On an example from Southwire Company, it is shown how to evaluate a cable pulling project in Cableizer. Special focus is on the useage of imperial units, weight correction factor, vertical bends, and vertical shafts.
Posted 2021-05-31
Categories: User guides, Validation
The following calculation example from Southwire is an extract from their Power Cable Installation Guide with copyright from 2005. The calculation example is found on pages 23ff in the section "Typical calculation for cables in conduit". This example validates our cable pulling module and has been added to our test cases, ensuring that cable pulling calculations won't unexpectedly and unnoticeably change.
In the Cable tab of the cable pulling module the cable is selected, which in the example is a THHN single-conductor 4/0 AWG copper cable. In the Cableizer cable editor, the conductor area $A_c$ can directly be selected as 4/0 AWG by changing to the AWG/kcmil input choices (instead of Metric). Typical values of conductor diameters $d_c$ for this cross section can conveniently be selected for AWG sizes up to 2000 kcmil from UL 1581 and beyond from ASTM B8-11 standards.
While the conductor area can be selected from AWG/kcmil values, all other inputs/outputs are currently only supported in metric units in Cableizer and need to be converted to/from imperial units.
The following table shows the corresponding inputs regarding the cable selection for the Southwire example, which covers the case where three identical unbound cables are pulled together. Permissible pull force and maximum sidewall pressure have been manually set to the same values as in the Southwire installation guide.
| Parameter | Symbol | Southwire | Cableizer |
|---|---|---|---|
| Cable weight | $m_{tot}$ | 0.711 lbs/ft | 1.06 kg/m |
| External diameter of cable | $D_e$ | 0.626 inches | 15.9 mm |
| Permissible pull force | $F_{ppc}$ | 5079 pounds | 2259 daN |
| Maximum sidewall pressure | $F_{rad}$ | 500 pounds/ft | 729.7 daN/m |
In the Conduit tab of the cable pulling module the conduit (EMT trade size 2 inch) is defined. The parameters include the inner diameter of duct, the conduit ratio, the conduit fill in percent, the conduit clearance in percent and the conduit jamming ratio.
Cableizer does then verify if the installation is feasible and calculates the weight correction factor and sidewall pressure factor as shown in the following table:
| Parameter | Symbol | Southwire | Cableizer |
|---|---|---|---|
| Inner diameter of duct | $Di_d$ | 2.067 inches | 52.5 mm |
| Conduit ratio | $CR_{pull}$ | 3.3 (cradled) | 3.3 (cradled) |
| Conduit fill | $CF_{pull}$ | 27.5 % | 27.5 % |
| Conduit clearance | $CC_{pull}$ | 1.424 inches | 36.2 mm |
| Conduit jamming ratio | $CJ_{pull}$ | 3.3 | 3.47 |
| Weight correction factor | $f_{wc}$ | 1.25 | 1.25 |
| Sidewall pressure factor | $f_{rad}$ | n.a. | 0.58 |
The calculations in Cableizer do well agree with the calculations in the Southwire installation guide. The conduit jamming ratio slightly differs due to the fact that Southwire is using the inner duct diameter while Cableizer is adding a 5% margin to that value. However, in both cases the risk for jamming is estimated as very small. Southwire does not explicitly state a sidewall pressure factor, but implicitly uses the same value as Cableizer in their calculations. Consequently, both sidewall pressure and weight correction factor have been applied to the calculation.
In the Routing tab of the cable pulling module the example route is composed of sections and bends. Bends have an angle $\phi_{arc}$ of 90° and a radius $r_{arc}$ of 914.4 mm (36-inch sweep elbows with an inside radius of 2.91 feet). The friction coefficient $\mu_{dyn}$ is 0.4 all along the route. The following images show the Southwire conduit layout and the Cableizer cable route (in 3D):
The results show a good agreement with the Southwire installation guide figures, both regarding the pulling force $F_{pull}$ and the sidewall bearing pressure $F_{rad}$. In bend 3, both forces exceed the respective limits:
| Type | Radial forces [daN] | Pulling forces [daN/m] | ||
|---|---|---|---|---|
| Southwire | Cableizer | Southwire | Cableizer | |
| Section 1 | 426 pounds (189.5) | 190.4 | ||
| Bend 1 | 187 pounds/ft (272.9) | 275.3 | 934 pounds (415.5) | 418.0 |
| Section 2 | 1041 pounds (463.1) | 465.7 | ||
| Bend 2 | 457 pounds/ft (666.9) | 712.3 | 2282 pounds (1015.1) | 1081.4 |
| Section 3 | 2335 pounds (1038.7) | 1105.2 | ||
| Bend 3 | 1026 pounds/ft (1497.3) | 1518.3 | 5119 pounds (2277.0) | 2305.2 |
| Section 4 | 4906 pounds (2182.3) | 2210.1 | ||
Part of the small differences in the above table can be explained by the fact that Southwire is using approximate equations for both horizontal bends and the vertical concabe downbend (bend 3). The following two figures show the pulling forces and radial forces along the cable route for the forward pulling direction. The red lines show the applicable force limits.
In Cableizer, you can also simulate a pull in backward direction. In this case the maximum pulling force is reduced to 1443.7 daN and does not exceed the force limit anymore. The radial force is also considerably reduced to a maximum value of 825.5 daN/m, which is only slightly above the limit.
The forces are reduced despite the fact that the cable has to be pulled up the vertical shaft at the start of the cable route! But having the bends close to the start of the route instead of close to the end of the route significantly reduces the overall pulling forces.
