Conduit fill percent that allows wires to bind when pulling

[JDBrown]

What conduit fill percent increases the risk of binding when attempting to pull wires?

I remember reading years ago that you have to be careful when up-sizing conduit for long runs because it's possible to increase the size just enough that it allows the wires to shift when being pulled, causing them to bind up mid-pull. It's been 10 or 15 years since I last dug into this topic, and of course I can't find my old notes (what a surprise).

From what I remember, there was a certain range of conduit fill (less than the maximum fill in the Code) that would allow the wires to shift and bind while being pulled. The advice was to make sure the conduit fill was either below or above that range, while still being less than the max allowed fill percentage.

I don't recall any of this being written in the NEC; as I recall it was a design guideline I came across when learning how to calculate pulling tension. Searching Google all morning hasn't yielded anything useful.

Is this ringing a bell for anybody? Any resources you can point me to?

 

[infinity]

Take a look at Chapter 9, Table 1, Notes 1 and 2 for some info about jam ratio.

 

[JDBrown]

Take a look at Chapter 9, Table 1, Notes 1 and 2 for some info about jam ratio.

Well I feel silly, it was in the NEC book after all! Thanks very much!

 

[infinity]

Well I feel silly, it was in the NEC book after all! Thanks very much!

Happy to help.

 

[LarryFine]

This fill calculator includes a jam-ratio warning for three conductors:

http://www.electrician2.com/rveltrain05/rf_calculator.html

 

[electrofelon]

Jamming issues can be avoided by using plexed conductors. I have ordered in plexed conductors, when it wasn't a standard size I could get in a URD. One example was a 800 foot pull of Pratt URD (250-250-3/0) in 2" PVC. The southwire calculator lists that as 32% fill and a significant jam probability.