Jam Ratio

[Smart $]

No these are two different things. 40% is the conduit fill. And the jam-ratio is the ratio of inner-diameter of the conduit to the outer-diameter of cable.

EXAMPLE: what is the jamming ratio (or jam ratio) for three single conductor THHN #2AWG cables in a 1" rigid metal conduit?

SOLUTION: The inner diameter of 1" conduit = 1.063". The outer diameter of one (not three) single conductor #2AWG cable = 0.3840. so the jam ratio is 1.063/0.3840=2.76.

As jam ratio of 2.76 lies outside the preblemtic range (2.8-3.2) so the installation is acceptable.

Hmmm....

Isn't it 1.05 times the diameter ratio.

FWIW, I did some excel calculating and the jam ratio will never apply to 6 or more conductors because of the 40% fill maximum.

 

[don_resqcapt19]

Hmmm....

Isn't it 1.05 times the diameter ratio.

FWIW, I did some excel calculating and the jam ratio will never apply to 6 or more conductors because of the 40% fill maximum.

A lot of the information out there uses 1.05 factor, but the Informational note to Table 1 of Chapter 9 does not. I understand that the 1.05 factor is to take into account the "egg" shapping that can occur when you bend a 90.

Informational Note 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.