Cable Pulling Tension Calculations

Don’t get caught-out with half-installed cables because you didn’t calculate the pulling tension

Introduction

Prior to installing cables it is recommended that the route be inspected.

This will:

  • Avoid bends and pulling tensions that exceed the limits of the cables.
  • Avoid clearance issues and jamming of the cables inside conduits.
  • Tell you which direction of pull (forward or reverse) will be the best.
  • Assist with placement and pulling force requirements for winching or push/pull machinery.

Maximum tension

Tension builds in cable pulls.  In straight sections it is added from one section to the next.  In bends, incoming tension is a multiplier. Generally, the highest tension is at the end of the pull however this may not be the case when there is a significant downhill section or a Push/Pull device in the pull.  Tension is not multiplied when the cable is pulled around a bend through free turning sheaths or rollers.

Maximum permissible tension of a cable is determined by its construction and is specified by the manufacturer.

Sidewall pressure

Sidewall pressure is a measure of the normal force pushing a cable against the conduit wall in a conduit bend.  Sidewall pressure depends on the tension coming out of the bend and the radius of the bend.  Sidewall pressure is specific to each bend. If there is no bend, there is no sidewall pressure.

Maximum permissible tension of a cable is also determined by its construction and is specified by the manufacturer.

Calculating tension and sidewall pressure

There are various complex equations used for calculating the tension and sidewall pressures along the following section types of a cable run:

  • Straight or horizontal.
  • Slope up/down.
  • Horizontal bend.
  • Downward or upward bends.
  • Large radius bends.
  • Rollers
  • Push/pull machines.

Calculating cable clearance and jamming ratio

Where there are more than two cables being pulled at the same time into a conduit then it is important to calculate the cable clearance and jamming ratio.

This will help to ensure the cables do not get stuck inside the conduit along the route.

Example calculation:

Conclusion

Pulling from either direction will not result in tension limit or sidewall pressure limit being exceeded.

Pulling from Location H requires a 20 % lower pulling force than pulling from Location A.

Use of a pulling lubricant results in a 67 % decrease in total tension.

In this example the cable pull consists of straight and horizontal bends along a total length of 450 m.

The cables are 3 x single core, Cu 500 mm2, XLPE/PVC with specifications as follows:

Overall diameter = 37 mm

Weight = 491 kg/100 m

Maximum permissible tension per cable = 10.6 kN

Maximum sidewall pressure = 1450 kg/m

The 3 cables will be installed inside a 125 mm diameter conduit.

Pulling from Location A:

Coefficient of friction = 0.5, dry 125 mm conduit.

Location Tension (N) Sidewall pressure (N/m)
A 0 0
B 1497 0
C 2233 78
D 2757 0
E 6127 214
F 6875 0
G 8065 282
H 8439 0

Coefficient of friction = 0.25, lubricated 125 mm conduit

Location Tension (N) Sidewall pressure (N/m)
A 0 0
B 748 0
C 916 32
D 1178 0
E 1760 61
F 2134 0
G 2606 91
H 2793 0

Pulling from Location H:

Coefficient of friction = 0.5, dry conduit.

Location Tension (N) Sidewall pressure (N/m)
A 0 0
B 374 0
C 569 20
D 1317 0
E 2933 102
F 3457 0
G 5153 180
H 6649 0

Coefficient of friction = 0.25, lubricated 125 mm conduit.

Location Tension (N) Sidewall pressure (N/m)
A 0 0
B 187 0
C 239 8
D 613 0
E 920 32
F 1182 0
G 1445 50
H 2193 0
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Advanced tension and sidewall pressure calculations.

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