4 Strategies for a Successful Cable Pull on New Construction Projects

With construction projects breaking ground worldwide, electrical engineers can maximize both the safety and efficiency of cable pulls by taking a proactive approach to installation. By embracing the power of planning software, they can ensure a longer life for the cable and save time and labor hours in the office and the field.

“Planning is a place where engineers should take time to look at various ‘what if’ scenarios to make a good decision for the actual installation,” said Sheri Dahlke, vice president of research and development for Polywater®.

Through a proactive approach to cable pulling, engineers can plan a conduit system that limits the number of cable splices and/or underground structures, which will save the project both money and time. “They can also plan safe pulls that will not exceed the tension limits of the cable or equipment, which will create a safe work environment,” said Wendy Peterson, solutions engineer coordinator for Polywater.

Before the field crews even put their pipe in the ground, engineers can follow these four best practices during the planning phase.

Manage Cable Fill to Prevent Jamming and Installation Delays

The U.S. National Electrical Code (NEC) requires different cable fill maximums based on the number of conductors: 40% or less fill for three conductors, 31% for two conductors and 53% for one conductor.

Jamming can occur in designs with three or more cables when they have a combined diameter close to the same as the inner diameter of the duct. To safely install cables, there must be clearance, which allows space between the cable and duct wall.

“When the three cables are bundled in a triangular formation, they may fit into the same duct nicely,” Dahlke said. “Then, when the cables go into a cradled or side-by-side formation, they become squeezed or jammed, especially as they go around a bend.”

“In this situation, engineers can either increase or decrease the duct ID, which [either allows more space or] holds the cables in the triangular formation so they cannot open enough to cause jamming,” she said. “To determine the proper cable fill, engineers can use the Polywater Pull-Planner® software, which calculates the percentage [of conduit] fill based on the conduit and cable sizes. The program will assess the probability of jamming based on three cables, lined up side-by-side, which jam while being pulled around a bend or a sweep,” Peterson said.

Optimize Bend Radius to Control Tension and Sidewall Pressure.

Another way to increase efficiency on new construction projects is to predict the maximum length of pull distance while remaining under the cable manufacturer’s maximum tension and sidewall pressures. This best practice minimizes splices and underground infrastructure like vaults.

“Extra vaults are expensive,” said Pat Estenson, vice president and general manager of the communications division for Polywater. “Splices also cost money and are potential failure points.”

Most electrical engineers who design underground systems may correctly assume that the smaller or tighter the radius and the larger the degree of the bend, the higher the sidewall pressure. At the same time, they may predict that increasing the bend radius will have a significant effect on reducing tension, but that is not always the case.

“There is typically some tension reduction, but not as much as most seem to assume,” Estenson said. “Calculations in very large bends can show a large increase in tension due to the length of the bend but may not show much of an increase in sidewall pressure. This is because the cable may be heavy enough that it is riding on the bottom of the duct and not being pulled up against the side.”

Because the degree of the bend and the radius of an angle can increase sidewall pressure, changing or modeling bend radius in the planning software like Pull-Planner allows users to reduce sidewall pressure and tension.

Choose the Right Cable Pulling Lubricant to Reduce Friction.

“All cable lubricants perform differently based on their formulation and their interaction with cable and conduit material types. Engineers must specify lubricants based on their best performance with the lowest COF (coefficient of friction) and the highest friction reduction properties,” Peterson said.

“If it is not specified by the engineer, it is left up to the contractor, who may not have a full understanding of the difference in lubricant performance or cable jacket compatibility,” she said.

Engineers should trust the COF database in Pull-Planner to determine the lubricant with the best friction reduction given the cable jacket and conduit in the application.

“These numbers are the result of real lab tests,” Estenson said. “There may need to be a safety factor added to accommodate for field conditions, but the comparison between various lubricants remains consistent.”

Another factor is the silicone-enhanced lubricant’s ability to cling to cable through water and continue to reduce friction. Lubricants without silicone wash off the cable easier through water, which reduces or eliminates friction reduction.

“Most cable pulling projects are underground, and water is typically a reality,” Estenson added.

Engineers must also consider lubricant’s direct impact on COF, and in turn, tension. By selecting the right lubricant for the cable jacket and conduit type, they can significantly reduce COF. The Pull-Planner provides a Polywater lubricant tested COF database, which allows users to select various COF values and model cable pulls using different lubricant types.

Plan the Optimal Pull Direction to Minimize Risk and Cost.

Engineers must perform advanced modeling to determine which direction to pull the cable.

“If you can choose the direction of the installation, start with the bends to get the hard stuff out of the way first,” Dahlke said. “You want the lowest tension going into the bend because this is an exponential multiplier.”

Installers often base pull direction on logistics, such as which truck will be pulling the cable reel or pulling machine. They also consider whether it’s necessary to block lanes of traffic or if equipment will need to be positioned off-road.

Because installation equipment like powered reels, rollers, sheaves, and pushers can influence tension and sidewall pressure, the pull direction can be critical to keeping the tension below the determined max tension.

“The implications of pulling the wrong way are shorter cable pulls, more splices, more underground structure, more money, more time, and possible cable damage,” Peterson said.

By using planning software, engineers can model the components prior to installation, obtain the predicted pull tension, prepare for an efficient and safe cable pull in the field, and enhance confidence in the reliability and longevity of the system.

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