Chapter 9 note 7

[yired29]

An existing 2? Rigid Metal Conduit contains ten #8 THHN copper conductors, how many #8 XHHW copper conductors can be added to the conduit?

(a) 23
(b) 22
(c) 27
(d) none of these


Chapter 9, Table 5
#8 THHN = 0.0366 x 10 = 0.366
#8 XHHW = 0.0437
Chapter 9, Table 4 Article 344
2? RMC @ 40% = 1.363
1.363 ? 0.366 = 0.997
0.997 / 0.0437 = 22.8

The answer to this question in the book says 23 more conductors.
I believe the answer is 22.
I don't think you can use the .8 rule from note 7 unless all conductors are the same size and insulation. Maybe I'm misunderstanding this.

What do you think?

 

[Dennis Alwon]

I would have to agree with you. Why they need to be the same size, I don't know but note & seems clear. 22 conductors would be my answer

 

[glene77is]

Why they need to be the same size, I don't know ...

IMO,
Note (7) and surrounding are dealing with total dimensions of conductors, for the purpose of handling the problem of "jamming".

IMO,
There is no simplistic answer to this problem of "jamming" ,
(1) as immediate thermal conditions will affect 'jamming',
(2) the introduction of 'slickem' will affect 'jamming'.
Both are somewhat unpredictable since the problem
is 'jamming' as the cable trundles and flops around a bend.

NEC states "...based on common conditions of
proper cabling and alignment of conductors where the
length of the pull and the number of bends are within reasonable
limits. It should be recognized that, for certain conditions,
a larger size conduit or a lesser conduit fill should
be considered."

IMO,
Use of the same conductors for this "common" application table is a method of simplified presentation.

I think that sometimes the code is too ticky,
and sometimes it almost falls flat.

Just an opinion.

 

[infinity]

I would have to agree with you. Why they need to be the same size, I don't know but note & seems clear. 22 conductors would be my answer

I agree too. There not the same size so clearly you cannot round up.