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.

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).]

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!

Guys,
Ahh..
The jam ratio! Lots of thought went into this idea. I think it refers to 3 wires, though. Not two.

Redsy,

That's my best shot...


Bill

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.

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.

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,
Sounds like some serious archive searching, at least on this issue. It makes me wonder if anyone actually pursues code history to this extent.