Precautions During Cable Pulling
All guides should be in the form of large diameter, smooth-surfaced, free-turning sheaves or rollers to prevent damage to the cable jacket when guiding the cable from the reel to the duct mouth or trench. Guide tubes or chutes must be smooth, burr-free working surfaces with the largest possible bend radii and be securely anchored if used. Mounting the cable reel in sturdy jacks, leveling the reel shaft and lubricating the reel arbor holes with grease will lead to low cable tension. If breaking the reel needs to happen, it should only be done to prevent reel over-run when the pull is slowed or stopped, or on steep downhill runs where cable weight is enough to overcome cable-duct friction.
This information also applies for rack or tray installations. The following points should be noted when making such pulls:
1. Cable support rollers should be spaced close enough so that the cable’s normal sag, even when under tension, will not result in tray dragging.
2. The cable rollers should be contoured so that the cable will not ride off the end of the roller or be “pinched” into a sheave contour diameter that is smaller than that of the cable.
3. When rollers or sheaves are used to guide the cable through the bends, it is important that enough of them be used to guide the cable in a smooth curve of the desired radius from tangent point to tangent point. If this is not done, the cable may be “kinked” around the radius of each roller.
The cable may be paid off the reel and laid into the trench as the reel is moved along the length of the trench in cases of direct-burial installations. When this happens, the cable is laid on a bed of soil or sand.
If the cable needs to be pulled through the trench, the best way to do so is to support the cable on temporary rollers so that the cable does not drag over the soil or sand bed. If one does not have rollers handy, sacks filled with very fine sand or other fine powdery material may be used as an alternative to support the cable and keep it from dragging on the trench bed during the pulling process.
To reduce pulling tensions and damage on cables, lubricants may be used in conduit. When using cable or pulling lubricants, one should avoid compounds that may contain oils or greases since they may damage the cable jackets. Also, pulling lubricants that contain micro-spheres or micro-balls should be avoided for medium-voltage cable installations. These kinds of lubricants are meant to be used on low-tension pulls that are not representative of power cable pulls.
Most commercially available pulling lubricants can be used with little worry for compatibility. But, one does need to keep in mind that some pulling lubricants react poorly to some cable jacket compounds, which may lead to ruining the cable jacket. It is best to avoid damage to the cable jacket by consulting the cable manufacturer regarding lubricant compatibility with specific jacket compounds.
Pulling Eyes & Grips
Pulling eyes attached to the cable conductor(s) are used for large, heavy cable, or for cables where the pulls are very long or contain numerous bends. To use the pulling eye, one must fasten it directly to the conductor(s) on the end of the cable by soldering the copper conductors into a socket-type eye, or by mechanically compressing the aluminum conductors into an aluminum eye. Then, a tape seal or heat-shrinkable tube is placed over the eye-cable joint to provide a reliable weather-tight seal for the cable during pulling. For armored cables, the armor needs to be properly secured to the eye to insure the reliability of the cable during pulling. Generally, pulling eyes tend to be installed at the factory; however, they can also be installed in the field.
Often times, woven wire pulling grips, generally called “baskets” are used to pull armored cables; they are well suited for pulling smaller size voltage cables, or where the pulls are fairly short. Special measures need to be taken if using “baskets” on Interlock Armored cables to avoid damage to the cable or problems in making the pull. The puling grip may tend to stretch the armor if the grip is not properly secured to the cable. The following method for preparing the cable and attaching the grip is advised:
1. Select the grip size that fits the cable diameter or armor best. Determine the length of the gripping portion of the grip.
2. Find two points on the end of the cable. The first is 75% of the grip length from the end; the second is 100% of the grip length from the end.
3. Get rid of the sheath/armor, and also the outer jacket if there, to the first mark. Do not damage the core of the armor. If need be, secure the armor at the cut point with friction tape before cutting.
4. Apply four, 3-inch long, tight wrappings of friction tape. Place this tape on a) the end of the core, b) on the core to the edge of the sheath/armor, c) on the jacket, or sheath/armor to the edge of same, and d) on jacket, or sheath/armor where the last 3 inches of grip will be.
5. If cable will be exposed to moisture during the pull, seal the cut ends of the conductors with sealing mastic and vinyl tape, or heat-shrinkable cap(s).
6. Place the grip on the cable and secure it tightly by “milking” it from the cable end towards the end of the grip.
7. Clamp the back end of the grip to secure it to the cable with a steel hose clamp, such as the “Band-It” types, or a tough steel wire that is firmly applied.
8. Over the clamp apply a tape wrapping to smooth it and prevent drag during the pull.
CAUTION: The ends of cable pulled this way will not be entirely safe from water. If this is an issue, properly applied pulling eyes should be used.
*NOTE: The force applied by pulling a grip may damage or disrupt the underlying cable, so it is best to cut off the section immediately below the grip as well as the three feet of cable behind the grip before fixing together.
Maximum Pulling Tensions
To prevent damage to the cable, pulling tensions for installing electrical cables should be maintained as low as possible. This may be done through proper use of size ducts or conduits, by avoiding long pulls, and avoiding runs that may contain sharp bends or severe changes in elevation.
The following maximum allowable pulling tension must not be passed when pulling cable by the method indicated.
A. Pulling Eye
| Tmax = Maximum pulling tension (lb)
CA = Conductor Area (cmil)
N = Number of conductors being pulled
CTC = Conductor Tension Constant
|For CTC with aluminum compression eyes or bolts use:
0.011 — Copper conductor
0.008 — Aluminum Stranded conductor
0.006 — Aluminum Solid conductor
|For CTC with filled eyes or bolts use:
0.013 — Copper conductor
0.011 — Aluminum Stranded conductor
0.008 — Aluminum Solid conductor
Pulling Eye Maximum Pulling Tension (lbs)