|
1 members (Scott35),
23
guests, and
17
robots. |
|
Key:
Admin,
Global Mod,
Mod
|
|
|
|
Joined: Mar 2001
Posts: 2,056
OP
Member
|
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.
|
|
|
|
|
Joined: Oct 2000
Posts: 4,132 Likes: 4
Member
|
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).]
Bill
|
|
|
|
|
Joined: Oct 2000
Posts: 5,393
Member
|
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]](https://www.electrical-contractor.net/ubb/biggrin.gif)
|
|
|
|
|
Joined: Mar 2001
Posts: 2,056
OP
Member
|
Guys,
Ahh..
The jam ratio! Lots of thought went into this idea. I think it refers to 3 wires, though. Not two.
|
|
|
|
|
Joined: Oct 2000
Posts: 4,132 Likes: 4
Member
|
Redsy, That's my best shot... ![[Linked Image]](https://www.electrical-contractor.net/ubb/confused.gif) Bill
Bill
|
|
|
|
|
Joined: Mar 2001
Posts: 2,056
OP
Member
|
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.
|
|
|
|
|
Joined: Oct 2000
Posts: 5,393
Member
|
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.
|
|
|
|
|
Joined: Nov 2000
Posts: 2,148
Member
|
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)
|
|
|
|
|
Joined: Oct 2000
Posts: 5,393
Member
|
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.
|
|
|
|
HCE727
Delaware County, PA, USA
Posts: 187
Joined: November 2005
|
|
|
|
