Q: The TIA/EIA-568A standard calls for no more than 25 ft/lbs of pulling tension on Category 5 cable. I find this concept ambiguous and, as the training director for our company, difficult to teach to my crews. The dictionary defines "foot-pound" as a unit of work equal to the force of one pound acting through a distance of one foot in the direction of the force. As cable is pulled into a conduit, tray or any other raceway, the mass being moved increases, thereby causing the force to increase to cover the same distance of one foot. I have also seen documents that use "25 lbs/foot."
How did the Telecommunications Industry Association arrive at this 25 ft/lb number and how can I calculate it?
Santa Fe Springs, CA
A: The pulling tension requirement in the TIA/EIA-568A standard is specified in "10.6.3.2 Cabling Practices . . . . The maximum pulling tension for 4-pair, 24 AWG horizontal UTP cables should not exceed 110 N (25 lbf) to avoid stretching the conductors during installation." The requirement is 110 Newtons of pulling force in metric units or 25 lbs pulling force in imperial units. The unit "lbf" stands for pounds force.
I contacted Paul Kish (Nordx/Cdt, St. Laurent, QC), chairman of the TR41.8.1 working group responsible for writing TIA/EIA-568A, and asked how they arrived at "25 lbf." According to Kish, the value of 25 lbf was derived using the tensile strength characteristics of copper. He explained that the rule of thumb is that copper conductors can withstand about 10,000 psi (pounds-per-square-inch) without significant deformation.
The cross-sectional area for 4 pairs of 24 AWG copper is:
A = 8*(0.020)*(0.020)*3.14159/4 = 0.0025 square inches.
By multiplying the tensile strength of 10,000 psi by the cross-sectional area of copper, you get a pulling tension of 25 lbf.
"I believe 25 lbs is a safe value to work with," says Kish. "Pulling tensions up to 40 lbs should not be detrimental to the cable; however, this still needs to be validated."
He recalls witnessing an experiment on a sample of 4-pair cable that was pulled, applying different amounts of tension to the cable. In this experiment, he noted that the cable itself broke between 90 and 110 lbf, significant stretching of the copper was evident at 70 lbf, and very little change occurred for pulling tensions less than 50 lbf.
Prompted by considerable discussion about whether the maximum pulling force of 25 lbs is too conservative for a 4-pair, 24 AWG copper cable, the TR41.8.1 task group on cabling installation practices plans to request that the experiment be repeated using a larger sampling of cables while monitoring all of the cable transmission parameters.