the purpose of 310.15(B)(6)

[Twoskinsoneman]

Why the difference between a "main power feeder to a dwelling" and one to a garage? I understand it's code but practically what drives allowing the smaller conductors on the dwelling unit circuit?

 

[barbeer]

I assume (dumb) you mean a detached structure!? I would almost classify(more than likely will get heat from this) all 1&2 fam dwellings to be wired per 310.15(B)(6) including detached structures with a separate service. Services or feeders that is.........if a branch circuit exit to 310.16.

 

[iwire]

I assume (dumb) you mean a detached structure!? I would almost classify(more than likely will get heat from this) all 1&2 fam dwellings to be wired per 310.15(B)(6) including detached structures with a separate service. Services or feeders that is.........if a branch circuit exit to 310.16.

No, Table 310.15(B)(6) can not be used to size the conductors to a panel supplying a garage, regardless if it is attached or not.

That table can only be used for the conductors supplying 100% of the power to a dwelling unit. The reason for this is that an entire dwelling unit has a know amount of diversity where any other application has unknown load diversity.

The wording is being changed in the 2008 to make this clear.

Here it is from the 2008 ROP regarding the use of Table 310.15(B)(6).

Short version....

Panel Statement: The panel agrees that the present wording is ambiguous. It is the panel’s intent that this allowance apply only to conductors carrying 100% of the dwelling unit’s diversified load.

Long Version....

6-61 Log #194 NEC-P06 Final Action: Accept in Principle
(310.15(B)(6))
____________________________________________________________
NOTE: The following proposal consists of Comment 6-40 on Proposal 6-41
in the 2004 May Meeting National Electrical Code Committee Report on
Proposals. This comment was held for further study during the processing
of the 2004 NATIONAL ELECTRICAL CODE. The recommendation in
Proposal 6-41 was:

Revise as follows:

(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders.
For individual units of one family, two-family and Multifamily dwelling
units, conductors, as listed in Table 310.15(B)(6), shall be permitted as
120/240-volt, 3-wire, single-phase service-entrance conductors, service
lateral conductors, and feeder conductors that serve as the main power
feeder to a each dwelling unit and are installed in raceway or cable with
or without an equipment grounding conductor. For application of this
section, the main power feeder shall be the feeder(s) between the main
disconnect and the lighting and appliance branch-circuit panelboards(s).
The feeder conductor to a dwelling each unit shall not be required to be
larger than their service-entrance conductors. The grounded conductor
shall be permitted to be smaller than the ungrounded conductors,
provided the requirements of 215.2, 220.22, and 230.42 are met.

Submitter: Frederic P. Hartwell, Hartwell Electrical Services, Inc.

Recommendation: Accept the panel action in principle. Clarify the
permissible application of the multiple feeder allowances as one of the
following four options:

1) “… the main power feeder shall include the feeder(s) serving only loads
associated with a single dwelling unit and running to but not originating in the
lighting and appliance branch-circuit panelboard(s) serving the dwelling unit.”

OR

2) “… the main power feeder shall include the feeder(s) serving only loads
associated with a single dwelling unit and running to the lighting and appliance
branch-circuit panelboard(s) serving the dwelling unit.”

OR

3) “… the main power feeder shall include the feeder(s) serving only dwelling
loads and running between the main disconnect and the lighting and appliance
branch-circuit panelboard(s) serving the dwelling unit.”

OR

4) “… the main power feeder shall include the feeder(s) serving only dwelling
loads and running to but not originating in the lighting and appliance branchcircuit
panelboard(s) serving a particular dwelling unit.”

Substantiation: By clarifying that this note applies to dwelling units within
multifamily housing, which is well advised, the proposal raises important
questions as to exactly which panelboard feeders are within the scope of this
allowance. Options 1 and 2 exclude feeders that are comprised of dwelling
loads, but that serve multiple dwelling units. Options 3 and 4 allow such a
feeder. Options 1 and 2 as a group and options 3 and 4 as a group sort out
whether this allowance applies to subpanel feeders within a dwelling unit.
Dwelling unit subpanel loads do not present the same diversity as dwelling unit
panels serving the entire dwelling unit, and thereby undercut one of the
traditional supporting assumptions underlying these allowances. However, all
of these interpretations are possible given the ambiguous “(s)” endings on the
word “feeder” and “panelboard.” CMP 6 needs to clarify exactly which feeders
qualify for this allowance.

Panel Meeting Action: Accept in Principle
Remove the 2 sets of parentheses and the duplicate “s” on panelboards so that
the section reads:

(6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For
individual dwelling units of one family, two-family, and multifamily dwellings,
conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240-volt,
3-wire, single-phase service-entrance conductors, service lateral conductors,
and feeder conductors that serve as the main power feeder to each dwelling
unit and are installed in raceway or cable with or without an equipment
grounding conductor. For application of this section, the main power feeder
shall be the feeder between the main disconnect and the lighting and appliance
branch-circuit panelboard. The feeder conductors to a dwelling unit shall not be
required to have an allowable ampacity rating greater than their serviceentrance
conductors. The grounded conductor shall be permitted to be smaller
than the ungrounded conductors, provided the requirements of 215.2, 220.61,
and 230.42 are met.

Panel Statement: The panel agrees that the present wording is ambiguous. It is
the panel’s intent that this allowance apply only to conductors carrying 100%
of the dwelling unit’s diversified load.

Number Eligible to Vote: 11
Ballot Results: Affirmative: 11

 

[charlie b]

For application of this section, the main power feeder shall be the FEEDER between the main disconnect and the lighting and appliance branch-circuit PANELBOARD.

It seems to me that the only change in the upcoming 2008 version is that the two words I show in the above quotation in BOLD ALL CAPS have been changed from plural to singular. Is that really enough to clarify the ambiguity? Is it likely to be clear to any reader that this section only applies when 100% of the dwelling unit?s load will flow through the conductor under consideration? How will this be interpreted for a two-family or multi-family dwelling unit?

By the way, had anyone else noticed, as I noticed for the first time just a few moments ago, that the 2005 version has a duplicate plural indication: ?panelboards(s)?? :grin:

 

[iwire]

It seems to me that the only change in the upcoming 2008 version is that the two words I show in the above quotation in BOLD ALL CAPS have been changed from plural to singular.

More has changed than that, but the strike throughs and underlines indicating the other changes did not carry over from the ROP pdf.

Later I will provide a link to the pdf at the NFPA so anyone can see it for themselves.:smile:

 

[kbsparky]

The short answer to all this is you can't use a 100 Amp breaker on a #2 AL wire feeding a garage. Got to use a 90 Amp breaker instead :grin:

 

[iwire]

Here is the link I said I would get.

Article 285-338 ROP 1 MB .pdf

 

[Twoskinsoneman]

The short answer to all this is you can't use a 100 Amp breaker on a #2 AL wire feeding a garage. Got to use a 90 Amp breaker instead :grin:

I think everyone agrees. However this thread is really a question as to why. The only answer given so far is "an entire dwelling unit has a know amount of diversity where any other application has unknown load diversity".

What is not clear to me is why the ampacity of a cable should change because the the load diversity is known?

 

[winnie]

What is not clear to me is why the ampacity of a cable should change because the the load diversity is known?

IMHO this is the crux of the problem with the way 310.15(B)(6) is written.

The _ampacity_ of the cable is not changing. Rather you are using a cable of thermal ampacity X with an OCPD of of trip rating Y, with Y > X.

There are other code situations where you see the same permission, eg. OCPD for transformers, motors, and welders. In these cases, the combination of the OCPD and the connected load is considered to sufficiently protect the conductors.

A significant difference between a home and a motor is that a motor will have its own overload protection, whereas a home does not have this overload protection, you simply have load diversity.

IMHO 310.15(B)(6) should not be written in terms of particular cable types and sizes, but instead in terms of conductor ampacity. Rather than saying that 2/0 Cu can be used for a 200A residential service, it should say that conductors of 175A allowable ampacity may be protected at 200A and used for residential services of up to 200A calculated load.

-Jon

 

[iwire]

What is not clear to me is why the ampacity of a cable should change because the the load diversity is known?

The ampacity of the cable has not been changed.

The way we are allowed to use it has changed.

The NFPA knows that a service or feeder for an entire dwelling unit when calculated using Article 220 will result in an service size that is larger then needed.

The cable will not be loaded to the amount that the service calculations come up with.

Also consider you rarely do service calculations that result in exactly 100, 150, 200 amps. Whatever the calculations come out to we round up to the next size.

 

[hillbilly]

As a example....I still read 310.15(B)(6) to mean.......

Say I mount a meter can outside on a dwelling and run a (code compliant) 3 conductor SE cable into a Main Breaker (or fused disconnect) that's mounted inside (on the opposite side of the wall).


From that main, (IMO) I can feed any number of lighting and appliance (sub)panels and use Table 310.15(B)(6) to size the feeders to those panels.

I realize that the rule states "3 wire", but it also states that the "main power feeder" is the feeder(s) "between the main disconnect and the lighting and appliance branch circuit panelboards(s)".
That "feeder" will be 4 wire because it must include a grounded conductor.

In my opinion, the fact that 310.15(B)(6) specifies "lighting and appliance panel boards" as the panels that can be fed using this rule, instead of "all" panels or more specifically "power panels" is what (to me) implies that the load diversity will allow smaller feeder conductors.

If I'm wrong, I would like to be shown where, and have it proven to me.
This has always (to me anyway) been a point of conjecture, and I don't see the new changes to the wording of the rule as providing a clear answer.

Just my Opinion

steve

 

[iwire]

From that main, (IMO) I can feed any number of lighting and appliance (sub)panels and use Table 310.15(B)(6) to size the feeders to those panels.

Steve you can not do that, that is really not an opinion but a fact based on the code making panel has said about this section of code.

Once you break it up into multiple panels they are no longer main power feeders.

Look again at what the CMP stated.

Panel Statement:

The panel agrees that the present wording is ambiguous. It is the panel?s intent that this allowance apply only to conductors carrying 100% of the dwelling unit?s diversified load.

So you can size the service conductors to a meter main with T310.15(B)(6) because they carry 100% of the load.

You can run one feeder from that meter main into a panel using T310.15(B)(6) because they carry 100% of the load but as soon as you hit the first lighting and appliance panel T310.15(B)(6) is finished.

Any feeder beyond that will have to use table 310.16 as they do not carry 100% of the dwelling units power.

 

[charlie b]

What is not clear to me is why the ampacity of a cable should change because the load diversity is known?

It is not often that I get to disagree with Bob. :smile: Sorry to say, Jon, that this time you are coming along for the ride.

The ampacity of the cable is not changing.

and

The ampacity of the cable has not been changed.

I submit that the ampacity of the conductors does change. Look at the definition of ?ampacity.? It includes the phrase, ?under conditions of use.? The way I see it, the conditions of use for which Table 310.15(B)(6) applies are different than the conditions of use for which Table 310.16 applies.

The design process remains the same. First you determine the load, the ?required ampacity? of the conductors. Then you select conductors that have at least that ampacity. Finally you select an OCPD to protect those conductors.

 

[winnie]

Charlie, I see your point, and if we could differentiate the continuous ampacity (from table 310.16) versus the 'residence non-continuous ampacity' (from table 310.15(B)(6) ) than I would agree with you.

However the definition of ampacity is 'The current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. '

-Jon

 

[Twoskinsoneman]

Once you break it up into multiple panels they are no longer main power feeders..

As long as the current NEC uses the current terminology

"

For application of this section, the main power
feeder shall be the feeder(s) between the main disconnect and
the lighting and appliance branch-circuit panelboards(s)."

I don't see how we can say Steve's example is wrong​

 

[charlie b]

Charlie, I see your point, and if we could differentiate the continuous ampacity (from table 310.16) versus the 'residence non-continuous ampacity' (from table 310.15(B)(6) ) than I would agree with you.

You lost me there. Neither Table 310.16 nor the text that refers to reader to that Table inserts the concept of "continuous." Neither Table 310.15(B)(6) nor the text that refers to reader to that Table inserts the concept of "residence non-continuous."

 

[Twoskinsoneman]

Look again at what the CMP stated.


Quote:
Panel Statement:

The panel agrees that the present wording is ambiguous. It is the panel?s intent that this allowance apply only to conductors carrying 100% of the dwelling unit?s diversified load.

Just to be picky, this statement just says "conductors", unless I'm trying to be too literal, why can't the word "conductors" in the sentence be descirbing the conductors of two feeders which are carrying the entire load through two panels.

 

[charlie b]

I don't see how we can say Steve's example is wrong

We can because the presence of the plural "(s)" can be interpreted at least two ways. I believe the intended way is the way it is being changed - to eliminate the plural. I believe the intent of the plural was to address cases of multi-family dwellings, in which a single feeder to each dwelling can be sized per 310.15(B)(6), but in which there were more than one dwelling unit.

 

[Twoskinsoneman]

We can because the presence of the plural "(s)" can be interpreted at least two ways. I believe the intended way is the way it is being changed - to eliminate the plural. I believe the intent of the plural was to address cases of multi-family dwellings, in which a single feeder to each dwelling can be sized per 310.15(B)(6), but in which there were more than one dwelling unit.

I really do see your point. In fact I don't necessarily believe it should be dones Steve's way, I just don't think it's clear enough to say he's wrong.

Why wouldn't the word "main disconnect" also be plural if it was refering to multi-family dwellings?

 

[iwire]

Charlie

You lost me there.

I agree with what Jon said here

However the definition of ampacity is 'The current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. '

If I was to treat T310.15(B)(6) as a true ampacity rating of the conductors I will overheat the conductors when loaded to that higher ampacity.

Call it an ampacity table if you must but IMO that is misleading as 'changing the conductors rating' is not why that table works.