310.4 Conductors in Parallel

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tcook99

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Can table 310.15(B)(6) be used to size parallel conductors for a single family dwelling
ie; parallel 2/0 for 400 amps.
I think you must use table 310.16, parallel 3/0 for 400 amps. Since table 315.15(B)(6) sizes are for 120/240-volt, 3 wire single phase and parallel conductors shall comply with table 310.15(B)(2)(a) which refers to table 310.16 to ajust for more than 3 current-carrying conductors.
Could use some help to present my case. Thanks
 
tcook99 said:
Could use some help to present my case. Thanks
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I agree with you. This has been discussed on this forum before, and not all participants will agree with you. Do a search, and you may find a couple threads.

To support your case, all that I think you need to do is to take note that that table does not give ampacity values. So you can't take two of a conductor, and add ampacities.

 
I also agree with you. T 310.15(B)(6) says nothing about parallel conductors. As far as I can see you must either 3/0 paralleled or 400KCM.
 
Proposal 6-74 in the 95ROP was made to add the following to Note 3 to the ampacity tables. "Application of this Note shall not be permitted for conductors connected in parallel".
The substantiation stated that the purpose of the proposal was to get CMP 6 on record as the the use of parallel conductors for Note 3 applications. The substantiation went on to say; "Actions by CMP6, whether in acceptance or rejection of this proposal will establish for the record the position of the panel. Panel 6 rejected the Proposal with this statement: "Conductors 1/0 and larger are permitted to be paralleled by section 310-4. This would apply to Note 3." Note 3 is now 310.15(B)(6).
 
iwire said:
I think they should point to the words in Table 310.15(B)(6) to support that statement. :smile:
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It is my opinion that panel statements are only one step removed from a Formal Interpretation. In other words the only thing that holds more weight on a code question is a FI.
 
don_resqcapt19 said:
It is my opinion that panel statements are only one step removed from a Formal Interpretation. In other words the only thing that holds more weight on a code question is a FI.
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I agree to a point but if the words written in the adopted version of the NEC do not support the panels view then they are just another opinion.

As it stands today the 310.15(B)(6) basically says 'you can use these conductors for these size services as laid out in the table' There is nothing in that table that even remotely mentions or shows parallels.

In my opinion the panel statement is wrong.:smile:
 
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iwire said:
In my opinion the panel statement is wrong.:smile:
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I would go so far as to say the panel?s statement was ?wrong.? But I do think it was not relevant. All they said is that we are allowed to put cables in parallel. That does not tell us what we achieve by doing so.

We basically believe that if a conductor has a given ampacity, under the temperature and other conditions in which it is installed, then if we put two such conductors in parallel (and in separate parallel conduits), the ampacity doubles. I might note that nothing I have ever seen in the NEC actually tells us that, but we act upon that belief anyway.

But in the case of Table 310.15(B)(6), there are no ampacity values included in the tabulated values. So if we place two conductors in parallel, there is no ampacity value to double. I do not know the panel?s intent here, and I think I will write up another proposal to pin this question down.
 
iwire said:
As it stands today the 310.15(B)(6) basically says 'you can use these conductors for these size services as laid out in the table' There is nothing in that table that even remotely mentions or shows parallels.
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The code doesn't say we can't so we can. Telling me that I can do something in no way acts to prohibit me from doing something else.
 
don_resqcapt19 said:
The code doesn't say we can't so we can. Telling me that I can do something in no way acts to prohibit me from doing something else.
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Neither does telling you there is an exception to a rule grant you permission to create other exceptions to suit your own design wishes. This is more along those lines, though it doesn?t actually involve the word ?exception.?

Start with 240.4, where it tells us to protect conductors against overcurrent in accordance with the ampacities given in 310.15.

Next, 310.15(A)(1) says that ampacities are per the tables or per engineering judgment. Let?s leave that last option out, for now.

Table 310.16 says that a 2/0 copper with 75C insulation has an ampacity of 175.

Despite this ampacity value, Table 310.15(B)(6) allows us to use a 2/0 copper conductor as a feeder for a 200 amp load. It doesn?t tell us that the 2/0 has an ampacity of 200, when used in this application.

So how can we conclude that we could use a pair of parallel 2/0 conductors for a feeder with a 400 amp load? I have a conductor set with a combined ampacity of 350. I am told to protect it at 350 amps. What tells me it?s OK for a 400 amp feeder? Nothing tells me that. So I can?t use it without violating 240.4.
 
charlie b said:
Neither does telling you there is an exception to a rule grant you permission to create other exceptions to suit your own design wishes. This is more along those lines, though it doesn?t actually involve the word ?exception.?

Start with 240.4, where it tells us to protect conductors against overcurrent in accordance with the ampacities given in 310.15.

Next, 310.15(A)(1) says that ampacities are per the tables or per engineering judgment. Let?s leave that last option out, for now.

Table 310.16 says that a 2/0 copper with 75C insulation has an ampacity of 175.

Despite this ampacity value, Table 310.15(B)(6) allows us to use a 2/0 copper conductor as a feeder for a 200 amp load. It doesn?t tell us that the 2/0 has an ampacity of 200, when used in this application.

So how can we conclude that we could use a pair of parallel 2/0 conductors for a feeder with a 400 amp load? I have a conductor set with a combined ampacity of 350. I am told to protect it at 350 amps. What tells me it?s OK for a 400 amp feeder? Nothing tells me that. So I can?t use it without violating 240.4.
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Judge Chamberlain Haller: Mr. Gambini?
Vinny Gambini: Yes, sir?
Judge Chamberlain Haller: That is a lucid, intelligent, well thought-out objection.
Vinny Gambini: Thank you, sir.
Judge Chamberlain Haller: Overruled.
 
don_resqcapt19 said:
The code doesn't say we can't so we can. Telling me that I can do something in no way acts to prohibit me from doing something else.
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Don,
The text in 310.15(B)(6) says,

conductors as listed in Table 310.15(B)(6) shall be permitted
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I see nothing in that table about parallel sets.

To me 'as listed in the table' means just that, nothing else.
 
charlie b said:
What tells me it?s OK for a 400 amp feeder?
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What would you consider to be the correct way to size substituting two parallel condcutors for the same 400a feeder?
 
LarryFine said:
What would you consider to be the correct way to size substituting two parallel condcutors for the same 400a feeder?
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The correct way would be to select conductors that each have an ampacity, per Table 310.16, of 200 amps.
 
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