Residential-400 amp sub panel fed off 200 amp service

[jwoody]

I've just done an inspection of this pv system where the single line has all the distribution removed from the main panel and relocated to the new 400amp load center, which is fed off 200 amp brkr and is the only ocpd in the main panel, other than the main brkr. I know the engineer did this because its a large pv system and it needs to conform to the section regarding busbars under 705.12. My interpretation of this section is that the busbar rating would still be the main busbar (200a) but I dont know PV systems all that well and our engineer has explained her interpretation to me, which of course overrides me. Either way, it visually looks funny having a large 400amp fed from a (less than 1/2 the visual size) 200amp service. Is this ok? Is there any section in the code book that talks about ratings and would contradict a sub panel that large on a smaller service?

 

[JakeS]

I could be utterly wrong but I believe the 400amp sub would be fine being fed by the 200amp breaker and 705.12 would still be based off of the MSP bus x 1.2 - main breaker size giving you your potential back feeding breaker size. Maybe the 400amp sub panel was laying around or he plans to upgrade the main at some point ? Maybe a expert can lay down some knowledge.

 

[jaggedben]

It is okay. 705.12(B)(2)(3)(c) explicitly allows the main to be used this way. And there is no code section that I know of that prohibits using conductors or equipment that are higher rated than needed, as long as proper overcurrent protection is provided. People oversize stuff all the time.

 

[PWDickerson]

Agreed. I don't see any issue with the design. Though it seems like an expensive solution. I am guessing a line side interconnection was not allowed by either the POCO or AHJ. Or maybe line side bussing wouldn't allow for it. That would have been a much more cost effecting solution. I also might have explored installing a 200A breaker inclosure between the meter and panel and interconnecting at a feeder tap.

 

[jwoody]

The average bus rating on a 200amp service is 225. This PV system @ 125% is 70amp. 705.12 (3) Busbars 2) (PV brkr) 70 + (main brkr) 200 = 270. Busbar rating 225 @ 120% = 270. Cannot go over pv + main. Just barely meets code. Thanks for your help guys

 

[wwhitney]

The point of removing all the distribution breakers from the service panel is to avoid having to apply the 120% rule to the service panel. Instead you can use the "sum of all breakers" rule, which excludes the main breaker. With a single 200 amp distribution breaker in the service panel, the service panel is good for up to 160 amps (* 125%) of on site generation backfeed.

The subpanel is sized at 400A to provide plenty of capacity for generation interconnection under the 120% rule.

Cheers, Wayne

 

[electrofelon]

The point of removing all the distribution breakers from the service panel is to avoid having to apply the 120% rule to the service panel. Instead you can use the "sum of all breakers" rule, which excludes the main breaker.

And I do this trick quite frequently. It is no different than a line side connection, but "looks like" a 230.40 exception #2 install with the PV connected load side to one of the service conductor sets in a panel board which often is also a "PV combiner".

 

[jaggedben]

The average bus rating on a 200amp service is 225. This PV system @ 125% is 70amp. 705.12 (3) Busbars 2) (PV brkr) 70 + (main brkr) 200 = 270. Busbar rating 225 @ 120% = 270. Cannot go over pv + main. Just barely meets code. Thanks for your help guys

I'm glad you agree the installation is okay, but I wonder if you're still not fully reading the code. The "120% rule" is no longer the only option for qualifying a panelboard. For any given panelboard, one can use 705.12(B)(3)(2)(a)(b) or (c). (b) is the 120% rule. (c) is the rule being used in the main in your example.

Panelboards in series can each use a different rule.

Also it may be the case that many 200A panels actually contain 225A bubars, but in my experience most are not marked as such and so can't be treated as such.

 

[jaggedben]

And I do this trick quite frequently. It is no different than a line side connection, but "looks like" a 230.40 exception #2 install with the PV connected load side to one of the service conductor sets in a panel board which often is also a "PV combiner".

That sounds to me like something else.

 

[electrofelon]

That sounds to me like something else.

We have debated this before. The fact is that, depending on some specifics, a supply side "PV disconnect" could be indistinguishable code wise from a second service disconnect installed per 230.40 ex #2. If utilizing a panelboard with 230.40 exception 2 and qualifying the busbar per the "sum of all breakers excluding the main" rule, you get all the "benefits" of a supply side connection.

 

[jaggedben]

We have debated this before. The fact is that, depending on some specifics, a supply side "PV disconnect" could be indistinguishable code wise from a second service disconnect installed per 230.40 ex #2. If utilizing a panelboard with 230.40 exception 2 and qualifying the busbar per the "sum of all breakers excluding the main" rule, you get all the "benefits" of a supply side connection.

In this case the 'sum of all breakers' rule is being used load side on an ordinary meter main with one service disconnecting means. I can't see how that that 'looks like' a 230.40 ex #2 install, and it is certainly not 'indistinguishable code wise.' Supply side and load side rules are completely distinguished and mutually exclusive in the code. I can see that, like with a supply-side connection, you can use the 'sum of all breakers' to get the same 'benefit' of backfeeding up to the rating of the service. But I think that mentioning 230.40 has nothing to do with that and is confusing. Hopefully I'm not just adding more noise by trying to clarify.

 

[electrofelon]

In this case the 'sum of all breakers' rule is being used load side on an ordinary meter main with one service disconnecting means. I can't see how that that 'looks like' a 230.40 ex #2 install, and it is certainly not 'indistinguishable code wise.' Supply side and load side rules are completely distinguished and mutually exclusive in the code. I can see that, like with a supply-side connection, you can use the 'sum of all breakers' to get the same 'benefit' of backfeeding up to the rating of the service. But I think that mentioning 230.40 has nothing to do with that and is confusing. Hopefully I'm not just adding more noise by trying to clarify.

Sorry, I probably caused some confusion. My comment wasnt really based on the entire situation in the OP, just the part about having only one breaker in a panelboard. I also meant to quote post #4 and was trying to say it is a work around if someone doesnt allow line side taps.

I dont want to hijack the thread, but I still say a supply side PV disco is code wise not always distinguishable from a "normal" 230.40 ex 2 disco. What makes you able to call it one but not the other?

 

[jaggedben]

I agree that a 230.40 Ex 2 install is essentially indistinguishable from a 705.12(A) supply side connection. In fact the code is very unclear which of those sections should govern.

However both of those are completely separate code-wise from a 705.12(B), load side "sum of all breakers" panelboard, which was what the OP was asking about. The code is very clear that this is governed by completely separate rules.

 

[electrofelon]

I agree that a 230.40 Ex 2 install is essentially indistinguishable from a 705.12(A) supply side connection. In fact the code is very unclear which of those sections should govern.

However both of those are completely separate code-wise from a 705.12(B), load side "sum of all breakers" panelboard, which was what the OP was asking about. The code is very clear that this is governed by completely separate rules.

Ok, bear with me here. Consider these options and tell where/when my reasoning becomes flawed

1. Supply side connection with fused "PV disconnect" that feeds a MLO combiner panelboard that has, say, three inverters in it.

2. Supply side connection with a circuit breaker enclosure used as the "PV disconnect" that feeds a MLO combiner panelboard that has, say, three inverters in it.

Do we both agree that, if grouped, we could code wise call both #1 and #2 "2 service disconnects per 230.40 exception #2" instead of a supply side connection?

Then if so, is there any reason I couldn't combine the two pieces of equipment from #2 into a main breaker panelboard? Why does it have to become a load side connection?

 

[wwhitney]

Then if so, is there any reason I couldn't combine the two pieces of equipment from #2 into a main breaker panelboard? Why does it have to become a load side connection?

The panel bus in the panelboard has to meet the 705.12 "load-side" connection rules, whether it is in a separate enclosure from the "service breaker" [to be purposefully ambiguous about whether that's a 230.40 Ex 2 service disconnect or a (2020) 705.11 supply side connection] or not. I don't really understand the distinction you are trying to make.

But everything you describe is different from the OP, where the solar will be interconnected in the 400A panel, which also has all the loads, so I'm not clear on why you are bringing this up in this thread. You seem to have implied previously this question has something to do with the OP's configuration, which it does not.

Cheers, Wayne

 

[jaggedben]

Well, the thread may now be well hijacked, but I can't resist a discussion.

Ok, bear with me here. Consider these options and tell where/when my reasoning becomes flawed

1. Supply side connection with fused "PV disconnect" that feeds a MLO combiner panelboard that has, say, three inverters in it.

2. Supply side connection with a circuit breaker enclosure used as the "PV disconnect" that feeds a MLO combiner panelboard that has, say, three inverters in it.

Do we both agree that, if grouped, we could code wise call both #1 and #2 "2 service disconnects per 230.40 exception #2" instead of a supply side connection?

Yes we agree.

(Sorry if you read my previous response, I misread your #1).

Then if so, is there any reason I couldn't combine the two pieces of equipment from #2 into a main breaker panelboard? Why does it have to become a load side connection?

There's no important difference between a main breaker panelboard and a separate disconnect with panelboard. However in either case the disconnecting means ahead of the busbar changes the rules which apply to the busbar.

 

[wwhitney]

No, we don't agree. In #1 you added three new service disconnects. In #2 you only added one new service disconnect.

I don't see how you can call the 3 breakers in the MLO panel in example #1 "service disconnects". If there is a service disconnect in that example, there's just one of them, the fused disconnect on the utility side of the MLO panel.

Cheers, Wayne

 

[jaggedben]

I don't see how you can call the 3 breakers in the MLO panel in example #1 "service disconnects". If there is a service disconnect in that example, there's just one of them, the fused disconnect on the utility side of the MLO panel.

I misread his post at first and have edited mine. Sorry for the confusion.

 

[wwhitney]

So, as a related question, other than the issue of a 7th "service breaker", would there be any substantive change in what configurations are and aren't allowed if we just deleted (2020) 705.11 and put in an informational note directing the reader to 230.40 Ex 2?

Cheers, Wayne

 

[electrofelon]

So, as a related question, other than the issue of a 7th "service breaker", would there be any substantive change in what configurations are and aren't allowed if we just deleted (2020) 705.11 and put in an informational note directing the reader to 230.40 Ex 2?

Cheers, Wayne

That has always been my stance: get rid of all the "line side tap" confusion and just have all PV connections fall under 230.40 ex 2 and be load side. Using "sum of all breakers except main" rule (and assuming this panelboard was added just for PV) you essentially have all the capacity as a supply side would.

I think that was my point of this whole diversion, I know it doesn't really apply to the OP, I just got distracted by some other posts and ended up hijacking the thread, sorry.