This usually occurs when you have three or more individual conductors lying side by side in a single plane. When sizing your raceway system, you should always consider the possibility of cables jamming or wedging.

Use the following equation to find the weight correction factor: What does this mean to you? It means you should assume the conductors would sit in the cradle position (unless you are pulling triplexed individual conductors from a single reel), because this will yield a higher and more conservative pulling tension calculation. When you have three single conductors of equal diameter and weight (which is the most common scenario), you can expect a higher weight factor for the cradle position than the triangular position. The equations in Table 1 for determining weight correction factor for specific installations are based on the inside diameter of the raceway and the outside diameter of the conductor. Weight correction factor is important because when you pull two or more conductors in a raceway, the sum of the forces developed between the conductors and the raceway is always greater than the sum of the individual conductor weights. Now that you know the cable positioning, it's necessary to determine how the conductors' weight will affect the pull. In this case, this ratio is:īecause this ratio results in a number greater than 3.0, the individual conductors will sit in a cradle configuration in the conduit. Use the ratio of the conduit's inside diameter (D) to the cable's outside diameter (d) to find how the single conductors will sit in the conduit. To determine how the conductors will sit in the conduit, refer to Table 4 for the inside diameter (D) of the 5-in. If you pull quadruplexed individual conductors from a single reel, the multiconductor cable will also sit in this position. 2): This position occurs when you pull four individual conductors from four separate reels, and their D/d ratio is less than 3.0.

This position is the least favorable because it yields the worst-case scenario of drag during the pull. 2): This position may occur when you pull three individual conductors from three separate reels, and their D/d ratio is between 2.5 and 3.0. If you pull individual triplexed conductors from a single reel, they also will sit in this position. 2): This occurs when you pull three individual conductors from three separate reels, and their D/d ratio is less than 2.5. By using the ratio of the raceway's inside diameter (D) to the conductor's outside diameter (d), you can find what geometric position you can expect to see.Īlthough the position of one cable is easy to predict (see option A in Fig. In addition, the positioning affects the weight factor. 2)) has a unique effect on the amount of frictional force, or drag, the conductors endure during the pull. It may not seem important, but each cable's geometric position ( Fig. This conduit size will allow you to slip under the allowable percentage of conductor fill by 10%. In this situation, Table 4 (Rigid Metal Conduit) in Chapter 9 of the NEC calls for a 5-in. You can calculate the total area of the three MV cables using the following equation: Table 1 in Chapter 9 of the NEC, states the allowable percentage of conductor fill is 40%. With these requirements in hand, check with the cable manufacturer and you'll find the MV cable you need has an outer diameter (d) of 1.60 in. After referring to NEC Table 310.73, size the feeder at 500kcmil. The client has also stipulated you use 90☌, single-conductor, grounded neutral, MV cables with tape shielding XLPE insulation and an overall PVC jacket. The client has required all feeders on the site to be pull-in-pull-out type in galvanized rigid steel (GRS) conduit. Suppose you're involved in a design/build project for a paper mill and your client needs a 400A, 15kV feeder to run as shown in Fig. Now, let's see how these factors apply in a sample pulling tension calculation. Mathematical proficiency aside, you do have to know the following installation parameters:

However, with the help of a few calculations and a working knowledge of arithmetic, you can prevent problems in recently energized feeders by calculating the maximum allowable pulling tension for any installation - and you don't even need to know calculus. Even if your crew has taken all the necessary precautions in paying out cable and handling the reels, a cable pull can still go sour if you damage the cable's outer insulation during the process.

Cable pulls