Proceed With Caution When Pulling Cable, or Learn the Hard Way

There are basically two ways one can learn how to do something — the hard way and the easy way. I prefer the latter.

That brings to mind one such incident many years ago involving cable pulling. I was an apprentice installer for a communication systems company that was trying to save money by not buying the existing cabling, but instead pulling cable through the existing ductwork, in parallel to the existing cable.

After much sweat, frustration, pain and broken cable runs, this installation company failed to provide what they had promised. The project collapsed and the installation company went into bankruptcy, leaving yours truly with some bounced payroll checks.

The moral of this story is to pay attention to the information in this article to avoid a similar experience. Improper cable pulling can be very costly, while proper practices can make a big difference in your system installs.

Listen to What Cable Experts Say
According to cable industry experts such as Joe Mosey, technical support specialist for Belden-CDT, the top problem they see is damage from too much tension being used when pulling cable. Mosey says, “The recommended minimum bend radius and maximum allowable tension information is put on every box of cable because of its importance.”

I recently asked Tom Fredericks, vice president of sales and marketing for American Polywater Corp., to tell me what the top three mistakes made by technicians when pulling cable. American Polywater ( is a major manufacturer of cable-pulling lubricants. Below are Fredericks’ top three picks.

Top 3 Mistakes of Cable Pulling
The biggest mistake is not using any or enough cable-pulling lubricant. Technicians have to remember that exceeding tension limits can cause irreversible damage to cable, such as scuffed or torn jacket, stretched conductors or broken strands. Additionally, a low-pulling tension can ease stress on the arms and pulling equipment.

How much lubrication to use can be estimated by a basic formula: Quantity (gallons) = conduit length (feet) X conduit insidediameter(inches) X 0.0015. An example would be 350 feet in a 2-inch conduit would need a little more than 1 gallon of lubricant. Additional lubricant may be needed for old conduit, cabling through multiple bends or water, or a high conduit fill.

The second biggest mistake is not measuring or exceeding the maximum allowable tension or sidewall pressure when pulling in conduit. A suggestion for controlling maximum pulling tension is the use of breakaway swivels or a dynamometer, which measures pulling tension and is built into some cable pulling machines.

While maximum pulling tension is marked on wire boxes, it can be a complicated matter to calculate the pulling tension for runs due to the type of cable jacket, temperature, size of conduit, type of conduit, number of bends and radius of bends, sidewall pressure and conduit fill. Software, such as the Pull Planner™ 2000 from American Polywater Corp., can be used by planners to help calculate cable pulling demands ahead of time.

Trying to interpret the pulling tension specified by a cable manufacturer can be confusing. If you look at a box, it may tell you the tension is 110N. What is 110N? Time to take a moment and go back to high school physics.

The “N” stands for units of Newton. A Newton is a metric measure of force equal to 105 dynes or 4.45 pounds of force (lbf). To convert a Newton force (N) to a pound force, divide it by 4.45. For example, 110N would be a mere 25 lbf. If a technician is not careful, that can easily be exceeded with a single arm pull. Remember to always attach your pulling line or rope to the yarn-like “strength member” provided in fiber-optic cable.

Two cable types of particular concern are the areas of fiber-optic and category (Cat 5e/6/7) cable. For example, if cable is exposed to too much tension and the fiber becomes distorted, it will probably fail cross-talk certification tests (see picture in September issue).

The higher frequency transmission demands of data transmission over Cat 5e/6/7 cable requires that a consistent twist be maintained in this cable and any distortion could cause a failure of certification testing. One tip is to use a 25-lbf breakaway swivel or a tension regulation link made from 25-pound fish line.

The third biggest mistake is exceeding the recommended conduit fill and minimum bend radius restrictions. Make sure to follow fill guidelines set by the current National Electrical Code (NEC) and the specifications of engineers and cable manufacturers. A good rule of thumb for multiple cabling in conduit is not to exceed a 40-percent fill ratio.

Slipping and Sliding Through
To understand more about pulling cable through conduit, we have to understand the relationship of pulling tension to the friction of the cable against the conduit walls. This friction relationship is often referred to as the coefficient of friction or COF.

A formula for estimating pulling tension is T = W x L x N x COF, where T equals tension in pounds, W equals cable weight in pounds/feet, L equals length of run in feet, and N equals number of cables. The above formula is for a straight pull. As it might be expected, the COF increases considerably with bends in the conduit and the formula for that is more complex.

In fact, the pulling tension of a pull with just a few bends can be reduced by a factor of five to 10 by lowering the COF by 50 to 60 percent. This COF reduction can mean a 25- to 100-pound lubricated pull vs. a more than 500-pound unlubricated pull. Remember that NEC guidelines call for conduit bends between two pull points to not exceed 360º.

Several Choices for Lubricants
There are many types and flavors of cable lubricants. Some of the first lubricants were made of clay or soap. Many of the earlier versions would leave a residue when they dried that would often cement the cable to the bottom of the conduit.

Today’s popular lubricants come either as a wax base with Teflon flavors, or a water base (up to 90 percent) that allows for a very low COF, good coating and no residue after drying, thereby making for easy cable removal. The consistency is either a liquid form than can be pumped into underground conduits or a gel-type that will cling easily to vertical and horizontal interior runs and can be applied easily by hand.

One final warning about lubricant and cable type compatibility — some wax-based lubricants may create problems when used with polypropylene cable jackets like those in fiber-optic and communication cables. However, some manufacturers of wax-based lubricants claim their compounds have been modified to now work with polypropylene. Using the wrong lubricant with your cable my cause degradation of the cable jacket years down the road.

When working with fire-retardant cable runs, make sure you use a fire- retardant lubricant as any residue left in the conduit may create an additional fire hazard. Some of the wax-based compounds may cause the spreading of fire in a conduit.

Always check with cable manufacturers on lubrication specifications.


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