7 Tips for Pulling Cable
The following suggestions – though not all-inclusive – will give greater assurance of success for pulling cable.
Be sure there is adequate clearance between conduit and cable. Clearance refers to the distance between the uppermost cable in the conduit and the inner top of the conduit. Clearance should be 1/4 inch at minimum and up to one inch for large cable installations or installations involving numerous bends. How to calculate conduit fill.
Jamming is the wedging of three cables lying side by side in a conduit. This usually occurs when cables are being pulled around bends or when cables twist.
The jam ratio is calculated by slightly modifying the ratio D/d. A value of 1.05D is used for the inner diameter of the conduit because bending a cylinder creates an oval cross-section in the bend.
- If 1.05D/d is larger than 3.0, jamming is impossible.
- If 1.05D/d is between 2.8 and 3.0, serious jamming is probable.
- If 1.05D/d is less than 2.5, jamming is impossible but clearance should be checked.
Because there are manufacturing tolerances on cable, the actual overall diameter should be measured prior to computing the jam ratio.
Use adequate lubrication of the proper type to reduce friction in conduit and duct pulls. Grease and oil type lubricants should not be used on nonmetallic sheathed cables. There are a number of commercially available wire pulling compounds (many of which are UL Listed) that are suitable for use with polymer jacketed cables. They usually consist of soap, talc, mica or the like, and are designed to have no deleterious effect on the cable. Graphite and other electrically conducting lubricants should not be used on nonshielded cables rated 2 kV and above. These materials can lead to tracking of the cable jacket.
Avoid sharp bending of the cable at the first pulley in overhead installations by locating the payoff reel far enough away from the first pulley that the lead-in angle is kept relatively flat.
After installation, check that end seals are still intact and have not been damaged to the point where water could enter. Apply plastic or rubber tape to help protect against invisible damage if the cable will be subjected to immersion or rain. This is particularly important if there will be a delay of some time between the pulling operation and splicing and terminating.
When installing interlocked armor cables in cable tray, use a sufficient number of rollers to prevent the cable from dragging on the tray, which might result in excessive tension. Avoid sharp bends in the cable by using a conveyor sheave with multiple small rollers at all 45- and 90-degree bends.
Keep adequate tension on the messenger in aerial cable installations to prevent sharp bends at pulleys. Do not release the tension on the messenger until it is secured to poles on both ends.