Cable Pulling Tension Formulas

Cable pulling tension formulas: tension on a straight run from weight, length and friction, the increase around a bend, and the limit from the conductor area.

The formulas

w is the weight of the cable or cables in kg per metre, g = 9.81 m/s², L the length in metres and μ the coefficient of friction, about 0.3 to 0.5 for cable in a conduit with lubricant, and higher without it. For a bend the tension multiplies, so a long pull has the bends as near the end as possible. A vertical or sloping run adds or subtracts the weight along the slope. σ is the allowed pulling stress of the conductor in N/mm², n the number of conductors and A the area of one conductor in mm². Take σ and the maximum pulling tension from the cable maker, and note that a pulling grip on the sheath has a lower limit.

Worked example: straight run

A cable of 6 kg/m pulled 80 m with μ = 0.4.

1. T = 0.4 × 6 × 9.81 × 80 = 1,884 N

Worked example: then a 90° bend

1. θ = 90° = π / 2 = 1.571 rad, so the factor is e^(0.4 × 1.571) = 1.874
2. T2 = 1,884 × 1.874 = 3,531 N

Worked example: limit on a three-core cable

Three conductors of 95 mm² and an allowed stress of 50 N/mm².

1. Tmax = 50 × 3 × 95 = 14,250 N, so the pull above is within the limit

Tension on a straight run

Frequently asked questions

Why does a bend raise the tension so much?

The cable presses on the conduit as it turns, which adds friction. The effect is exponential, and each bend multiplies the tension by the same factor.

How can I reduce the pulling tension?

Use cable lubricant, rollers, a shorter pull, a larger conduit, fewer bends, or pull from the end that gives the lower tension.

What is the difference between the pulling eye and the grip?

A pulling eye on the conductors uses the strength of the metal. A grip on the outer sheath has a lower limit and is used for short pulls.

Is the maximum tension the same for every cable?

No. It depends on the conductor material, the construction and the method of pulling, so use the value from the cable data sheet.

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