conduit fill

[gene senter]

in the note(s) to table 1 in the 2011 NEC. with one conductor a conduit is filled 53%,two conductors 31%, three conductors 40%. my question is how these percentages came about and why? thank you for any help.

 

[infinity]

The two notes following the table shed some light on the subject. Here's a good thread from a few months ago.

Welcome to the Forum.

http://forums.mikeholt.com/showthread.php?t=143966&highlight=jam+ratio

FPN No. 1: Table 1 is 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.
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.