Cable Pulling Tensions Calculator

Tutorial Video

Cable Pulling Calculations Explained

Cable pulling calculations are essential for ensuring the safe and efficient installation of cables in conduits or ducts, particularly in lengthy or complex runs. These calculations determine the maximum pulling tension, sidewall pressure, and required lubrication to prevent cable damage during installation. Factors considered include cable weight, length, conduit bends, and friction coefficients. By accurately assessing these variables, engineers can select appropriate pulling techniques and equipment, ensuring that cables are not subjected to excessive mechanical stress, which could compromise their integrity and performance.

ℹ️Proper cable-pulling calculations help avoid installation issues, reduce the risk of cable failure, and ensure compliance with industry standards.

Standards for Cable Pulling

AS/NZS 3000:2018, also known as the Australian/New Zealand Wiring Rules, provides detailed cable pulling and installation guidelines to ensure safety and reliability. The standard emphasises avoiding excessive strain on cables, maintaining minimum bend radii, using appropriate supports and fastenings, and handling cables carefully to prevent mechanical damage. It also stipulates using lubricants to reduce friction when pulling cables through conduits and requires adherence to the manufacturer’s recommendations on pulling tension and installation practices. Additionally, the standard mandates environmental considerations and post-installation verification and testing to confirm the integrity of the installed cables. By following these guidelines, the integrity and longevity of electrical installations are maintained, ensuring compliance with regulatory requirements.

Checks Before Pulling Cables

Before installing cables, it is recommended that the route be inspected to:

• Avoid bends and pulling tensions that exceed the limits of the cables.
• Supports in contact with cables under pressure from changes in direction shall not have sharp edges.
• Avoid clearance issues and jamming of the cables inside conduits.
• Decide which direction of pull (forward or reverse) will be the best.
• Decide placement and pulling force requirements for winching or push/pull machinery.

⚠️Detailed pulling studies are required before the finalisation of the cable route.

Minimum Bending Radius

According to AS/NZS 3000:2018, the bending radius recommended by the cable manufacturer shall be observed. Where the manufacturer’s information is not available, the following minimum internal radii may be considered suitable:

Cable Pulling 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.

Cable pulling tension is the main parameter to evaluate when assessing cable installations. Knowledge of the pulling tension is essential to planning the cable laying and assessing the suitability of the cable design, route design, and installation methodologies.

The highest tension is at the end of the pull. However, this may not be true when a significant downhill section or a Push/Pull device is involved. Tension is not multiplied when the cable is pulled around a bend through free-turning sheaths or rollers.

The tension required to pull cables can change significantly depending on the pulling direction. In general, lower pulling tensions are encountered when feeding the cables into the end of a duct run with the largest number of bends or when cables are fed into the uphill side of a cable run.

ℹ️According to a survey from CIGRE TB 889-2022, the “average” maximum pulling lengths are 850 m in urban areas and 1185 m in rural areas. The absolute maximum reported pull lengths were 2200 m for urban areas and 3700 m for rural areas, but these required highly specialised equipment and techniques for a successful pull.

Maximum Allowable Cable Tension

The maximum permissible cable tension is determined by its construction and specified by the manufacturer.

Several equations for estimating maximum allowable cable pulling tension are available, which can be used without more accurate data from the manufacturer.

The following equation is for maximum pulling tension based on pulling by the conductor, but there are other equations for pulling by a basket-weave grip over the cable outer jacket.

Where:

= maximum pulling tension N (lbf)

= conductor area in square mm (circular mils)

= number of conductors

= 70.2 MPa (0.008 lbf/cmil) for soft annealed copper

= 52.7 MPa (0.006 lbf/cmil) for hard aluminum (alloy 1350-H16)

The maximum nominal pulling tension that a single-core cable can withstand is 22,250 N. Due to the unevenly distributed forces of multicore cables when bending, this rating becomes 44,500 N for a three-core cable.

Cable Sidewall Pressure

Sidewall pressure measures 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.

ℹ️There are no sidewall pressures if there are no bends in the pull.

Maximum Allowable Sidewall Pressure

For single conductors, multiple conductors, triplexed power, and multi-conductor control or power cables, the Maximum Allowable Sidewall Pressure (MASP) is between 4380 N/m and 7300 N/m of bend radius, depending on the cable's material.

Instrumentation cables have MASP ranging from 4380 N/m to 7300 N/m of bend radius, which relies on the construction and material of the cable.

Armoured cables typically have a MASP of 4380 N/m of bend radius or lower.

Calculating Tension and Sidewall Pressure

There are various complicated equations used for calculating the tension and sidewall pressures for 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 more than two cables are pulled simultaneously into a conduit, it is essential 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.

Cable clearance is the distance between the top of the uppermost cable inside the conduit and the inner top surface of the conduit. It should be at least 10% of the conduit’s inner diameter or one inch for large cables or installations involving numerous bends. There are multiple cases checked for the cable clearance calculation depending on the number and formation of the cables, which are:

1 Cable

2 Cables

3 Cables (Trefoil)

4 Cables (Diamond)

Where:

= inner diameter of the conduit

= average outer diameter of the cables

Jamming is the wedging of cables when pulled into a conduit, which usually occurs because of crossovers when the cables twist or are pulled around bends. The jamming ratio can be calculated by using the equation:

Where:

= inner diameter of the conduit

= average outer diameter of the cables

The jamming probability is based on the jamming ratio and is given in the figure below:

Tension from Pulling Cables Off a Drum

Reel back tension is the force required to pull the cable off the reel. The tension needed to pull the cable from a reel will depend on the cable size, weight of the first lap of the cable on the reel, the stiffness of the cable, and the type and condition of the reel payoff stand used. The tension force for pulling a cable off the reel in a horizontal position can be approximated using the following equation:

Where:

= tension from the cable reel, N

= weight per unit length of the cable, kg/m

= length of the cable, m

= acceleration due to gravity constant (9.8 m/s2)

= basic coefficient of friction (typical values between 0.5 to 1.0)

Correctly Unwinding Cable from the Drum

When unwinding or winding power cables from or to a cable reel/drum, the top figures below show the correct methods and the bottom figures show approaches to avoid.