[RE-wrenches] Gray code area

William Miller wrmiller at charter.net
Tue Jul 29 09:14:23 PDT 2008


Friends:

There are two issues brought up here:  Designation of a load center as a 
"grid-tie combiner" and point of connection.

Grid-tie combiner:  The concept is to designate a load center as a 
"grid-tie combiner" and not allow any load breakers to be installed.  While 
I completely agree with the concept, you have to convince your 
AHJ.  Putting this load center in a location where only competent workers 
have access may help your case.

Point of connection:  There is some confusion in the original e-mail.  What 
is a "feed-through" breaker box and what are "feed-through" lugs?  I looked 
in the NEC and that combination of words occurs only twice and neither case 
seems to apply.

I assume that some breaker panel on the premises feeds loads.  All such 
load centers, be they sub-panels, main panels or meter/main panels, need to 
satisfy 690.64(B).  Every buss that receives locally generated power and 
also feeds loads must comply.  Even if there is not a breaker in a given 
panel directly from an inverter, if there is a breaker feeding a sub-panel 
that accepts inverter power, that panel must comply.

I have posted a drawing that illustrates this concept at 
http://mpandc.com/case_studies/case_studies.html  Click on the Point of 
Connection link.  Note that the sub-panel does not comply because only 25 
amps are allowed and 29 amps are connected.  The meter/main does comply.

I hope this explanation helps shed some light on the subject.

William Miller

At 02:21 PM 7/28/2008, you wrote:
>Drake and all,
>
>If I understand your thread, you are connecting through an extra 200-A
>breaker in a box with two 200-A services. If this service was originating
>from the utility, everything would be good and you would be making a
>690.64(A) supply side service connection. If the connection at the main
>service is in fact feed-through lugs with no service disconnect, then the
>200-A breaker the PV is connected to is still a line-side connection and
>everything is fine--690.64(B) does not apply. I assume this system has 6,
>20-A breakers for a total of 120-A of supply fed through a 200-A breaker
>(actually the 200-A breaker doesn't even matter--all that matters is that we
>are less than 200-Amps).
>
>I may have misunderstood something here, but 690.64(B) is a load side
>connection section and must have loads in order to be considered load side.
>With no loads on the line side of the service disconnect, we are simply
>creating another service disconnect (see previous discussions on numbers of
>disconnects and service definitions).
>
>The bummer with the way the code is written is that as soon as a load is
>added, everything changes. We are working on a proposal that would make that
>situation easy, but right now it is a pain. The 2008 NEC in 690.64(B) is a
>huge step forward to allow small PV systems, relative to the service, to
>connect to the load side of the service legally without having to change out
>the service equipment. There are those that still oppose this idea, so we
>need to continue to fight to keep what we have and to expand it beyond the
>120% limit. Objections still get raised routinely, and those concerns must
>be silenced with test data and good engineering analysis.
>
>Bill.
>
>
>-----Original Message-----
>From: re-wrenches-bounces at lists.re-wrenches.org
>[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Drake
>Chamberlin
>Sent: Monday, July 28, 2008 1:03 PM
>To: RE-wrenches
>Subject: Re: [RE-wrenches] Gray code area
>
>Kent and Jerry,
>
>Thanks for the ideas.  The idea of a label is a good one.  Since the
>building in which the box is located is fed by a completely separate
>service, no loads should ever be tapped from it. The inspector should
>be contacted first, since this method is not completely in line with
>the code rules.
>
>The breaker cannot really be down sized, since it feeds the
>residence.  The solar feed would be about 46 amps, or about 23% of
>the rating of the mains.  Can we use adjustable trip breakers to meet
>the 20% requirement?  If we use them, do we need certification
>concerning their amperage capacity?
>
>The problem with more spaces being available on the bus bar might be
>addressed by altering the bar.  But, I'm guessing that this would
>void its listing.
>
>Drake
>
>
>At 03:45 PM 7/27/2008, you wrote:
> >Drake,
> >
> >I agree that taking it up with your inspector ahead of time is a
> >good idea, but it is possible he may not accept the argument that
> >the panel isn't for loads if there are additional spaces for
> >breakers that someone could use later on.
> >
> >How much inverter capacity (amps) are you connecting to the panel?  Is
> >it possible to change out the breaker for a smaller one or de-tune
> >the trip setting of the breaker to
> >bring you in compliance with the sum of breakers rule?
> >
> >
> >Cheers,
> >
> >Jerry Caldwell
> >
> >Recurrent Energy
> >
> >
> >
> >----- Original Message ----
> >From: Kent Osterberg <kent at coveoregon.com>
> >To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
> >Sent: Sunday, July 27, 2008 7:49:11 AM
> >Subject: Re: [RE-wrenches] Gray code area
> >
> >Drake,
> >
> >The writer's of section 690 haven't covered all of the possible ways to
> >make a safe installation.  Or all of the needs for a large system.  You
> >should take it up with the AHJ first, but I suspect that it will be
> >approved.  After all, the 20% rule (and now with NEC 2008 at the bottom
> >of the bus too) was intended to prevent overload of the bus bars.  The
> >20% rule shouldn't apply if there aren't any loads in the panel.
> >
> >Consider this possibility: you have six 3-kW inverters to install on
> >this 200-amp panel.  Do you have six points of interconnection with the
> >utility on the bus bars?  Or do you have one point of interconnection at
> >the line side of the 200-amp breaker?  I think it is really the second
> >and I think that is what the electrical contractor was thinking. You may
> >need to label the panel to make it clear that it isn't for loads.
> >
> >Kent Osterberg
> >Blue Mountain Solar
> >www.bluemountainsolar.com
> >
> >
> >Drake Chamberlin wrote:
> > > We are installing a PV system where an electrical contractor left us an
> > > easy way to interface with the grid.  The contractor does extremely
> > > clean work which exceeds NEC requirements, and the local code
> > > authorities are very happy with him.
> > >
> > > What he left us was a 200 A, feed through breaker box that feeds no
> > > branch circuits.  The building that the breaker box is in is fed by a
> > > separate, commercial service.  The feeder on the feed through lugs, from
> > > the box, goes to a residence where the wiring feeds a 200 A service
> > > disconnect breaker for a residential electrical service.
> > >
> > > The sole purpose of the breaker box is to give us a place for the PV
> > > input.  The potential code issue is that the PV input goes over the 20%
> > > limit.  But there is no way the bus can be overloaded, since it is
> > > protected by a 200 A breaker on the load side.  Since the building is
> > > commercial, and fed by a separate service, there would be no reason to
> > > tap any loads from the box in question.
> > >
> > > This doesn't look code compliant, from the wording of the NEC.  Do you
> > > think we should use the bus bar and backfeed a breaker, or tap in ahead
> > > of the 200 A breaker to strictly meet code requirements.
> > >
> > >
> > >
> > > Drake Chamberlin
> > > Athens Electric
> > > OH License 44810
> > > CO License 3773
> > > 740-448-7328
> > > 303-328-5533
> > >





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