power cable pulling calcs

[KVX10]

1. In regards to pulling tension calcs, I am trying to determine the jam ratio pulling 4 cables through 6" conduit.

Cables used are:
3-750kcmil diameter:1.73 per cable
1-4/0kcmil diameter:.98 per cable
I am using the following formula:
Ratio= 3D/n1d1+n2d2
Where D=inner diameter of conduit
n1=number of cables
d1=diamter of cable
therefore: Jam Ratio=(3*6.065)/((3*1.73)+(1*0.98))
=2.94
In seeing this, the probability of jamming is high. Is this calc correct?

2. In regards to the weight correction factor for 4-cables, I am trying to determine a weight correction factor. The weight of the by using the following formula:

W=1+{2*[d/(D-d)]^2}
Where D=inner diamter of conduit
d=diamter of individual conductor
W=Weight correction factor

My question about this is: if I'm using the four cables mentioned above, do I use this formula separately for both sizes of cable and then add them together?
Thanks in advance.

 

[alehman]

For 3 same size cables,
jam ratio = conduit ID / cable OD
Jam ratio between 2.5 and 3.2 is a moderate or higher risk

Generally you can ignore the ground when calculating jam ratios. Several cable manufacturers have excellent papers on this topic.

 

[7anoter4]

I think alehman is right. You don't have to take the neutral cable into consideration.
So the jam ratio will be 6.036/1.73=3.49 [a very small probability].
Also w [weight correction] will be 1.4 [for 4 cables]

Extract from EPRI NP-7485s Power Plant Practices to Ensure Cable Operability
Jamming
Jamming can occur when three single cables of similar size are pulled into a conduit when
the summation of the cable diameters approximately equal the diameter of the conduit. Jamming results when the center cable is forced between the two outside cables while being pulled around a bend and the jackets of the outer cables are forced against the conduit causing a braking action.
There is a specific range of cable diameters (d) to conduit diameters (D) for which a jamming
concern exists. This range of D/d is 2.8 to 3.1. Above this range, the cables will lay in parallel
around a bend without jamming against the sides of the conduit. Below this value, the center
cable will stay in a triangular or cradled configuration with respect to the outer cables and not be able to be forced between them.
Jamming is undesirable since cable insulation may be damaged and extra effort will be
required to eliminate the jam. The jam ratio should be checked for the particular cable
configuration before proceeding with the cable pull.
For weight correction factor see:

http://www.southwire.com/Southwire/StaticFiles/Text/envirotectPowerCableInstallManual.pdf