Jam Ratio calculation for 4 cables in a conduit

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neerdesh

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Ann Arbor, MI
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Electrical Engineer
I have a 4 cables running in a 6" conduit (ID = 6.03 inches)
3 x 750 Kcmil AC - OD: 2.111 inches
1 x 2/0 Gnd - 0.402 inches

How do I calculate the jam ratio in this case?
I believe the Jam Ratio formula is = 1.05 * (ID conduit)/(Average OD of cables) = 1.05* [(6.03)/((2.111*3 + 0.402)/4)] = 3.76

Is this accurate? Do I consider the gnd cable in this calculation?
 
Just curious why you need to calculate the jam ratio? Aren't the NEC 40% fill limit requirements enough?
 
I thought that was only an issue where there are 3 conductors in the raceway.
Chapter 9, Table 1 ....
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.
Click to expand...
 
We pass the NEC 40% fill requirements. However if I only consider the 3 x 750 cables, the Jam ratio then becomes 1.05*(6.03/2.11) = 3.00
Informational Note 2 in NEC Chapter 9 states "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"

This jamming scenario is likely to take place at the bends, where the raceway may be slightly oval. This can happen even if 40% fill is observed. One conductor could slip between the other two conductors causing a jam.

I want to know the correct formula for calculating the jam ratio of 4 cables.
 
neerdesh said:
We pass the NEC 40% fill requirements. However if I only consider the 3 x 750 cables, the Jam ratio then becomes 1.05*(6.03/2.11) = 3.00
Informational Note 2 in NEC Chapter 9 states "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"

This jamming scenario is likely to take place at the bends, where the raceway may be slightly oval. This can happen even if 40% fill is observed. One conductor could slip between the other two conductors causing a jam.

I want to know the correct formula for calculating the jam ratio of 4 cables.
Click to expand...
I have never seen one for anything other than 3 conductors or cables.
 
How does the physics of jamming work? If 3 identical wires in a conduit are apt to jam (bad jam ratio), does adding a 4th smaller wire help, hinder, or make no difference?

Cheers, Wayne
 
wwhitney said:
How does the physics of jamming work? If 3 identical wires in a conduit are apt to jam (bad jam ratio), does adding a 4th smaller wire help, hinder, or make no difference?

Cheers, Wayne
Click to expand...
IMHO adding an additional conductor helps prevent the jamming. Its a matter of geometry. draw a circle to represent the conduit. Then draw an infinity symbol representing the conductors inside. Its the physical space limitations that can cause the jamming. :)
 
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