Most of us in the electrical industry have seen electricians install cables in conduits. The general idea is to install a pulling line, then fix the line to the cables to be installed, apply lubricant, and pull…. somehow.
However, the force required can be large, and can make things dangerous. It is not uncommon for electricians to be injured and equipment or installation materials to be damaged during this operation. The cable insulation can also be damaged, leading to catastrophic faults with potential for injury, electrocution or extensive downtime years later after construction is complete. I recall working for an electrical contractor as a summer job when I was 15 years old. The foreman had me driving a pickup truck that was attached to a pulling line at the mouth of a conduit high up on an exterior wall. Over and over I backed the truck up, then pulled forward using momentum to advance the cables a few feet further into the conduit. As I drove forward, the upward component of the tension became high enough to lighten the back end of the truck and cause the wheels to spin. The foreman told me to “keep doing that” and went back inside, presumably to check whether the guy feeding the cable bundle had lost an arm into the conduit yet. Despite my supervisor’s apparent lack of proper equipment and knowledge about cable installation, I was having great fun and was immensely proud to have been suddenly promoted from “tool gopher” to “company truck driver”. Life was good.
To my surprise, I later discovered that cable pulling is meant to be a more refined operation than that performed by the guys in the truck pull at the county fair. With the benefit of some education and experience I learned there are several important considerations rooted in math, physics and various published industry standards. How much tension will be required? What sidewall force can the conduit withstand? What pulling equipment should be used, or can we safely perform a manual pull? Will lubricant be required? What do the static & kinetic coefficients of friction tell us about stopping in the middle of a pull?
In short, pulling tension is related to the simple friction equations that we all learned in high school physics. However, the geometry makes them a bit more complicated when dealing with conduit runs that go up, down and around corners. A simplified form of the pulling equations is shown below.
We analyze each straight and curved section of the conduit in series, using the tension out of one section as the tension into the next section. The tension in the line at the outlet of the last section is our pulling tension, a very useful number which allows us to select a strong enough rope, and determine what pulling equipment is needed. Even the most burly electricians won’t be able to win a 1000 lb tug o war – so keep them off the workers comp dole and don’t let them try!
One observation from the equations is that elbows hurt. For those who aren’t math people, that exponential means that every bend in the conduit causes a dramatic increase in tension. You might have guessed that. But a less obvious product of this analysis is the determination of preferred pulling direction. As an example of this, consider the figure below. Should we pull from A to C or C to A?
The answer is not intuitive, and if you ask a room full of electrical workers this question you are likely to get a mixed response (there will be no shortage of colorful supporting theories however). Looking at our equations, one can see that the math we use to calculate pulling tension is a nonlinear operation. That is, we calculate A-B tension and B-C tension separately, and it matters a lot which one comes first. In this case it turns out that pulling from C to A requires less tension. Barring other circumstances such as accessibility, the pull in this direction will be less likely to injure electricians, and damage equipment or materials. One can construct a spreadsheet or use a program to make this calculation fast and simple for use during design, or on the jobsite. A little extra time tapping numbers on the iPad can save a lot of frustration, blood, and sweat during the pull!
Other cable pulling tips;
- Consider feeder reels & spools at the inlet to reduce incoming tension
- Use pulling equipment that allows smooth, continuous adjustable pulling speed. Restart tension after an unplanned stop can be much higher than pulling tension, possibly causing the pulling rope to break.
- Use a tension meter to predict an accident before it happens.
- Always have 2 way communication between inlet and outlet locations
- Use a lubricant and ensure that the manufacture indicates compatibility with your cable jacket type.
Need help with pulling calculations? Had an electrician injured on the job? Need some training on cable pulling best practices? Contact Kleinholz Inc. today for a free consultation.
Jim Fink, P.E.