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#127726 - 10/16/01 07:10 AM Why oh why?  
Redsy  Offline
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
Why is only 31% conduit fill allowed for 2 wires, as opposed to 40% for 3?
I assume that it is precautionary, in the event that only 2 wires of a 3phase system are used, and the magnetic fields aren't cancelling. However, for all the commentary in the Handbook, this is not mentioned.
Anyone else.


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#127727 - 10/16/01 02:44 PM Re: Why oh why?  
Bill Addiss  Offline
Member
Joined: Oct 2000
Posts: 3,878
NY, USA
Redsy,

My guess is that it would be because 2 wires together that took up 40% fill may be too tight to pull through a conduit because they would be like >> oo or 8 << and together might approach the inside conduit diameter with the 40% fill limit. It would be a tight one for sure! Straight pulls might not be so bad, but 360 degrees would be another thing.

Does that make sense?

Bill


[This message has been edited by Bill Addiss (edited 10-16-2001).]


#127728 - 10/16/01 08:49 PM Re: Why oh why?  
sparky  Offline
Member
Joined: Oct 2000
Posts: 5,306
Redsy,
I see FPN #2 70-561, to back up the angle Bill's offered. Other than that the #'s do make me wonder if any specific mathematical logic was followed.
CMP9,,,
35!
no..29!
34 ?...
30
31.., an' it's break time!
[Linked Image]


#127729 - 10/16/01 10:06 PM Re: Why oh why?  
Redsy  Offline
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
Guys,
Ahh..
The jam ratio! Lots of thought went into this idea. I think it refers to 3 wires, though. Not two.


#127730 - 10/16/01 11:02 PM Re: Why oh why?  
Bill Addiss  Offline
Member
Joined: Oct 2000
Posts: 3,878
NY, USA
Redsy,

That's my best shot...

[Linked Image]
Bill


#127731 - 10/17/01 07:12 AM Re: Why oh why?  
Redsy  Offline
Member
Joined: Mar 2001
Posts: 2,056
Bucks County PA
Bill,

It's not anything that really concerns me. It's just something I have wondered about this in the past, and now, with this forum I thought I'd bounce it around.
Thanks.


#127732 - 10/17/01 07:22 AM Re: Why oh why?  
sparky  Offline
Member
Joined: Oct 2000
Posts: 5,306
Redsy,
We are, as electricians, always complying to code, and left wondering about the rationale. The handbook only goes so far, the only other resource would be a formal interpetation.
The thing is, some codes, and specifics , are probably so old there's no one left to explain it's orgin.


#127733 - 10/17/01 12:47 PM Re: Why oh why?  
resqcapt19  Offline
Member
Joined: Nov 2000
Posts: 2,148
IL
sparky,
A formal interpretation would be of no help in finding out the "why" as the NFPA requires that requests for a formal interpretation be submitted in a manner that will allow the NFPA to answer the question with either a "yes" or a "no". The only way that I know of would be to find the TCR (now known as ROP) and the TCD (now known as the ROC) for the proposal that became the rule.
Don(resqcapt19)


Don(resqcapt19)

#127734 - 10/17/01 07:55 PM Re: Why oh why?  
sparky  Offline
Member
Joined: Oct 2000
Posts: 5,306
Don,
Sounds like some serious archive searching, at least on this issue. It makes me wonder if anyone actually pursues code history to this extent.



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