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

Thanks for all the answers, guys. My head is spinning a little less now.


Ok, it's forgotten. But it's still confusing. That's why I looked up the definition of a feeder in the first place. I guess this article modifies the NEC definiton of a feeder, right?

I agree with you Roger that this article should be rewritten. IMO, it should be rewritten to allow Table 310.15 B6 to pertain to feeders from the main panelboard as well.

Confession: The subpanel I added at my home a few years ago is #2 AL SER protected by a 100 amp breaker. No, I'm not going to change it to a 90, in case anyone is wondering.

Peter

[This message has been edited by CTwireman (edited 04-08-2004).]

Triple, yes I know your just "jerking me around" and I'll give you credit to be able read and translate my post as I meant it.

Look at where I said that you seemed to have (in a blanket) put all your faith in.

As to having the privilege of being a moderator here, I was never asked to patronize anyone.

Roger




[This message has been edited by Roger (edited 04-08-2004).]

(Alas I do not have my copy of the NEC handy, so I am working from memory...I promise I'll check it later)

This issue comes up because the 'load calculations' produce a single number (KVA), which we presume is a real value, rather than a nominal value which is used to size conductors and OCPD. A 12ga conductor is _not_ a 20A conductor; it is a conductor with a particular time/temperature curve, with particular characteristics in extreme overload, etc. In general use, it is limited to operation on a 20A circuit breaker, again with its own time/temperature curve, with various restrictions on its use.

For things like welders or electric motors, the various articles describe how you can use one value for the conductor ampacity, and a _different_ number for the OCPD rating, this based upon the time characteristics of the load, the internal overload protection, etc.

The same applies to 'dwelling' loads. To supply sufficient power for a 'dwelling' you need conductors of a given ampacity and OCPD of a given rating...but rather than have load calculations which produce these two separate numbers, we have a load calculation which produces a single number, and then table 310.15(B)(6) which gives different conductor sizes to use for the single number produced by the load calculation.

If I do a load calculation on a residence, and it comes out to 82A, then it is certainly _not_ the case that this residence will be drawing 82A continuously. The 82A number simply tells us what OCPD is required and what size wire is required. This is rather different from a single device which draws 82A continuously, and both the conductor and OCPD sizes will be different for these two applications.

My interpretation of the matter: 310.15(B)(6) applies any feeder which supplies the dominant load of a single dwelling, and is intended to better match the load characteristics of a dwelling. It does not apply to feeders which do not have the same overall characteristics of a dwelling, including feeders within dwellings, eg. for air conditioning or a hot tub.

If there is a subpanel in this dwelling that does _not_ carry a 'dwelling type' load, then 310.15(B)(6) would not, and could not, apply. For example, a subpanel to a workshop is rather different from a subpanel which includes the kitchen, the air-conditioning, the heating, the lighting and the various house branch circuits. So, for example, if you have a main disconnect at the meter, and then feeders to two subpanels, one a workshop in the garage, the other the 'main circuit breaker panel' for the house (meaning the panel that has the branch circuit breakers for all of the residence circuits), then 310.15(B)(6) would apply to the feeder to the residence side of things but not to the feeder to the garage.

Then, in addition to the loads which 310.15(B)(6) is designed to properly size the conductors for, there is language that _sometimes_ extends this sizing to other feeders in residential occupancies. In particular, there is the wording that says that no feeder need be larger than the service entrance conductors. I take this to mean that you can size the service entrance conductors using 310.15(B)(6), then continue to use _this_ size for any subpanel feeders.

For example, a 200A subpanel in a home with 200A service. The feeder to the subpanel need not be larger than the service entrance conductors, even if normally you would have to use 310.16 and use larger conductors. This makes sense in that the current to the subpanel must be less than or equal to the main service current. However this note does not extent 310.15(B)(6) to other feeders. For a 100A subpanel to a home with 200A service, 310.15(B)(6) could not be applied to the subpanel feeder.

-Jon

[This message has been edited by winnie (edited 04-09-2004).]

It is very clear here that the NEC book does a very poor job of explaining its position. I understand that outside sources cannot be enforced as if from the NEC. However, in cases such as this, what else is there? How many of you have bought books on grounding or motors and relied on the info they provided?

I don't have a very large library of electrical books. I'm sure many ECN members have much larger ones. Since nobody has come forward (in this thread or in previous ones) with a source of opposition, then I can only assume such a source does not exist.

Not a single published person agrees that the residential table can be used for subpanels?????????????????????????????

How can that be?

Use your common sense and an answer is readily apparent.

I say just the opposite, use your common sense and the answer is more elusive.

If you have a combo meter / service disconnect outside and a panel inside this panel inside is a 'subpanel' (separate grounded and grounding conductors) you may use the table for the conductors running between the service disconnect and this 'subpanel'.

Your right the wording is tough.

Bob

Iwire, you took my words out of context. When I said, "use your common sense" I was only referring to the fact that not a single person has written a book supporting your side of the issue.

The NEC does not (to my knowledge) define what a subpanel is. The situation you describe would not be a subpanel in the eyes of table 310.15 (B)(6). Since the conductors you are referring to would carry the entire supply to each residence then the afformentioned table could be used.

Iwire, since you apparently don't believe in the use of sources outside the NEC then how can the word subpanel even be in your vocabulary? Do you have a selective memory;...only information that supports your personal position is absorbed?

I hope that nobody takes offense to my comments. I feel that "the truth is out there". So if your view is the truth then where is it? Do you want me to believe that the truth is a secret or perhaps a conspiracy? If it is not meant to be hidden, then where is it?

I believe that Roger and Iwire both have more written resources available to them then I do. So please take 20 minutes, refer to them, and get back to this thread with your newfound information.

Do I have to beg yet again


[This message has been edited by triple (edited 04-10-2004).]

Triple, that was good on the subpanel thing.

The application you describe (the 320 meter socket with two panels and two feeders) is without a doubt one way to apply this table.

Now as the only thing that matters is what is written in the code book (as you just nailed me with ) not what other sources say, lets really look at the wording here, totally forget what we may think the intent is, just what is written down.


As it is written in this section there is nothing to prevent me form using this for 10 feeders and panel boards one after the other instead of side by side like your 320 example.

In both cases these panels will not be suppling the main power to the building, in your 320 example each panel would be suppling about half the power.

You, me, and the authors of the books can only go by what its written in that article beyond that it is time for an official interpretation.

What I said about common sense was not a swipe at you at all, what I mean is electrically it makes no common sense that tis can apply to the main panel and not an additional panel.

Both panels sizes and loads are calculated by the same methods.

Like I said before take the table right out and have all of us use 310.16.
Bob

Our local inspector has a book of proposed changes to the NEC. I don't know what it is called but I'm sure one could find it at NFPA. But, we thumbed through it to try to pick up on some of the intentions of the CMP. There was a proposal to make an exception to the recpt. requirement on a 2' section of wall if that section of wall was buried behind the swing of a door. Then it shows why they rejected the proposal with an explaination.

The reason I bring this up, if anyone has this book, there may have already been a proposal to eliminate that section with a rejection and an explaination directly from the CMP. If so, (and hopefully it is clear enough) that may be enough to satisfy all parties.

Great point Jps1006. The way I first learned that 310.15 (B)(6) could not be used for residential garages was by reading the proposed changes. Someone suggested allowing that table’s use for such a purpose and the panel provided reasons why it could not. Is there a place where a person can easily look up all previous proposed changes and subsequent rejections on a given code issue (I know the most recent ones are always available but that may not be enough)? That may be the best way to resolve this issue once and for all.

Please people, I know I am not the only person with a book that deals with this subject. Quickly review those books and let us know your findings. Am I the only one that reads anymore? Does everybody else just slide through life assuming they already know it all?

Triple, do you really believe everything you read or hear?

Look at the NEC handbook, it is only opinions.

If you insist on being so insecure that you need a published document to substantiate your direction, than so be it.

If Tesla would have settled for the published words, we wouldn't be talking about this now.

Roger

[This message has been edited by Roger (edited 04-10-2004).]