Pulling Tensions

Pulling tension is a crucial factor when installing cables, especially in large-scale electrical systems or long cable runs. Pulling tension is important for:

1. Prevention of Cable Damage: Excessive pulling tension can damage the cable's insulation or even the conductors inside. This damage might not be immediately visible but can lead to serious issues such as short circuits, reduced insulation resistance, or even complete cable failure over time.

2. Maintaining Cable Integrity: Each cable has a maximum pulling tension specification, usually provided by the manufacturer. Staying within this limit ensures the cable maintains its physical and electrical properties throughout its service life. Exceeding this limit can stretch the conductors, deform the cable geometry, or damage the cable jacket, compromising performance.

3. Facilitating Easier Installation: Proper pulling tension, when controlled, makes the installation smoother. It helps the cable navigate through conduits, ducts, or around bends without getting stuck or twisted, reducing the risk of damage.

4. Avoiding Long-Term Issues: Excessive tension can cause strain on the cable over its entire length, leading to stress points that could result in future failures, especially under load. Controlled tension ensures the cable is laid correctly without introducing unwanted stress.

5. Compliance with Standards: Many industry standards and codes (like the NEC in the U.S.) have specific guidelines on the pulling tension allowed for different types of cables. Adhering to these standards ensures compliance and the long-term reliability of the installation.

The following recommendations are based on a study sponsored by ICEA and can be modified with experience and if more information is known:

Maximum Pulling Tension on Cable

1. With pulling eye attached to copper conductors, the maximum pulling strain in pounds should not exceed .008 times CMA.
2. With pulling eye attached to aluminum conductors, the maximum pulling strain in pounds should not exceed 0.006 times CMA. (Tm = 0.008 x N x CM) 
   Where: Tm = max. tension, lb.
   CM = cir-mil area of each conductor
3. With cable grip over non-leaded cable, the maximum pulling strain should not exceed 1,000 lbs. and may not exceed the maximum tension based on 0.008 or 0.006 x total conductor area.

Maximum Permissible Pulling Length (Lm = Tu/CW)

Where: Lm = pulling length, feet (straight section)
Tm = maximum tension, lb.
W = weight of cable per foot, lb.
C = coefficient of friction (usually 0.5)

Pulling tension must not exceed the smaller of these values:

1. Allowable tensions on the conductor
2. Allowable tensions on pulling device
3. Allowable sidewall pressure

 

Pulling Eye: T (Cu) = Number of Conductors x circular mils x .008
Basket-Group: T = 1000 lbs. per grip
Sidewall Pressure: p = 300 lbs. x radius of curve in feet