subpanel-residential

George Stolz · Feb 16, 2006

Re: subpanel-residential

Originally posted by DaveTap:
I'd guess the reasoning in 310.15 is a service should not draw anywhere near its rated capacity...

Sounds reasonable to me.

...and in most cases the conductors are either outside the stucture or encased in concrete where minimum damage would occur if they did overheat...

I disagree. There are no such provisions for these conductors. I run SER right through a dwelling to get to the panel. I'm not required to keep them outside or encase them.

suemarkp · Feb 16, 2006

Re: subpanel-residential

Originally posted by charlie b:

Originally posted by fastforward: So Charlie, are you saying that if this application was a 100A service.(which would be #4 wire feeding it.per.310.15(b)(6) )If there was a 100A sub somewhere else in the same dwelling. We would have to increase are wire size by (3) sizes to #1 per 310.16 (60 deg.)

Click to expand...

I agree with Infinity's statement that you only need to upsize from 4 to 3, but otherwise, yes, that is what I am saying. Or more precisely, that is what I believe the code is saying.

I don't think you have to upsize. We can add more flys to the ointment by looking at article 215. NEC 215.2(A)(4) Individual Dwelling Unit or Mobile Home Conductors says: Feeder conductors for individual dwelling units or mobile homes need not be larger than service-entrance conductors. Paragraph 310.15(B)(6) shall be permitted to be used for conductor size.

So it appears to me that if you have a 100A service with #4 copper, you can have 100A feeder to a subpanel also with #4 copper. Now the kicker for me is, does the last sentence apply to all feeders for dwellings?

[ February 16, 2006, 02:12 PM: Message edited by: suemarkp ]

fastforward · Feb 16, 2006

Re: subpanel-residential

Originally posted by suemarkp:

Originally posted by charlie b:

Originally posted by fastforward: So Charlie, are you saying that if this application was a 100A service.(which would be #4 wire feeding it.per.310.15(b)(6) )If there was a 100A sub somewhere else in the same dwelling. We would have to increase are wire size by (3) sizes to #1 per 310.16 (60 deg.)

Click to expand...

I agree with Infinity's statement that you only need to upsize from 4 to 3, but otherwise, yes, that is what I am saying. Or more precisely, that is what I believe the code is saying.

Click to expand...

I don't think you have to upsize. We can add more flys to the ointment by looking at article 215. NEC 215.2(A)(4) Individual Dwelling Unit or Mobile Home Conductors says: Feeder conductors for individual dwelling units or mobile homes need not be larger than service-entrance conductors. Paragraph 310.15(B)(6) shall be permitted to be used for conductor size.

So it appears to me that if you have a 100A service with #4 copper, you can have 100A feeder to a subpanel also with #4 copper. Now the kicker for me is, does the last sentence apply to all feeders for dwellings?

I think this answers my question.Art.100 Feeder is all circuit conductors between the service equipt.,etc.,or other power supply source and final branch-circuit overcurrent device.It also references 310.15(b)(6). ANYBODY DISAGREE

roger · Feb 16, 2006

Re: subpanel-residential

Lets change the scenario to a 200 amp service feeding a "Lighting and Appliance Branch-Circuit Panelboard", 310.15(B)(6) was used for the feeder (2/0 conductors) from a "Main Disconnect" to this "Lighting and Appliance Branch-Circuit Panelboard".

Now, from this "Lighting and Appliance Branch-Circuit Panelboard" we feed a 100 amp sub panel inside this dwelling, this 100 amp panel feeder can not use 310.15(B)(6).

Roger

charlie b · Feb 16, 2006

Re: subpanel-residential

Originally posted by don_resqcapt19: While it does imply that with the use of the word "main," that implication is negated by the definition of "main power feeder."

Nothing in the code's wording contradicts or invalidates the meaning of "main." That article defines (or I think "explains" is a better characterization) "main power feeder" as conductors that run from the service disconnect to the panel(s). That would not include conductors that run from one panel to another.

Originally posted by don_resqcapt19: There is nothing in the section to limit a dwelling unit to a single "main power feeder."

If you started at the service disconnect, and ran two separate feeders to two separate panels, both of which are in the same dwelling unit, then I would agree that both feeders could be sized per Table 310.15(B)(6). In that circumstance, the two feeders collectively would comprise the "main feeder" to that one dwelling unit.

But that is different from the original question. It described two sub-panels, each of which was fed from the main panel. Neither was fed from the service disconnect. Therefore, neither of these would be the "main" feeder to the dwelling unit.

Originally posted by don_resqcapt19: There is nothing to say that such a feeder must carry the complete or even the major portion of the load.

If one feeder supplies 100% of the loads, including all the loads on the "first panel" and all the loads on the downstream panels, and if a second feeder to a second panel supplies 50% of the loads, then you cannot say that both are "the main." You have a "main" and you have a "not main."

iwire · Feb 16, 2006

Re: subpanel-residential

Charlie are you offering that as fact or opinion?

charlie b · Feb 16, 2006

Re: subpanel-residential

Originally posted by fastforward: I think this answers my question. Art.100 Feeder is all circuit conductors between the service equipt.,etc., or other power supply source and final branch-circuit overcurrent device. It also references 310.15(b)(6). ANYBODY DISAGREE

I do. I think it answers your question incorrectly.

From the utility to the service disconnect, the conductors are "service conductors." From there to the first panel (or to the first set of panels, each of which is fed from the service disconnect), the conductors are called "feeders." From any panel to any other panel, or to an overcurrent device that is separate from a panel, the conductors are also called "feeders." But in the context of 310.15(B)(6), the "main power feeder" to a dwelling unit comes from the service disconnect to a panel (or a set of panels). In that same context, if conductors are not starting at the service disconnect, then they are not "main power feeders," and you can't use Table 310.15(B)(6).

charlie b · Feb 16, 2006

Re: subpanel-residential

Originally posted by iwire: Charlie are you offering that as fact or opinion?

The only thing that can be called "fact" is the text of the code itself. All else is opinion, regardless of who is doing the talking.

fastforward · Feb 16, 2006

Re: subpanel-residential

Charlie,
Doesn't art.100 definition for feeder and 215.2(a)(3)answer the original concept for you.If not,then why?

iwire · Feb 16, 2006

Re: subpanel-residential

Originally posted by charlie b:
The only thing that can be called "fact" is the text of the code itself. All else is opinion, regardless of who is doing the talking.

Thats refreshing to hear.

fastforward · Feb 16, 2006

Re: subpanel-residential

Originally posted by charlie b:

Originally posted by fastforward: I think this answers my question. Art.100 Feeder is all circuit conductors between the service equipt.,etc., or other power supply source and final branch-circuit overcurrent device. It also references 310.15(b)(6). ANYBODY DISAGREE

Click to expand...

I do. I think it answers your question incorrectly.

From the utility to the service disconnect, the conductors are "service conductors." From there to the first panel (or to the first set of panels, each of which is fed from the service disconnect), the conductors are called "feeders." From any panel to any other panel, or to an overcurrent device that is separate from a panel, the conductors are also called "feeders." But in the context of 310.15(B)(6), the "main power feeder" to a dwelling unit comes from the service disconnect to a panel (or a set of panels). In that same context, if conductors are not starting at the service disconnect, then they are not "main power feeders," and you can't use Table 310.15(B)(6).

Charlie,
Did you look at 215.2(a)(3)?Says nothing about main power conductors.Look at art.100 def. for feeder(to the final branch circuit overcurrent device.)Sub panels definitly included there per NEC definition.

DaveTap · Feb 16, 2006

Re: subpanel-residential

Originally posted by charlie b:
A feeder to a garage is not going to be the main power source to a dwelling. Nor will a feeder to a panel in a basement. 310.15(B)(6) does not apply to either situation.

Actually Charlie I get a lot of work from old houses being rezoned to allow basements and garages to be used as apartments. I always upgrade to multiple meters/disconnects but I've seen them upgrade the service from 100A to 200A replacing the old meter socket with a meter and outdoor lug through panel or combo using the existing 100 amp MBR panel and feeders then adding breakers in the combo for the new apartment panels. As I read it this does not meet code but is being done. Since they actually remove load from the existing MBR panel it should be safe. It does raise some interesting questions though. For instance in this case why does the NEC seem to allow the old 100A MBR panel to have smaller feeders when installed alone with only load side overcurrent protection than when installed after line side overcurrent protection. To me this has never made any sense. I still think service entrance conductors should be oversized not undersized. Then again I've seen the results of service conductors that have over-heated and always figured larger conductors with line side protection would have prevented it.

electricmanscott · Feb 17, 2006

Re: subpanel-residential

Dave in your scenario they are no longer garages and basements. They are now dwelling units and 310.15(B)(6) does apply to that units power feeder.

charlie b · Feb 17, 2006

Re: subpanel-residential

Originally posted by electricmanscott: Dave in your scenario they are no longer garages and basements. They are now dwelling units and 310.15(B)(6) does apply to that units power feeder.

I agree with this.

Edited to add: My agreement is contingent upon each of these two new "dwelling units" receiving its feeder from the main disconnect, and not from the service panel within the house.

[ February 17, 2006, 10:39 AM: Message edited by: charlie b ]

charlie b · Feb 17, 2006

Re: subpanel-residential

Originally posted by fastforward: Doesn't art.100 definition for feeder and 215.2(a)(3) answer the original concept for you. If not, then why?

It does not. The reason is that 310.15(B)(6) includes the word "disconnect" in a very critically important context:

For application of this section, the main power feeder shall be the feeder(s) BETWEEN THE MAIN DISCONNECT and the . . . panelboard(s).

If you are looking at a wire that does not have one of its endpoints at the main disconnect, then you are not within the scope of 310.15(B)(6).

fastforward · Feb 17, 2006

Re: subpanel-residential

Charlie,
It seems that you didn't even look at art.100 or 215.2 because it is pretty clear that you can run #4's for a 100A panel and then to a sub.It specifically refers to 310.15 for conductor size.Need NOT be larger than Service conductors.This art.215 speaks for itself and does not contradict art.315.Actually compliments it.

charlie b · Feb 17, 2006

Re: subpanel-residential

Originally posted by fastforward: Charlie, It seems that you didn't even look at art.100 or 215.2 because it is pretty clear that you can run #4's for a 100A panel and then to a sub. It specifically refers to 310.15 for conductor size.

OK. I have not been clear or complete in expressing my views. My bad. I'll try again.

Looking at 100 and 215.2, I conclude that they do not change anything I have been saying. The key word in 215.2(A)(3) (2005 NEC) or 215.2(A)(4) (2002 NEC) is the first word in the second sentence. The word is "paragraph," not "table."

Paragraph 310.15(B)(6) shall be permitted to be used for conductor size.

That does not authorize us to use the values in the table. It tells us to follow what the paragraph tells us to do. In that paragraph are the words I quoted in my earlier post. In that paragraph, we are told that we can use the table for conductors that start at the main disconnect and that end at panel(s). But that paragraph does not say we can use the table for conductors that start at one panel and that end at another panel.

On the subject of "feeders don't have to be bigger than service conductors," please note that the service conductors terminate at the main disconnect. Nothing in this thread has yet addressed the size of those conductors. They are provided by the utility and in accordance with the utility's rules. This thread has been discussing conductors downstream of the main disconnect. Nothing in any NEC section that has yet been brought up in this thread will back up an assertion that a feeder from one panel to a second panel need not be bigger than the feeder to that first panel.

fastforward · Feb 17, 2006

Re: subpanel-residential

Charlie,
I think your last point is clear on what you mean.HOWEVER, Does not the references in paragragh 310.15(B)(6).service-entrance and service-lateral mean the same thing as service conductors in a dwelling application.If so,Then if they are #4's per table 310.15(B)(6),then art.215 allows for the #4's for the sub..The more I look at this .The more I see that there is a thread.The thread is it doesn't make sense to have to install larger conductors downstream.I think these art.'s point this out.

suemarkp · Feb 17, 2006

Re: subpanel-residential

Originally posted by charlie b:

Paragraph 310.15(B)(6) shall be permitted to be used for conductor size.

Click to expand...

That does not authorize us to use the values in the table. It tells us to follow what the paragraph tells us to do.

If that is the case, then why did they even bother to add that last sentence to 215.2? Can't you use 310.15(B)(6) with out the explicit permission stated in 215.2? Seems redundant unless they were trying to expand or clarify the meaning or scope of 310.15(B)(6).

charlie b · Feb 17, 2006

Re: subpanel-residential

Fastforward: I think I see how we might be reading this differently. It has to do with the word "that." In my understanding of the English language, the word "that" refers back to the one thing that immediately preceded it in the list, not to the entire list, unless there is some clear separator that alters the meaning of "that."

I will restate the first sentence of 310.15(B)(6) in two versions. They have very different meanings.

Both versions start out with, "For individual dwelling units of one family, two family, and multi-family dwellings, conductors as listed in . . . " Now I'll show two versions:

VERSION 1:

Table 310.15(B)(6) shall be permitted as 120/240 volt, single phase . . .


. . . service entrance conductors that serve as the main power feeder to each dwelling unit. . .

and


. . . service lateral conductors that serve as the main power feeder to each dwelling unit. . .

and


. . . feeder conductors that serve as the main power feeder to each dwelling unit. . . .

VERSION 2:

Table 310.15(B)(6) shall be permitted as 120/240 volt, single phase . . .


. . . service entrance conductors. . .

and


. . . service lateral conductors. . .

and


. . . feeder conductors that serve as the main power feeder to each dwelling unit. . . .

Do you see the difference? I think you have been using Version 1. I have been using Version 2, and here is what Version 2 says to me:

If you have service entrance conductors that land on a panel in the house, then those conductors can be sized per the table. Furthermore, all feeders in the house (i.e., downstream of the panel) need not be larger than the service entrance conductors.

If you have service lateral conductors that land on a panel in the house, then those conductors can be sized per the table. Furthermore, all feeders in the house (i.e., downstream of the panel) need not be larger than the service lateral conductors.

If you have a feeder that starts at main disconnect that is remote from a panel in the house, but that lands on a panel in the house, then that feeder can be sized per the table. Furthermore, all feeders in the house (i.e., downstream of the panel) need not be larger than the service conductors that are upstream of the main disconnect. But you don't get to compare sizes of the feeders downstream of the panel with the feeder to the panel.

subpanel-residential

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