Cable Jam calcs

[wfaulkn]

Can anyone tell me where I can find information on cable jamming? Specifically, in the 2005 NEC handbook (Chapter 9 pg. 1169) there is a discussion about probable cable jamming for 3 cables of equal OD. Does this hold true for 3 cables of unequal OD?

 

[mdshunk]

Chapter 9, Table 1, FPN #2 is a lot more clear.

FPN No. 2: When pulling three conductors or cables into
a raceway, if the ratio of the raceway (inside diameter) to
the conductor or cable (outside diameter) is between 2.8
and 3.2, jamming can occur. While jamming can occur
when pulling four or more conductors or cables into a raceway,
the probability is very low.

It's all about the ratio, and less with the actual cable diameters. The better conduit fill calculator filll programs will give you a "jam warning". I've had it pop up a few times on mine.

 

[beanland]

Jamming

Jamming

Okonite provides some good references. It all has to do with conductors going into a sweep and, rather than being triplexed, becoming flat. If the three conductors outside measurement is about the same is the conduit inside measurement, they can jam while trying to go around the corner. The harder you pull, the more jammed they can get.