More on Conductor Fill

[JohnE]

One of my guys came in this morning and asked if I would "do the math" for conductor fill or use the tables in annex c. I told him that I'd do the math since I'm used to it and annex C wasn't always there, ( and also the preface to annex C stating that it is not part of this NFPA document), but why do you ask. He said that in a code class the previous evening, they had an example where doing the math for #14 THW resulted in 19.7 conductors being allowed in 1" SCH 80 PVC, but annex C shows only 13. I told him that can't be right. When I double checked, I found it to be true.

I ran 16 examples using #14 and #8 THW and THHN for 1" and 2" sch 80 pvc and emt. I found that every example using thhn correlated with annex C, but every example using THW was drastically different.

For example: #14 THW = .0139 sq. in. from table 5. 1" sch 80 = .275 sq. in @40% fill. So, .275/ .0139 = 19.7, so 19 conductors, no? Annex C (p. 698) shows 13 conductors.

I'd think I were doing something wrong, except his instructor came out with same values.

Maybe we are both doing something wrong.

Any Thoughts?

 

[charlie b]

Re: More on Conductor Fill

I checked your example, and agree with your results. I did not check the other THW examples, but I have no doubt you are right.

To quote Ben Franklin:

The fact is singular. You require the reason? I do not know it.

 

[JohnE]

Re: More on Conductor Fill

Charlie, thank you.

 

[charlie b]

Re: More on Conductor Fill

At the request of the originator, I am moving this thread to the Engineering / Calculations topic area.