[RE-wrenches] Gray code area

William Miller wrmiller at charter.net
Wed Jul 30 08:48:00 PDT 2008


Drake:

I'm sorry, but this isn't making any sense.  Here is what I understand:

1. There is a commercial building that you would install a grid-tie PV on 
the roof.
2. The commercial building has a service that you will not feed the 
grid-tie to.
3. This commercial building has a "feed-through" breaker panel that was 
installed for the PV to tie to.  This "Feed-through" box does not connect 
to the commercial service.

Where is the service that the PV system would feed?  In a separate building?

Where does the feeder connected to the bottom of this "Feed-through" panel go?

William Miller

At 10:24 AM 7/30/2008, you wrote:
>Thanks to All who have replied,
>
>First, to answer William's question
>
>"What is a "feed-through" breaker box and what are "feed-through" lugs?"
>
>A feed through panel is the type of service equipment often installed on 
>pre-manufactured homes, where a main switch is needed outside the 
>building, and a limited number of loads, fed from the outside, are also 
>supplied.  On the bottom of the busbar,  there are lugs equivalent to the 
>lugs for the service entrance cables.
>
>This is useful when feeder is to be installed to supply the breaker panel 
>in the premise that is equal to the total service amperage.  No breaker is 
>needed for the run to the subpanel inside, since the wiring will be 
>equivalent to the service cable.   It is similar in function to a 
>disconnect switch, but will typically have 4 to 8 breaker spaces.
>
>The situation in this project is that a feed through panel is in a 
>separate building from the building to be supplied from that panel.  The 
>building this panel is in has a separate electrical service, which is 
>commercial.  The service for the residence comes in through a separate 
>meter, located on this  same commercial building, then goes to the feed 
>through panel.  This panel was installed for us, for the sole purpose of 
>allowing a feed from the solar array.
>
>The issue here is that the busbar will have 200 amps from the main, and 
>over 40 amps from the solar.  No power is to be taken from the residential 
>service to feed the commercial building.   There is no possibility of an 
>overload on the busbar because the house has a main 200 A breaker that 
>will limit the load from the panel.
>
>It could be argued that since the line to the house will have up to 240 
>amps available on a 200 amp service, that is too much.  I would argue that 
>a standard service drop can supply 10,000 amps instantaneously.
>
>The box is strictly to be used as an AC combiner and feed for two 
>inverters.  By the letter of the NEC, it does not seem to be code.  I'm 
>going to use Kent's suggestion and use a label, after running it by the 
>AHJ beforehand.
>
>
>-Drake
>
>
>At 10:14 AM 7/29/2008, you wrote:
>>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





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