ECN Forum Forums Code Related Discussion NEC & other Code issues Table 310.15(B)(6)

I've been watching this thread with interest, as this seems to cause more stress than just about any issue. I believe Tom Horne and Jon/Winnie have the technically correct answer, but the code words are not clear.

Roger - would your answer be any different if the two 200A breakers were changed to 4 100A breakers on #4 copper (all from parallel lugs and assume each 100A feeder terminates in a 100A MLO lighting and appliance panelboard)?

In dwelings, there is some implicit diversity as the load increases (you don't see feeders under 100A in table 310.15(B)(6)). There is also some implicit diversity in a lighting and appliance panelboard. However, the 10% threshold to change from a power panel to a lighting and appliance panel seems way too small to me. In the largest panels, you only need four or maybe five 30A or less circuits with neutrals to make a lighting and appliance panelboard. You only need 1 in a 10 circuit panel.

If I was NEC god, you could leave the words to 310.15(B)(6) as is if either lighting and appliance panelboards were redefined to have more <=30A breakers (these are the most diverse loads) or you mandated that the target panelboard had some minimum number of circuits. I'd be inclined to decree you could use that table if there were 4 or more single pole breakers rated 20A or less -or- there are at least 6 breakers of any size. I would also consider tossing in a restriction that the largest breaker in the feeder target panel could be no larger than 50% of the feeder rating.

But I'm not NEC god...

Welcome to the forum Mark.
If tap rules were followed, any number of lighting appliance branch-circuit panels
can be fed using 310.15(B)(6) immediatly downstream of the service equipment.

If we look at a 30 unit condo, (any number can be used) from the Metering Equipment, and disconnects at this location, the individual feeders to the lighting appliance branch-circuit panel in each unit could use 310.15(B)(6).

Roger

So if multiple 100A panels are OK, what would be wrong with two 100A feeders off of a 200A panel each feeding half of a dwelling? How about 3 100A panels from a 320A disconnect each supplying 1 floor of a large house? This seems to be the issue -- at what point is a feeder supplying a diverse enough portion of a dwelling to qualify for 310.15(B)(6)?

This seems difficult to codify easily as it would be possible for the dumb electrician to put a bunch of electric heat, water heater, and one 15A outside branch circuit all in one panel fed with #4 copper. Conversely, if all of the general purpose branch circuits and all of the required ones that have no calculated load (outside, basement, bathroom, etc) were put on a 100A panel with #4 copper, you'd porbably never have more than half of that being used for any significant time.

Mark, nothing would be wrong with this if the 200 amp panel were a "Power Panel", if it were a "lighting appliance branch-circuit panel", 310.15(B)(6) has already been used.


This would be fine in that the feeders would fit the discription of "Main Power Feeders" per 310.15(B)(6).

Take note that my opinion of this issue is simply the way I interpret the poor wording of this article, which I think many of us agree is the case.

Roger

Guys I have to point out that the plural in "feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboard(s)" is meant to apply to multiple dwellings that are supplied with single phase 120/240 service. This is a very common situation in small apartment buildings. A good example is a two story and basement building code construction type III (masonry bearing walls with wood floors and roof framing; generally called ordinary), with two units on each upper floor, one unit in the basement and a basement laundry room. With a calculated load of 370 amps. The service entry conductors to the meter bank could be 400 MCM copper. The service entry conductors from each meter to the five apartment units could be number four copper. But the service entry conductors to the 100 ampere house panel that serves the laundry room, heating plant pumps, and so forth would have to be number three copper.

What is obvious to me is that the language of section 310.15(b)(6) needs to reflect a one dwelling unit one feeder approach. I've called the NFPA; since I'm a member; and I've gotten the forms for a request for formal interpretation. Short of a change in the code language that is the only authoritative way to resolve this. In my conversation with the engineer in the electrical section he assures me that the intent is one dwelling one set of conductors. Before you tell me that it is only his opinion please keep in mind that he is staff to the code making panel that is responsible for that section. He even pointed out that parallel conductors for these services cannot be sized under Table 310.15(B)(6).

One correct way to install the entrance conductors for a four hundred ampere service using two, two hundred ampere, panels is to equip one of the panels with double barreled lugs, run 600 MCM Al or 400 MCM Cu to those lugs from the meter can, then run 3/0 Cu or 250 MCM Al to the second panel under exception two to 230.40. But before going to the expense of that size of conductor remember that the size of the service entrance conductors serving more than one disconnect is governed by the calculated load rather than by the sum of the ratings of the two to six disconnecting means. If your calculation is for a 250, 300, or 350 ampere service then there is no need to use four hundred ampere conductors.
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Tom Horne


[This message has been edited by tdhorne (edited 04-14-2004).]

[This message has been edited by tdhorne (edited 04-14-2004).]

Hello Tom, now we have another controversy to the wording.

Let us know if you have any satisfaction with the formal interpretation.

As of right now and the wording provided, I don't agree with this person, (yes, I know he is a member of the CMP for this section) although the intent may be otherwise, it isn't written to reflect it.

This will take more than one question, and would almost take rewriting 310.15(B)(6) in a series of questions for any interpretation/s (note the plural) to put this to rest.

I have heard (hearsay only) that formal interpretations must be worded as YES or NO questions, this will cause confusion in itself.

BTW, I don't think parallel conductors are questioned since they are not mentioned in the table, and therefor could not be considered anyways.

Roger



[This message has been edited by Roger (edited 04-14-2004).]

Tom, you may be right, maybe you do have inside info on this, maybe your contact is 100% correct.

Here is the problem, all electricians are supposed to be able to use and apply the code without 'inside info'.

Right now there is no language in the code article to support (or dispute) your stated reasons for the (s).

As they do not tell us what the (s) is for, we can use it as we like.

And I agree, this table does not apply to parallel conductors.

Bob

I have been staying out of this until now but will give my opinion even though it’s not worth much.

I have debated this issue with Don a couple of times over the last couple of years and have always believed it to be exactly how Tom stated. In previous additions of the NEC the only feeder conductors that could be sized according to table 310-15(B)(6) had to carry the entire load of the service. I believe it was the 96 addition when the change was made to allow feeders that carry the main power to the dwelling to use the reduced sizes. Before the 96 addition of the NEC apartment feeders could not use this table and had to be sized according to table 310.16. In my area we always have over current protection in residential meter sockets. Often there are two 2-pole service disconnect breakers, 1 of the breakers will feed a panel in the house and the other will feed an exterior hot tub or swimming pool. In this case table 310.15(B)(6) can be used for the feeder supplying the panel in the house because it carries the main power for the dwelling since the pool or hot tub are not part of the dwelling.

Curt

The problem that I have with this section is that there is nothing in the definition of "main power feeder" that requires this feeder to supply the main power to the dwelling unit. There is nothing about the diversity of the load. The wording only requires one thing: that the feeder be between the main disconnect and the lighting and appliance branch circuit panels. It doesn't even say that this feeder must be directly between the main disconnect and the lighting and appliance branch circuit panels. If you just apply the words as written, then all feeders within the dwelling unit are "main power feeders". This is very poor code writing. Yes, the intent was that these feeders are required to provide the main power to the dwelling unit, but it is not easy to enforce intent. The inspector can only enforce what is written in the code book and the words as written permit all feeders that feed lighting and appliance branch circuit panels to the sized per Table 310.15(B)(6).

Don

I have submitted the request for formal interpretation. I'm told it will take a couple of months for that to be processed.
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Tom H

[This message has been edited by tdhorne (edited 05-04-2004).]