Downsizing a 100A main service disconnect?

[jayzbond007]

Hello,

I have a pv system which I am designing in Chino Hills, Southern California. The main service panel (msp) is a 100A rated with a 100A main service disconnect. The AHJ uses 100% rule on center-fed main service panels. So this means that I cannot backfeed anything into it. Can I downsize the main service disconnect to 80A? Is there anywhere in the NEC that the main service disconnect can be less than 100A? I understand the rating of the panel has to be 100A. I am avoiding upgrading the main service panel.

Any help is appreciated.

Thanks.

 

[iwire]

If I was the inspector I would expect load calculation to prove its ok

 

[shortcircuit2]

Hello,

I have a pv system which I am designing in Chino Hills, Southern California. The main service panel (msp) is a 100A rated with a 100A main service disconnect. The AHJ uses 100% rule on center-fed main service panels. So this means that I cannot backfeed anything into it. Can I downsize the main service disconnect to 80A? Is there anywhere in the NEC that the main service disconnect can be less than 100A? I understand the rating of the panel has to be 100A. I am avoiding upgrading the main service panel.

Any help is appreciated.

Thanks.

Connect as line side connection.

 

[Sierrasparky]

Hello,

I have a pv system which I am designing in Chino Hills, Southern California. The main service panel (msp) is a 100A rated with a 100A main service disconnect. The AHJ uses 100% rule on center-fed main service panels. So this means that I cannot backfeed anything into it. Can I downsize the main service disconnect to 80A? Is there anywhere in the NEC that the main service disconnect can be less than 100A? I understand the rating of the panel has to be 100A. I am avoiding upgrading the main service panel.

Any help is appreciated.

Thanks.

Who is making this decision the POCO or the city buidling official. I know you said AHJ. If the Local Buiding Official is saying this there needs to be an amendment to the Code by city ordinance and filed with the State Buildings and standards. If you feel they are in error and you feel you would be safe you could challenge those fools if there is no filed amendment.

 

[jaggedben]

The minimum service disconnecting rating in the NEC is 60A, I believe. So no problem there. However most center fed panels I've seen have a factory installed main breaker and you'd probably be violating the listing if you replaced it.

Also iwire is correct you should look at load calcs, and the AHJ would just be following the code if they asked for that.

A supply side connection (line side tap) or service upgrade may indeed be easier.

 

[don_resqcapt19]

Who is making this decision the POCO or the city buidling official. I know you said AHJ. If the Local Buiding Official is saying this there needs to be an amendment to the Code by city ordinance and filed with the State Buildings and standards. If you feel they are in error and you feel you would be safe you could challenge those fools if there is no filed amendment.

Based on the wording in the 2011 code, it would be my opinion that the inspector is correct when the main is at the center of the panel bus. When the main, is at the center there is no "opposite" end.

 

[ggunn]

Based on the wording in the 2011 code, it would be my opinion that the inspector is correct when the main is at the center of the panel bus. When the main, is at the center there is no "opposite" end.

We have had discussions about this before. Connecting to a center fed panel is problematic because there is the possibility that all the loads on one side can be on and the PV feeding from the other side with none of the loads on that side being on, which would be the same as putting the PV in at the top of a standard panel right below the main breaker. Hence the 100% rule.

 

[Sierrasparky]

Based on the wording in the 2011 code, it would be my opinion that the inspector is correct when the main is at the center of the panel bus. When the main, is at the center there is no "opposite" end.

I would think that would be up to interpatation.
It is my understanding that the both feeds not be next to each other.

besides I have yet to see a 100 amp panel that did not have a 125amp buss rating. Even the poor Zinsco 100a panels are rated at 125. So a 125 would suffice.

 

[GoldDigger]

I would think that would be up to interpatation.
It is my understanding that the both feeds not be next to each other.

A more correct interpretation of the rationale behind it is that you do not want to allow any loads to be connected on the same side of both feed breakers. So if one section of the center-fed bus is empty and blocked off you would be OK putting the PV at the far end of the other half. But the letter of the Code still does not allow for that.

 

[Sierrasparky]

However a 125 amp rated buss would allow a 100 amp main and a 20 amp PV at any location as I read the code.

 

[Zee]

SierraSparky brings up a good point: If indeed the busbar is rated at 125 A......then the center-fed aspect is not an issue. Can you demonstrate that is the rating, 007?

(The 100 A (E) main breaker plus your [unstated, but assumed 20 A PV breaker?] is 120 A total. You are well within the "100% rule".

 

[Smart $]

However a 125 amp rated buss would allow a 100 amp main and a 20 amp PV at any location as I read the code.

That is correct.

Unless the panelboard is rated not less than the sum of the ampere ratings of all overcurrent devices supplying it, a connection in a panelboard shall be positioned at the opposite (load) end from the input feeder location or main circuit location.

 

[Smart $]

However a 125 amp rated buss would allow a 100 amp main and a 20 amp PV at any location as I read the code.

SierraSparky brings up a good point: If indeed the busbar is rated at 125 A......then the center-fed aspect is not an issue. Can you demonstrate that is the rating, 007?

(The 100 A (E) main breaker plus your [unstated, but assumed 20 A PV breaker?] is 120 A total. You are well within the "100% rule".

That is correct.

One caveat...

The Code states "Unless the panelboard is rated not less than...". Note it refers to the panelboard rating... not the bus rating. Even if one can verify a 100A-rated panelboard has a 125A-rated bus, the AHJ can still disapprove without reproach (unless someone brings forth refuting authoritative documentation, such as an ROP or official interpretation).

 

[cadpoint]

The minimum service disconnecting rating in the NEC is 60A, I believe. ...

2011 (no shading), 2008 (has various shading through out -but not in respects to these two articles.)

Article 230.79 (D) All Others. .... not less than 60 Amperes.

Article 230.79 (C) One Family Dwellings ...of not less than 100 amperes. 3-wire.
This is the OP's problem.

 

[jaggedben]

besides I have yet to see a 100 amp panel that did not have a 125amp buss rating. Even the poor Zinsco 100a panels are rated at 125. So a 125 would suffice.

I've seen plenty of panels with 100A bus rating and breaker. Very common around here, although 125A may be slightly more common. Now as for center fed panels specifically, I'm not so sure. But when the OP clearly stated the ratings each at 100A I'm inclined to believe he has read the labels and that's what he found. I've seen much stranger things.

 

[Sierrasparky]

One caveat...

The Code states "Unless the panelboard is rated not less than...". Note it refers to the panelboard rating... not the bus rating. Even if one can verify a 100A-rated panelboard has a 125A-rated bus, the AHJ can still disapprove without reproach (unless someone brings forth refuting authoritative documentation, such as an ROP or official interpretation).

Since when does the main breaker size determine the rating of the panelboard!

If this was the case then the exception should never have been written.

 

[Sierrasparky]

I've seen plenty of panels with 100A bus rating and breaker. Very common around here, although 125A may be slightly more common. Now as for center fed panels specifically, I'm not so sure. But when the OP clearly stated the ratings each at 100A I'm inclined to believe he has read the labels and that's what he found. I've seen much stranger things.

Where is here?
I really cannot recall any panels that are 100 amp only rated. I have Murray, siemens and Sqd with 100a main breakers in them but the panel is rated at 125. Throw in a 125 and appropriate wire and you have 125. You need to look at the label.

Just the same last summer a friend was asking about solar at his home out of state. He was told that he would need to upgrade his panel because he wanted to back-feed his panel with 40 amps but he only had a 150 amp panel. He was told he needed to upgrade to a 200. I told him to find the label and photocopy it for proof because I was willing to be it was a 200 amp bus with a 150 amp main.
Sure enough the panel was a 200 amp buss with only a 150 amp main. Manufactures use standard sizes. The only place you might see exact labels that match the factory main is in some commercial paneboards.

 

[Smart $]

Since when does the main breaker size determine the rating of the panelboard!

If this was the case then the exception should never have been written.

I didn't say that.

 

[GoldDigger]

If the maximum main breaker size a panel will accept is 100A, I would have some justification for calling it a 100A panel even if it had a 125A bus.
So I do think that in a case like that the argument could go either way.
The root of the problem is that one section refers explicitly to the bus and wire ratings while the other just says "panel".

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[iwire]

I would think that would be up to interpatation.

Of course it is, but it seems the inspector has interpreted it as Don has so we are kind of wasting our time on it.