310.15(B)(6) - Parallel conductors

[pierre]

For a dwelling, can the conductor sizes in table 310.15(B)(6) be used for paralleling? Such as using parallel sets (6 conductors)of 4/0 copper THHN conductors in the same raceway, for a 400 amp service? Calculating for 6 conductors in a raceway, 250 amps X 2 = 500 amps X .8 = 400 amps.

Pierre

 

[earlydean]

Re: 310.15(B)(6) - Parallel conductors

The handbook says "For a single set....", and Table 310.15(B)(6) also ties the size of the service with the size of the conductor by the headings. I would say, "NO".

 

[pierre]

Re: 310.15(B)(6) - Parallel conductors

Earl
I am asking this question, because I have been asked the same question and was not sure how to answer. The handbook is a very good document, but not the formal interpretation of the NFPA. I see the handbook starts the sentence saying a single set, but I cannot see how they found that interpretation from the body of 310.15(B)(6). I am not too sure if I understand your second sentence.
As you can see, I used the amperage value of the table in .15(B)(6), and performed the conductor amperage adjustment.
Excluding the handbook, can you pick out in the NEC how they determined that it is one set?

I am not convinced that a parallel set of conductors cannot be used with the ampacity rating of the cable from table 310.15(B)(6), but... I do not know if it can .
Do you see any impending problems if one was to install this type of arrangement?

Two sets of 4/0 copper with the ampacity adjustment using table 310.16 would equal 368 amps. Table 310.15(B)(6) permits the use of 400 kcmil for a 400 amp service, and in table 310.16 400s equals 335 amps. I do not see an amperage value problem here, so what might the problem be.

Also the jurisdiction we work in permits two sets of 4/0 aluminum SE cable, one set to each of 200 amp panel setups for a 400 amp service. Using parallel 4/0 copper would seem to be a superior installation as far as amperage is concerned.

Anyones answer(s) would be appreciated, as they may open a 'door' I am not looking in.

Pierre

 

[charlie b]

Re: 310.15(B)(6) - Parallel conductors

I agree with Earl?s answer of ?no,? but I cannot offer a technical argument that clearly makes the case. I don?t know the history of the code development of the two tables in question. But I strongly suspect that they have different origins, and have different design bases. I think that the use of 310.15(B)(6) has to be restricted to the exact configuration described in the table and in the corresponding article. If you mix apples with oranges, you no longer have apples.

 

[Guest]

Re: 310.15(B)(6) - Parallel conductors

But, if you cut the apples in half you do get pears, right?

 

[tom baker]

Re: 310.15(B)(6) - Parallel conductors

the first edition of the 2002 is missing the conductor types allowed for Table 310.15(B)(6).
The table is not allowed to be used for all conductor types. the errata is available at the NFPA web site.

 

[tom ogryski]

Re: 310.15(B)(6) - Parallel conductors

I've got to vote no on paralleled conductors when using table 310.15(B)(6)

If you want to use the reduced sizes for a 400 amp service, I feel that you are limited to the two choices shown (400 kcmil for copper & 600 for aluminum). Anyone that wants to parallel conductors is going to have to use table 310.16.

If I've done my figures right, parallel 2/0 copper will have a heat output per foot of close to 190% of what the 500 kcmil will have, and it will be 150% higher than 400 kcmil.This is due to heat going up as the square of the current.

What this means as far as the operating temperature is concerned is more than I can figure out.

[ December 08, 2003, 03:41 PM: Message edited by: tom ogryski ]

 

[charlie b]

Re: 310.15(B)(6) - Parallel conductors

Originally posted by tom ogryski: If I've done my figures right, parallel 2/0 copper will have a heat output per foot of close to 190% of what the 500 kcmil will have, and it will be 150% higher than 400 kcmil. . . What this means as far as the operating temperature is concerned is more than I can figure out.

I concur with your 190% and 150% numbers. Clearly, the operating temperature would be higher. But it would be higher by less than the 190% or the 150%. The steady-state temperature is not a linear function of heat input.

 

[ty]

Re: 310.15(B)(6) - Parallel conductors

I would agree with the others if you were comming from a 400amp meter socket to a 400amp main breaker load center.
But, I doubt that to be the case. If you are going from a 400amp meter socket to (2) 200amp main breaker loadcenters, and this is in a dwelling, then I see no problem with running 2 sets of 4/0AL SEU according to the wording in 310.15(B)(6).

 

[pierre]

Re: 310.15(B)(6) - Parallel conductors

Tom and Charlie
I was not totally convinced that paralleling from the table values could not be done, until your last posts. Heat is one of our 'big' enemies and those heat values are enough to convince me.

Could you point me to a direction, or show me the calculations?
Thanks

Pierre

 

[charlie b]

Re: 310.15(B)(6) - Parallel conductors

During my navy career, I had to be able to calculate the temperature at various distances from a line that represents a source of heat. As you move away from the source, the temperature dropped off at different rates, depending on the materials through which the heat was flowing. In those days, the line was the centerline of a nuclear fuel rod. Nowadays, the line could be a conductor that is releasing heat in terms of I*2R. I do not remember how to do that type of calculation. But I do recall enough about the physical and mathematical processes of heat transfer to have made the earlier statement that the temperature would not rise in the same proportion as the rate of heat generation. If I needed to know anything about temperatures that result from heat sources, I would call for the services of a mechanical engineer.

 

[hurk27]

Re: 310.15(B)(6) - Parallel conductors

If you are running from a single 400 amp meter to two seprate 200 amp panels this would not meat the definition of paralleled conductor.

electrically joined at both ends to form a single conductor

This would be no more than two services taped off one service entrance cable 230.40 exception #2 being installed in same building.

I wouldn't think more than 2/0 would be required?

[ December 09, 2003, 09:36 PM: Message edited by: hurk27 ]