Center fed panels

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cowboyjwc

cowboyjwc

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Are any of you being called for installing solar on residential center fed panels?

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We were told at a seminar put on by John Wiles that this was a no no and that you could only size the PV at 100% and could not use the 120% exception in these cases. I started calling it and I have only one PV installer not argure with me about it and the one that didn't argue agreed that it was a correct call.

Hate being the lone wolf, but not if I'm correct.
 
I think the code is not entire clear; it contains no language that explicitly applies to this situation.

I've never "been called" for doing it. I've see dozens of installations, at least, where it was done, although as I think about it I couldn't guess how many of them brought the 120% rule into play (which, btw, is not an 'exception'.)

I think common sense and logic would argue the following:
-that if the sum of the ratings for the load breakers on the side of the main breaker opposite from the solar do not exceed 100% of the rating of the busbars, and
-if the solar breaker is installed at one end of the busbar (i.e. as far as possible from the main), then
-the installation is safe and satisfies the intent of the code, if not every possible interpretation.

Whenever I encounter a middle fed panel where the 120% rule would come into play, I always install the breaker on the end farther away from the main, if there's a difference. This has resulted in satisfying the criteria in the last paragraph in every case I can remember.

I'm not one to argue with John Wiles. But if he was saying that the current code disallows it, my response would be that the code ought to address this situation rather that rely on people's interpretation of language that seems to refer to a different situation (i.e. an end-fed panel).

I don't recall all the detail right now, but I think there might be some proposals for 2014 that will address this.
 
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It seems to me that as long as you put the PV backfed breaker at one of the ends of the busbars you would be OK. I don't see that it would be more likely to cause a hot spot on the bar than in the traditional configuration. It looks less likely to me, actually, but far be it from me to challenge Big John. :D
 
The crux of John's logic is probably found in 690.64(B)(7) in NEC 2008 and 705.12(D)(7) in NEC 2011. Basically the Code says that if the sum of OCPDs feeding the busbar of a panel exceeds the 100% rating of the busbar, then you have to install the PV OCPD at the opposite end of the busbar from the main OCPD. Here's the problem: If you have a center-fed panel, then you will never have the main OCPD and the PV OCPD literally at opposite ends of the busbar—it simply is not possible to do so—therefore, you cannot take advantage of the 120% allowance. (You might be able to downsize the main OCPD and create busbar capacity for the PV system that way.)
 
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Cowboy,
I am glad you brought this up, I had never realized that the center-fed panels i tied PV into may be overloaded under certain conditions!
btw - No. I was never called on it. Hell, there is a lot of stuff i should have been called on! PV is a new, dynamic field...... :ashamed1::blink:

Yes, I agree with Solarpro, that this causes a real problem.

Technically, word-for-word, you DO meet Code.
(SolarPro, 2011 NEC 705.12 (C) (7) says ".....a connection in a panelboard shall be positioned at the opposite (load) end from the input feeder location or main circuit location." Not opposite ends of busbar.
But that is semantics in this case.)

I mean what is the purpose of the relevant code here? To have currents supplying a busbar cancel each other out rather than ADD up on the busbar.

I think tieing into a center-fed panel does NOT meet the intent, and i am almost sure they neglected to consider such a scenario.
Let's say you place a pv breaker at one of the two ends of a center-fed busbar. Then, that current could be added to the main breaker's max current if it were flowing to supply power over to the opposite end of the busbar. (remember the PV current passes under the breaker along the busbar, not thru it) So you may have PV Amps + MAIN Amps flowing on the busbar section that is on the flip side of the Main breaker.

The surprise here for me was that it isn't the PV end of the busbar that is overloaded but the other end of the busbar.....
Not good.
 
What I'm curious about, throwing out the OCPD requirements and ratings,no matter which OCPD is carrying the full load or partial load what difference does it make where the load side of the OCPD attaches to the bus?

dick
 
Zee said:
So you may have PV Amps + MAIN Amps flowing on the busbar section that is on the flip side of the Main breaker.

The surprise here for me was that it isn't the PV end of the busbar that is overloaded but the other end of the busbar.....
Not good.
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Exactly.

Dick, the reason OCPD location matters is because the overload condition described above cannot occur if the PV OCPD and the main feeder are at opposite ends of the busbar/panel. Let's say you have a main OCPD at the top of a bus bar that is rated for 100% of the busbar rating. 705.10(D)(2) allows for the sum of the OCPDs feeding the busbar to equal 120% of the busbar rating, which means you can add an OCPD for a interactive PV systems that is sized up to 20% of the busbar rating. If these OCPDs are at opposite ends of the panel/busbar, when you analyze current flow in the busbar with full power through both of these OCPDs, there is actually no point on the circuit where the busbar is overloaded.

With a center fed-panel, you could have combined loads equal to your 120% inputs (Main + PV) at the end of the panel/busbar opposite the PV OCPD. In this case, the busbar under the main OCPD is overloaded under full power conditions.
 
Thanks,I was thinking that the load under normal conditions never exceeded the either OCPD rating,,I see from your explanation that is apparently wrong in that the total load is shared,so what happens if the normal supply has a failure,is there a load shedding feature?

dick
 
Thank you Solarpro, that was pretty much the explanation that we got.

I have signed off many like this because I did not know any better, the real problem get's to be when they do a service change and put in this kind of panel. It's also something that I can't verify at plan check.

There is an exception for the 120% rule. In the 2008 and 2011 NEC residential and commercial are treated the same. The exception is if the breaker is located on the opposite end of the busbar from the main then you can use the 120% for your calcs.

I do agree that the code is very vague on this and you cannot read it into it either.
 
One of my replies to this thread seemingly never got posted. I won't try to recreate it, but one point I made was that if the 2014 looks anything like the draft underconsideration, then 705 will see some major changes. Among other things, it may allow center fed panels under enginnering supervision.
 
jaggedben said:
One of my replies to this thread seemingly never got posted. I won't try to recreate it, but one point I made was that if the 2014 looks anything like the draft underconsideration, then 705 will see some major changes. Among other things, it may allow center fed panels under enginnering supervision.
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That's what I heard too. They are trying to rewrite it so that it is clear, but "under enginnering supervision" doesn't really clear it up for me.
 
jaggedben said:
One of my replies to this thread seemingly never got posted. I won't try to recreate it, but one point I made was that if the 2014 looks anything like the draft underconsideration, then 705 will see some major changes. Among other things, it may allow center fed panels under enginnering supervision.
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This would make sense. There's a difference between what is possible in theory and what is possible in reality. Residential loads are seldom continuous and residential panels rarely if ever see anything like the full rated current. In a residential setting, designing a PV interconnection in a center-fed panel to the 120% allowance should never cause a problem. An engineer should be able to evaluate the building load and load distribution on the busbar and make that determination.

Of course things in that panel could change in the future...but unless a residential panel were severely overloaded, it's really hard to imagine a continuous residential load (on the order of 120 A or 240 A) all fed from one end of a center-fed busbar.
 
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FWIW: Bill Brooks brings up a great point on a separate forum: Prior to the 2008 Code there was no breaker location requirement associated with applying the 120% allowance?because this allowance only applied to residential installations. The breaker location requirement was only added when the 120% allowance was expanded to include panels serving commercial loads.
 
So I'm arguing with my 5th or 6th unhappy contractor and suddenly my mind makes sense of the whole thing. If you could install the PV breaker in a panel with a midpoint main, then why couldn't you just install the PV breaker at the midpoint of a top or bottom fed panel?

Every solar installer knows you can't do that, so why is it so difficult to understand the other?
 
I was faced with this once.

The 200A panel was really two 100A bussbars, each with 100A breaker, all 4 poles of the breaker factory tied together.

The solution to installing two SB7000US inverters on the service was to install two 20A breakers on the ends of the two bussbar sets, two AC disconnect switches (30A), and two KWH meters. The AHJ and utility both accepted this solution. At that date the installer supplied the meter base and the KWH meter. Nowadays, the utility does not like this solution because they will be supplying a rather more expensive 'smart' meter with wireless reading at their cost.
 
I can't really argue with your interpretation of the current code. It's a valid interpretation within the vaguery of the language. I just think the current code doesn't really pass the common sense test. The more I think about it, the more I think 705.112(D)(7) ought to have the following exception:

Exception: A connection shall be permitted at one end of a center-fed panelboard if the sum of the ampere ratings of the load breakers on the opposite side of the input feeder or main circuit location does not exceed the rating of the panelboard.
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Oh well, maybe in another five years.
 
jaggedben said:
I can't really argue with your interpretation of the current code. It's a valid interpretation within the vaguery of the language. I just think the current code doesn't really pass the common sense test. The more I think about it, the more I think 705.112(D)(7) ought to have the following exception:



Oh well, maybe in another five years.
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And there's the rub. If they are trying to clear it up in the 2014 code, here in CA we won't see that until about 2016. That's quite a lot of arguing between now and then.:rant:
 
Cowboy,
It must be tough in your position to argue all the time.:blink:

As a compromise: where applicable, why don't you request they relocate all the big breakers (2p30's 2p40 2p50 breakers) in the space between the PV and main breaker?

Then you'd have a light load of 15's and 20's on the opposite end of the busbar, unable to draw too many amps.
 
Zee said:
Cowboy,
It must be tough in your position to argue all the time.:blink:

As a compromise: where applicable, why don't you request they relocate all the big breakers (2p30's 2p40 2p50 breakers) in the space between the PV and main breaker?

Then you'd have a light load of 15's and 20's on the opposite end of the busbar, unable to draw too many amps.
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Actually I hate to argue and it just wears me out. :happyyes:

Well that's probably where the "engineering supervision" comes in.
 
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