Paralleling Conductors

Status
Not open for further replies.
charlie b said:
...What I do object to is the attempt to apply the same reasoning to the use of a pair of 1/0 for a 350 amp feeder to an individual dwelling unit, by doubling the "ampacity" value in Table 310.15(B)(6).
Click to expand...
It appears that CMP6 does not agree.
6-74- (Article 310, Notes to Ampacity Tables of 0 to 2000 Volts, Note 3): Reject
SUBMITTER: R.W. Worthing, Auberry, CA
RECOMMENDATION: Add new text at the beginning of the second sentence to read:
Application of this Note shall not be permitted for conductors connected in parallel.
SUBSTANTIATION: At a recent Section meeting of the 1AEI, the question of the applicability of Note 3 to parallel conductors was raised. No consensus was established. Actions, by CMP 6, whether in acceptance or rejection of this Proposal, will establish, for the record,the position of the Panel.
PANEL ACTION: Reject.
PANEL STATEMENT: Conductors 1/0 and larger are permitted to be paralleled by Section 310-4. This would apply to Note 3.
NUMBER OF PANEL MEMBERS ELIGIBLE TO VOTE: 10
VOTE ON PANEL ACTION:
AFFIRMATIVE: 10
Click to expand...
 
Before I say why I disagree that they disagree, or why it is not relevant that they might disagree, can you tell me where these alleged "notes to ampacity tables" might be? I cannot figure out what the proposal meant, or what their rejection meant, without seeing the original text. But I don't see any notes to Table 310.16.
 
charlie b said:
Before I say why I disagree that they disagree, or why it is not relevant that they might disagree, can you tell me where these alleged "notes to ampacity tables" might be? I cannot figure out what the proposal meant, or what their rejection meant, without seeing the original text. But I don't see any notes to Table 310.16.
Click to expand...
You have to go back to the 93 code, this was a proposal for the 96 code. What was note 3 is now 310.15(B)(7).
 
A couple cycles ago, I submitted a proposal to explicitly prohibit the use of that table with parallelled conductors. They did not accept my proposal, and made a similarly vague statement to the effect that 310 does allow paralleling conductors. That, however, does not settle the issue. Here is what I mean:
  • We have one table that tells us that the "ampacity" of a 1/0 is 150 amps.
  • We have a rule that allows us to parallel conductors.
  • We infer that the ampacity of two parallel 1/0 is 300 amps.
  • We have another table that tells us we can use a 1/0 for a 175 amp service (or main feeder) to an individual dwelling unit.
  • That table does not tell us that the ampacity of a 1/0, under that specific set of "conditions of use," has been declared to be 175 amps.
  • Can we, or can we not, use two parallel 1/0 for a 350 amp service, under that same specific set of "conditions of use"?

I think we are left with no answer to that question.
 
Based on the proposal and panel comment, it is clear to me that the panel intends to let us use two sets of 2/0 copper for a 400 amp dwelling unit service. I don't see how you can read the proposal any other way.

Look at this way. The table says I can use 400 kcmil copper for a 400 amp dwelling unit service. The 75?C ampacity of 400 is 335 amps. The 75?C ampacity of two sets of 2/0 copper is 350 amps.
Assuming they are not in the same raceway, what logic would say that a cable with an ampacity of 335 amps is better than two cables with a combined ampacity of 350 amps for a 400 amp dwelling unit service.
 
don_resqcapt19 said:
Based on the proposal and panel comment, it is clear to me that the panel intends to let us use two sets of 2/0 copper for a 400 amp dwelling unit service. I don't see how you can read the proposal any other way.
Click to expand...

I do see the proposal your way.


I also think they are wrong as the words in the NEC do not support it in my opinion.
 
don_resqcapt19 said:
Assuming they are not in the same raceway, what logic would say that a cable with an ampacity of 335 amps is better than two cables with a combined ampacity of 350 amps for a 400 amp dwelling unit service.
Click to expand...

That is an excellent point assuming they are parallel conductors. I don't believe you can use 2/0 where you use two 200 amp panels. In this case 3/0 is needed, IMO because the diversity no longer exists.
 
iwire said:
I do see the proposal your way.

I also think they are wrong as the words in the NEC do not support it in my opinion.
Click to expand...

They could be, but in my opinion, the only thing that holds more weight in a code interpretation issue than a panel statement is a formal interpretation.
 
The problem with the panel statement is that it does not give a complete answer. They say that we can parallel conductors, and we already knew that. But what happens when we parallel conductors is never spelled out anywhere (not even in the ampacity tables), and this is one place where it should be spelled out. All I would ask is that the word "ampacity" be inserted somewhere in the table. As an alternative, a statement should be added to declare that two parallel conductors of the tabulated size can be used for a service of twice the tabulated rating.
 
charlie b said:
The problem with the panel statement is that it does not give a complete answer. They say that we can parallel conductors, and we already knew that. But what happens when we parallel conductors is never spelled out anywhere (not even in the ampacity tables), and this is one place where it should be spelled out. All I would ask is that the word "ampacity" be inserted somewhere in the table. As an alternative, a statement should be added to declare that two parallel conductors of the tabulated size can be used for a service of twice the tabulated rating.
Click to expand...

Whether we are talking about the resi table or commercial, AFAICT the whole paralleling ampacity idea is implicit and not explicit. Common sense yes, but I cannot find a definitive statement allowing or disallowing the math.
 
I usually caution that titles are not part of the Code itself; however, occasionally they are a reasonable basis for interpretations. As such, with the overall Section 310.15 titled Ampacities for Conductors Rated 0?2000 Volts, it would seem that Section 310.15(B)(7) is a description of the basic conductor ampacity for a very specific ?condition of use?.

The problem is there is no statement directly or by reasonable subordination to another Section in either the Section 310.15(B)(7) text or table for the ambient that was used to determine the Table ampacities. Therefore, there is no basis for corrections or adjustments based on different ambient conditions. Other adjustments not based on ambient conditions, such as more than three current carrying conductors in a raceway or cable, would still apply.

In my opinion, each member of a properly installed parallel conductor has "full" ampacity subject to the appropriate adjustments and corrections.
 
rbalex said:
I usually caution that titles are not part of the Code itself; however, occasionally they are a reasonable basis for interpretations. As such, with the overall Section 310.15 titled Ampacities for Conductors Rated 0?2000 Volts, it would seem that Section 310.15(B)(7) is a description of the basic conductor ampacity for a very specific ?condition of use?.

The problem is there is no statement directly or by reasonable subordination to another Section in either the Section 310.15(B)(7) text or table for the ambient that was used to determine the Table ampacities. Therefore, there is no basis for corrections or adjustments based on different ambient conditions. Other adjustments not based on ambient conditions, such as more than three current carrying conductors in a raceway or cable, would still apply.

In my opinion, each member of a properly installed parallel conductor has "full" ampacity subject to the appropriate adjustments and corrections.
Click to expand...

I would agree or disagree if I was smart enough to understand what you just wrote, but I am not.

Got a dummy version for us grunts?
 
jumper said:
I would agree or disagree if I was smart enough to understand what you just wrote, but I am not.

Got a dummy version for us grunts?
Click to expand...
Hopefully this is better:
  • Based on the overall Section 310.15 Title, Subsection 310.15(B)(7) and its Table are defining conductor ampacity for a very specific ?condition of use.?
  • The ?condition of use? described in Subsection 310.15(B)(7) and its Table is incomplete because a ?base? ambient temperature is not stated, and it can?t be reasonably inferred. Therefore, proper ampacity adjustments/corrections based on various ambient conditions can?t be made.
  • However, if proper ampacities could be determined, then the combined ampacity of parallel conductors is simply the ampacity of a properly ?adjusted/corrected? conductor times the number of parallel conductors.
 
Status
Not open for further replies.
Top