Have you ever pulled in conductors with a truck? You shouldn’t!
Why can some wire pulls be so hard to pull in first place? The factors that go into whether or not it will be an easy or a hard pull, and the reasons why, often go unnoticed. In this column we will explore some of the basics of pipe fill and conductor installation. While the 2023 National Electric Code is beginning to be ordered and mailed out, this month I felt like going back to some little understood basics when it comes to conductors installed in raceways. Hopefully you will find this information useful for your next “wire pull.”
A few general Code requirements for raceway installations can be detailed in Article 300 and are as follows:
In Chapter 3 of the NEC, ***.22 requires you to calculate the pipe fill of raceways in accordance with Table 1, Chapter 9. This is where you will find a table that looks similar to this.
|One conductor or cable||53%|
|2 conductors or cables||31%|
This Table is the maximum fill permitted for pulling conductors in and out of completed raceways as required in 300.18. If you are using a raceway like rigid metal conduit, type RMC, as a short section, to provide protection against physical damage of a single conductor or cable, then the 53% fill is not required. If it fits? It ships. You can use it. See Note 2 to Chapter 9.
Pipe fill percentages are set up to help prevent jamming of conductors while conductors are installed. There is a lot of science involved to the real application of conditions for each conductor installation which can be intimidating. I will try to simplify it for you without using hyperbolic trigonometry or other complex formulas. Yes, they are available for you if you want to take a deep dive into that. The following outline is the short list and is not in any particular order.
Make sure your pipe fill calculations are accurate and comply with Table 1, Chapter 9 at a minimum. Larger conduits are permitted and may be an option for longer runs when friction is evaluated to be extremely high. Leaving room for future pulls is optional but providing additional empty conduits should be the first choice. Consideration should be given for conductor types. For example, compact stranding, compressed stranding, solid or stranded, aluminum or copper and even copper-clad, aluminum. Although aluminum conductors must be sized larger than copper to carry the same current, they are lighter and less expensive. The actual manufacturers dimensions of the conductors are permitted to be used for pipe fill calculations instead of the table values. Otherwise, use Chapter 9, Tables 4 and 5 to calculate the proper pipe fill in compliance with Table 1.
Depending on the installation, you may be required to “prove” the conduit after installation. This would include pulling a specifically sized mandrel through the raceway before the conductors or cables are installed. It is a fact that 90 percent of failures to medium-voltage cables are the result of improper cable installation.
Using a lubricant or a wire-pulling compound during the installation is common during many wire pulls. Caution should be applied with using the proper type of lube. Use listed wire-pulling compounds. Don’t use detergents and soaps or petroleum-based greases and oils. In section 517.160, it requires not using wire-pulling compounds that increase the dielectric constant.
Specifically, branch circuits that are derived from an isolation panel as required for some healthcare facilities. Caution should be exercised with wire-pulling compounds around energized equipment and parts. Cleaning up and housekeeping should be exercised before energizing the exposed parts.
Proper feeding of cables while pulling cables is a good example of “push me, pull me.” Proper timing and rhyme should be established for each pull. Pulling slack off the reels and feeding it into the raceway can be very strenuous and requires good communication with the pulling side of the task. This is more common for hand pulls between two or more people than that of a long pull of large cables, but teamwork is a key factor for both. A pre-planning meeting should be part of any long pull requiring the use of mechanical equipment to install the cables or conductors. Everyone should know they have a stop work authority during the pull. Everyone should be aware of the hazards and only qualified persons should be leading the task. The team feeding the conductors should maintain a symmetrical pattern while the wires enter the conduit. Overlapping and twisting should be avoided.
Pulling or feeding conductors in energized equipment should also be avoided at all times, unless an energized electrical work permit (EEWP) in accordance with NFPA 70E, 130.2 is properly filled out and implemented. Otherwise, put the equipment into an electrically safe working condition (ESWC) before feeding or pulling. See NFPA 70E, 120.5 for more information
Three conductors installed in round raceways can jam if the ratio of the inside diameter of the raceway to the conductor outside diameter is between 2.8 through 3.2. Jamming is more prominent at bends where the conduit could be slightly oval. As you approach the ratio of 3 jamming could occur even in the straight portions of the run. To calculate this ratio, take the nominal inside diameter in inches (Ch. 9, Tbl. 4) and divide that by the conductor diameter in inches. (Ch. 9, Tbl.5)
NUMBER OF BENDS
The total number of bends should not exceed that of 360 degrees. This can be any combination of offsets, saddles, 45 degree and 90 degree bends. Friction is added to the pull and can be calculated to determine the force required to install conductors and cables safely and effectively. Stresses on pulled cables can be high. Too much force on cables or conductors can damage the conductor or the insulation and should be avoided. For longer pulls, reducing the number of bends may be beneficial if practical.
SIDEWALL BEARING PRESSURE
Table 2 in Chapter 9 of the NEC contain the minimum radii of conduit and tubing. The larger the radius of a raceway the less sidewall pressure. Though friction is still present, sidewall pressure can add forces which compress the insulation while installed. Medium-voltage cables are especially vulnerable because the shielding can be distorted and can be the cause of a fault in the cable shortly after the cable is energized. However, any damage to any conductor insulation should be avoided.
The pulling head or the point where the conductors are attached to the pulling equipment is very important. For hand pulls we often just bend the conductors around the fish tape and pull it.
However, think about the thickness of the head and the possibility of jamming. If the total number of conductors is doubled because you just bent the wires over the head of the fish tape, you will have more difficulty pulling that run. Tapered heads that do not tend to be much larger than the total conductor fill is a better option. For larger pulls, baskets, Kellum grips, and special cable pulling heads are better options.
Factory applied pulling heads are also available. Pulling conductors safely and effectively into the raceway should be the primary goal as opposed to pulling conductors out after a failed pull. This rarely results in a usable conductor afterwards.
As mentioned above, with the number of bends and the use of lubricants, friction is not a positive aspect to your wire pull. Different types of raceway materials can add or reduce friction to the pull.
Nonmetallic raceways can have ropes burn into sweeps during the pull and may require metallic sweeps or concrete encasement of the sweeps prior to pulling. The type of rope used is also important. Avoid twisted ropes. Use appropriately sized, double braided polyester ropes with factory spliced pulling eyes and approved pulling heads. Avoid splices and knots in the ropes. Knowing how long the pull is can help with determining the amount of friction that could be encountered. Utilize jet line with footages marked on the line to help pre-measure the run.
Pulling conductors into raceways can be designed and installed to make it easier or harder. Note 1 of Chapter 9, has an (I.N.) Information Note that suggests installing larger conduits or calculating a lesser fill to reduce stresses encountered from pulling wire. To reduce hazards from the wire pulling process, planning, and Code compliant rules should be implemented. Soft tissue injury can occur even from a simple two-person wire pull in the smallest raceways and the smallest conductors.
Many other factors go into a successful and safe cable pull and each pull is different and presents unique circumstances and situations that must be recognized and addressed. Cabletray is a unique raceway that may require special tools, skills, and knowledge to properly pull conductors and cables within a completed cabletray installations. Some basic knowledge and skills must be also used to help keep the mistakes minimal and the installation costs low.
May I suggest The American Electricians Handbook. In Division Nine this book offers many useful and proven tips for wire pulling and raceway installation. Make a plan, seek expertise, and use tools designed for the job and appropriate for the task. Plan on an (ORM) Operational Risk Management meeting to help identify any risks and hazards that could be encountered. Look for ways to eliminate the identified hazards. Establish a good communication system for the task. Give all team members the stop work authority.
Few of us remember the successful pulls where all went well, and no one got hurt. But all of us can recall a story where the pull failed, and the failure was costly, dangerous, and avoidable.
Hopefully, armed with this information, your next wire pull will be much easier, profitable, and ultimately safer for everyone.