the purpose of 310.15(B)(6)

[iwire]

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

The (s) will be removed in the 2008 NEC.

As you can see, and as Charlie pointed out the (s) can allow different interpretations of the section.

The advantage we have here is we have the panels statement of intent which is as close to an official interruption as we will likely ever get.

 

[charlie b]

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.

Sorry, Bob, but I have to disagree. It is an ampacity table, and it does change the conductors? rating. You may note that the title and other information at the top of Table 310.15(B)(6) do not use the word ?ampacity.? But the text of article 310.15(B)(6) does. By saying that feeders do not have to have an ampacity greater than the service entrance conductors, it is describing the information in the Table as being ampacity values.

I infer that running a load of 100 amps continuously through a #4 copper feeder that was installed per 310.15(B)(6) will not overheat the conductors any more than running 85 amps continuously through a #4 copper feeder that was installed per Table 310.16. Different conditions of use; different ampacity values.

 

[winnie]

You lost me there. Neither Table 310.16 nor the text that refers to reader to that Table inserts the concept of "continuous."

Agreed, however the definition of ampacity in article 100 specifically says "continuously", right before "conditions of use". This makes it very clear that variability of current flow cannot be one of the relevant "conditions of use".

My understanding of "conditions of use" is that it covers things such as ambient temperature, local thermal insulation, heating from other conductors, etc. which determine how much heat a given conductor can reject.

Note for example the difference between table 310.16 and 310.17. These are tables of ampacity for two different "conditions of use". 310.17 is for conductors with much better cooling that 310.16, and conductors of the same size and insulation are permitted to carry higher current.

Having said all of that, I now see that 310.15(B)(6) could be read as an ampacity table, if one makes the assessment that the cooling of 'main power feeders' in a residence is better than the cooling of similar installations that are not 'main power feeders'. However I believe that the reason for 310.15(B)(6) is that the main power feeder of a residence is normally loaded well below the article 220 calculated load for a residence.

 

[LarryFine]

I infer that running a load of 100 amps continuously through a #4 copper feeder that was installed per 310.15(B)(6) will not overheat the conductors any more than running 85 amps continuously through a #4 copper feeder that was installed per Table 310.16.

Why do my instincts disagree with this statement. Well, the gist of it, anyway. While 100a may not overheat the conductors, 100a would certainly warm them up more than 85a would.

 

[hillbilly]

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.

Steve doesn't think so either.
I was just giving my interpretation of the rule as written.

You won't find any of my jobs with a #4cu feeder on a 100A breaker.
I just bump it up to #2 and put it on a 125 .

steve

 

[charlie b]

While 100a may not overheat the conductors, 100a would certainly warm them up more than 85a would.

If both were installed under the same conditions, yes. But I can set up circumstances in which 85 amps in one wire would overheat, even damage a conductor. I can also set up circumstances in which the same size and type of wire would be barely warmed up at all, with a current of 100 amps. It is all about "conditions of use."

 

[winnie]

I can set up circumstances in which 85 amps in one wire would overheat, even damage a conductor. I can also set up circumstances in which the same size and type of wire would be barely warmed up at all, with a current of 100 amps. It is all about "conditions of use."

What I don't see is a significant difference in the "conditions of use" between the main power feeders of a residence and any other large conductors in a residence, or the service entrance conductors for a residence and the service entrance conductors for a small store, or a feeder in a residence.

The requirements for 310.15(B)(6) talk about conductor type, but say nothing about ambient conditions, thermal insulation, or conductor installation practise. 310.15(B)(6) may be applied if the ambient temperature is 40C. It may be applied if the conductors are buried in the soil, or buried in fiberglass, or in pipe on the wall in the sun. Nothing about 310.15(B)(6) suggests that conditions of use would provide any better cooling than those assumed for 310.16

Thus I maintain that the difference between a main power feeder for a residence and any other general load is not the "conditions of use" of the conductors, but instead the electrical load placed on those conductors.

-Jon