of the source
> circuits should apply. For PV systems a 125% factor will already apply to
> each inverter circuit.
>
> While it seems logical that the conductors are an extension of the bus bar
> and should be treated the same way, 705.12(D) isn't about the conductors -
> it's about the bus bars. The conductors are protected by the breakers --
> 80-amp breakers and wire with 80-amp ampacity.
>
> Getting into the example further, I see flaws in it. If the inverters are
> 7500-watt 240-volt, the output current would be 7500/240 = 31.25 amps and
> 40-amp breakers would be adequate. Then there would have been no issues,
> even going directly into the main panel. If the inverters are 7500-watt
> 208-volt, the output current is 7500/208 = 36 amps and the 50-amp breakers
> make sense. That means the bus bars and feeder conductor have a continuous
> current of 72 amps.  That means neither the 80-amp breaker nor 80-amp wire
> is sufficient because 72 x 1.25 = 90 amps. Now the example doesn't resolve
> the limitation of backfeeding at the main. Opps!
>
>
> Kent Osterberg
> Blue Mountain Solar
>
>
>
>
>
>
> Jason Szumlanski wrote:
>
>   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<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<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<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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