It’s a fact of life; most electricians will have to pull cable at some point during their careers.
Although it might seem like an easy task, wire pulling can be an arduous and delicate procedure that must be handled with care. Not taking precautions can create a dangerous situation that could result in short circuits, arcs, or even worse, a fire.
Before beginning a commercial or industrial job that involves pulling wire through conduit, take a few minutes to answer basic questions to decide how to do the job quickly, safely, and correctly.
Electrical conductors are put through a lot of stress when pulled through conduits but managing that stress well can make the job move along much more safely and smoothly.
The first type of stress is pulling tension, which is the force applied to a conductor as it is pulled through the conduit. If a worker tugs on the conductor with too much force, the electrical wiring can become damaged or break. When too little tension is applied to the wire, it may get jammed or, at the very least, make for a slow and inefficient pull.
If you’re running cable through conduit, don’t stop applying pressure to it after the pull begins. Constant stopping and starting can put the conductors under undue stress, especially in areas where they may become stuck, like at 90-degree angles.
Another force to keep in mind is sidewall friction/pressure. This is related to the friction exerted on conductors as they are pulled through bends and around corners. If too much friction is applied to the wire as it’s pulled through a bend, it can become damaged and expose the conductor. If this happens, the wire may spark, arc, or short out.
Tension and friction problems created during the pulling process can lead to conductor failures. Depending on where the work occurs, there may be even more stringent rules that dictate how the wiring is installed. For example, hospitals rely on having a steady, safe electrical supply to continue caring for patients, so they have stricter guidelines than those for an office building or commercial property.
No matter where the project occurs, NEVER exceed the maximum pulling tension suggested by the wire manufacturer.
Before jumping in and haphazardly pulling wire through hundreds of feet of winding conduit lines, it’s worth asking yourself a few questions to make the job simpler in the long run.
Shorter runs are naturally easier to pull because there isn’t as much distance involved to build up a dangerous friction coefficient. Even if they’re entirely straight lines, longer runs still make the job more difficult because the wire will continue to build friction throughout the run.
Conduits come in all shapes and sizes, so knowing what type is being used can help determine how difficult the pull may be. Electrical Metallic Tubing (EMT), for example, is typically straight. Other conduit options like Electrical Non-Metallic Tubing (ENT) or Flexible Metal Conduit (FMC) can bend, leading to more twists and turns for conductors to navigate through.
When the conduit bends, it applies added friction to the conductors. As conductors are pulled through the conduit, they will rub against the inside walls of the bend and potentially damage the wires. In some cases, a client may warn against continuous runs that exceed a certain number of degrees total.
One conductor is easier to pull through a conduit than three or four. As more conductors are pulled through the conduit, the greater the chances are of jamming or other damage occurring.
A single conductor doesn’t pose much of a problem, but issues may arise as you add more wires. For example, a three-conductor pull could start in a triangle pattern but collapse into a cradle pattern, increasing the friction coefficient and opening the door for potential damage.
The larger the bend, the more tension that needs to be applied. More tension means more pressure applied to the conductor as it makes its way through.
Conductors give off heat when they’re in use, so there needs to be enough room inside the conduit for the resulting heat to dissipate. If electrical contractors ignore conduit fill limits or assume they’re fine when they aren’t, the conductors could exceed their insulation heat ratings and melt.
According to NEC Code 300.17, the following are the limits allowed for conductors based on size and number:
It may not seem like an issue when considering how much space is left inside the conduit, but the rules also allow for easier installation and removal of the wires without damaging them.
No electrician wants to deal with the hassle of a jam preventing them from getting the job done. Luckily, there are a few simple ways to prevent jams, keep wires from getting damaged, and even save a little time in the process.
One of the best ways to avoid causing tension damage to conductors is by using a tensiometer. This helpful tool measures the amount of pressure applied to a wire to figure out whether too much tension is present.
If the wire is holding too much tension, it could break. It may also create too much friction in conduit bends, resulting in physical damage to insulation and potentially exposed conductors.
There is no shortage of cable-pulling lubricants on the market today. Each type of lube has a different application or situation where it works best, so keep the conditions in mind before selecting the right lube for the job.
When used inside conduits, lubricants reduce the friction applied to the wire throughout the pull and especially around bends. It can also help the job move faster as the overall amount of tension is lowered.
Instead of laboring to pull single conductors through a conduit several times, tray cable combines multiple conductors into one bundle, then wraps them in an added layer of insulation and an outer jacket. The layers of insulation make the conductors less susceptible to damage, preventing the risk of conductors becoming exposed and causing an issue.
Versatile and tough, tray cable is a multi-purpose type of bundled wire that can be used in a variety of conditions, including indoors and outdoors, thanks to its sunlight and corrosion resistance.
Tray cable can be pulled very quickly through conduit, saving everyone time and money.
This type of work doesn’t need to be laborious. There are plenty of tools and products available to make the job as easy as possible.
Before pulling wire or cable through a conduit, take a moment or two to consider the conditions and know where problems are likely to occur. Being careful and understanding the situation can go a long way toward protecting electrical wiring and running cable correctly, smoothly, and safely.
Subscribe to the Kris-Tech Blog