Medium-voltage (MV) cable pulling is not just slopping on lubrication and applying brute force. An MV cable is much like a piece of sophisticated switchgear, with various components interacting together to safely handle voltage and current. Damage any of these components during the pull, and the likelihood of a cable failure turns to an unfortunate reality.
So, how should you approach MV cable pulling? Our suggestion involves using a step-by-step approach. This means first evaluating the type and configuration of raceway you intend to use (existing or new), and seeing how the MV cables will be affected. This first part of the approach is like the "design" portion of the design-build construction technique.
Then, look at the anticipated pulling tension, which leads you to the best type and style of pulling grip and the best pulling location. This second part is like the "build" portion.
Know what the cables look like in the raceway
Since the majority of MV cable installations involve single-conductor cables in an electrical raceway (rigid metallic or nonmetallic), you should begin your approach here. Depending on their number, these cables will occupy the raceway in unique geometric ways, as shown in Fig. 1.
This is important because the particular configuration has a direct effect on how much frictional force, or drag, you place on the cables during installation. To find out the configuration, use the ratio of the inside diameter of the raceway (D) to the outside diameter of an individual cable (d). We call this the D/d ratio.
Obviously, one single conductor cable rests in the raceway as shown in Fig. 1 (option A). A triangular configuration, as shown in Fig. 1 (option B), occurs when you pull three individual conductors from three separate reels, and their D/d ratio is less than 2.5. If the ratio is 2.5 to 3.0, the conductors could again assume the triangular configuration, or may position themselves into the cradle configuration, as shown in Fig. 1 (option C). If the ratio is over 3.0, the three conductors are very likely to lay in this same configuration. The diamond configuration, as shown in Fig. 1 (option D), occurs when you pull four individual conductors in parallel from separate reels, and their ratio is less than 3.0. If you're pulling triplexed or quadruplexed single-conductor assemblies from one reel, the configurations will always be triangular for the former and diamond for the latter.
Add a fudge factor for cable weight
Each of the above configurations interacts uniquely with the new or existing raceway. You can also use the specific configuration's D/d ratio per the specific configuration to determine a fudge factor called weight correction factor.
When you install two or more single- conductor cables in a raceway, the sum of the forces exerted between the cables and the raceway is always greater than the sum of the individual cable weights. So, you need to apply a weight correction factor, which effectively increases the weight of cables. This helps in calculating a more accurate required pulling tension.
To find the weight correction factor for the intended installation, use the applicable equation shown in Table 1.
In the usual case of three single conductor cables of equal diameters and weights in a raceway, the weight correction factor is typically higher for the cradled configuration than for the triangular configuration. For example, a 40% raceway fill in a cradled configuration gives you a weight correction factor of 1.441, as compared to a value of 1.222 for the triangular case.
So, unless your cable is made up of triplexed single conductors, you should assume the cradled case, since it will yield higher values in calculated pulling tensions. This obviously results in a more conservative design.
Don't crush the cable's sidewalls
When you pull a cable through a bend in a raceway system or around a sheave, pressure develops between the cable and the bend or sheave. This pressure, known as sidewall bearing pressure, or SWBP, dramatically affects your power distribution feeder design. This directly relates to the radius of the bend, pulling tension on the cable, and weight of the cable. (The cable's weight is relatively small in comparison to the pulling tension; as such, we omit it in the SWBP calculation.)
This performance parameter is usually expressed in terms of the tension out of the bend (in lbs) divided by the bend radius (in ft). Obviously, the calculated result is really a unit of force per unit length.
You can use the equations in Table 2 to calculate SWBP values of specific cable/raceway configurations. These equations highlight the specific conductor subjected to the maximum crushing force for each of the configurations noted in Fig. 1. In the cradle configuration, this is the center conductor; for the diamond, it's the bottom most conductor; and for the triangular, it's the bottom two conductors.
Looking at these equations, you see as the bend radius is increased, the amount of SWBP to which the cable is subjected decreases.
Table 3 lists recommended SWBP values for various cable types and constructions. Based on these SWBP limits, you increase the size of a bend so you can apply a greater pulling tension out of that same bend. For example, suppose you calculate the tension in pulling three single conductor 600V XLPE cables around a bend is 3600 lbs. Then, the minimum bend radius is 3600 lbs divided by 1200 lbs/ft (from Table 2), or 3 ft.
Don't jam the cables
You have to consider the potential for jamming or wedging cables during the pull installation when you're sizing a new raceway or verifying if an existing raceway is of sufficient size. Jamming usually happens when three or more cables lay side by side in a single plane. As you pull cables through a bend, the curvature of the bend tends to squeeze the cables together, causing a jam that can result in insulation damage.
You normally won't have a jamming problem if you're pulling one or two single conductors, or a single multiconductor cable with an overall jacket. Also, nonjacketed multiconductor cables made up of triplexed or quadruplexed single conductors won't jam.
To find out if you may have a jamming problem, use the ratio of the raceway inside diameter (D) to the individual conductor's outside diameter (d). One special note: Since bends are not round but oval in section view, you must increase the diameter measurements by 5% (by a factor of 1.05).
Here are some jamming guidelines for you to follow:
- If 1.05 times D/d is less than 2.5, jamming is not possible.
- If 1.05 times D/d is less than 3.0 but greater than 2.8, jamming is very probable.
- If 1.05 times D/d is greater than 3.0, jamming is not possible.
Another special note: Avoid jam ratios of 2.8 to 3.2 for extruded dielectric cables.
Don't overbend the cables
A cable's minimum bending radius is directly related to its construction and directly affects cable insulation.
Two values are used in defining a minimum bending radius: One describes the radius of a cable bend made by training a cable in its place after its installation; the other defines a bend a cable can safely negotiate without damage during the pulling process.
As you would expect, the pulling radius is always greater than the training radius for the same cable because you expose the cable to greater physical stress during a pull.
Pulling radius. MV cables with overlapping tape shielding systems are particularly susceptible to damage if you don't use properly sized bends. If the radius is too small, the overlapping layer of tape shielding separates as the cable progresses through the bend. As the cable straightens out, the tape shielding buckles as it tries to come back together. In fact, it may actually cut into the cable's insulation. Since this cable construction usually features an overall jacket, you probably won't notice the damage, and the cable will have a reduced service life.
A similar concern applies to cable constructions having a spiral interlocking metallic (aluminum or galvanized steel) jacket. These jackets tend to pull apart if you force the cable to conform to an improperly sized radius, such as a too-small sheave.
For 600V and MV power cables (through 35kV) with metallic shielding or armor, use the following minimum pulling radii:
- Multiconductor interlocked armor cables: seven times overall cable diameter, except as noted for shielded conductors. Refer to Table 3 (on page 32) where a minimum pulling radius greater than seven times "d" is required.
- Flat tape and wire armored cables: 12 times the overall cable diameter. * Metallic tape shielded cables: 12 times the overall cable diameter.
- Concentric wire strand shielded cables: Refer to Table 3 except where a minimum pulling radius greater than seven times the cable diameter is required.
Training radius. The Insulated Cable Engineers Association (ICEA) has set recommended values of minimum training radii to bend cables for permanent training. These are shown in Table 3 (600V through 35kV) and Table 4 (over 35kV). Remember these limits do not apply to duct or conduit bends, sheaves, or other curved surfaces around which you pull power cables under tension. In all cases, the minimum radii specified here refer to the surface of the cable in contact with the bend and not the axis of the cable.
Give cables enough headroom
You must make sure there's enough clearance (C) between the uppermost cable and the inside top of the raceway to ensure a safe and easy pull. For a straight pull, the clearance can be as small as 1/4 in. and still be adequate. For more complex pulls, however, you'll need between 1/2 in. and 1 in.
Use the equations noted in Table 5, based on worst-case configurations, to find out the clearance distance for a given cable/raceway configuration. To compensate for variations in cable and raceway diameters, and the oval shape of raceway sections at bends, the nominal diameters are increased by 5% (by a factor of 1.05).
Don't pull too hard on the conductor
Maximum allowable pulling tension depends on three limiting factors; it cannot exceed the smaller of any of the following:
- Maximum tension on the cable's copper or aluminum conductor;
- Maximum tension on the pulling device (pulling eye or cable grip); or
- Maximum cable SWBP.
The maximum tension you can place on a cable's conductor is 0.008 times the circular mil (cmil) area of the conductor for copper and 0.006 for aluminum. Base these values on the tensile or yield strength of the respective metals and a 2.4 safety factor. In other words, you divide the respective material yield strength by 2.4 to find out the resulting maximum tension you can apply directly to the conductor material. This gives you a built-in safety cushion.
Use the equations noted in the SIDEBAR: How Much Tension Is Too Much For A Conductor Material? to calculate the maximum conductor material pulling tension.
Don't kill the pulling eye or cable grip
If you're pulling large kcmil conductors, their allowable maximum pulling tension is in the thousands of lbs. This large amount of tension can exceed the capability of your pulling equipment. Consider the capability of the pulling device along with the conductor's performance characteristics.
Obviously, the pulling device manufacturer's recommendations are important. As such, you should follow recommended maximum pulling tensions on cable conductors with pulling eyes, as shown in Table 6.
Conventional pulling eyes. These devices, whether compression or epoxy filled, have a maximum capability of 14,000 lbs. If you need additional capacity, you can use soldered pulling eyes, which allow a maximum of 16,000 lbs.
Basket grips. Here again, you should follow the manufacturer's recommendations. Table 7 lists the recommended maximum pulling tensions on cable grips with copper and aluminum single and multiconductor cables.
To calculate how much tension you can apply on a cable grip when pulling a lead sheath cable, see the SIDEBAR: Using A Cable Grip On Lead Sheathed Cable.
SIDEBAR: How Much Tension Is Too Much For A Conductor Material?
Yes, there is a limit as to how much tension you can place directly on aluminum or copper - as defined by the following equations:
Copper: Tm = 0.008 x n x cm
Aluminum: Tm = 0.006 x n x cm
"Tm" is the maximum allowable tension applied to the cable conductor, "n" is the number of cables, and "cm" is the individual conductor area in circular mils.
You'll have to adjust these equations to reflect the number of cables taking the tension. For example, if you're pulling three single-conductor cables (either cradle or triangular configured), n=2, since two of the cables may sustain the total pulling tension. If you're pulling more than three cables of equal size, the maximum tension you can directly apply to the conductor material is 60% of the total value.
SIDEBAR: Using a Cable Grip on Lead Sheathed Cable
You can find the maximum pulling tension on lead-sheathed cable when using a cable grip by using the following equation: TsubM = (4712) x (t) x (d - t), where t is the sheath thickness (inches) and d is the overall diameter of the cable (inches).