Bending Radii and Pulling Tensions

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Training is the positioning of the cable when NOT under tension. "Bending" is under tension. The minimum bending radius applies to the inner surface of the cable and not to the cable axis. "Sidewall pressure" or loading is the radial force exerted on a cable when pulled around a bend or sheave. Excessive sidewall pressure will crush the cable.

SIDEWALL LOADING

Where T is tension out of a bend, in pounds and R is the radius of the bend, in feet

1/C per conduit:

3-1/C cradled:

3-1/C triangular:

Typical Sidewall Loadings Lbs/Ft
600v nonshielded control 300
600v & 1kV nonshielded EP and XLP 500
5 & 15 kV Unishield & EP/XLP 500
23-35 kV power 300
Interlocked armored cable (All Voltage Classes) 300

MINIMUM BENDING RADII

Power Cables without Metallic Shielding, up to 600v
The minimum bending radii for both single and multiple-conductor cable with or without lead sheath and without metallic shielding are as follows:

Thickness of Conductor Insulation in Mils Minimum Bending Radius As A Multiple of Cable Diameter
Overall Diameter of Cable In Inches
1,000 and less 1,001 to 2,000 2,001 and over
155 and less 4 5 6
170-310 5 6 7
325 and over – 7 8

Power Cables with Metallic And Lead Covered Shielding (all voltages)

  1. For Tape Shielded Cables, the minimum bending radius is twelve times (12) the overall diameter of the completed cable.
  2. For Wire Shielded Cables, the minimum bending radius is eight times (8) the overall diameter of the completed cable.

Power Cables Over 600 Volts, Non-Shielded, Non-Armored

  1. The minimum bending radius for all cables is eight times (8) the overall cable diameter.

Interlocked Armored Cable, All Voltages

  1. The minimum bending radius for cables with non-shielded conductors is seven times (7) the armor O.D.
  2. The minimum bending radius for cables with shielded conductors is twelve times (12) the diameter of one phase conductor or seven times (7) the armor O.D., whichever is larger

PULLING TENSIONS

The following recommendations are based on a study sponsored by the ICEA. These recommendations may be modified if experience and more exact information so indicate.

  1. Maximum Pulling Tension on Cable
    1. With pulling eye attached to copper conductors, the maximum pulling tension in pounds should not exceed 0.008 times cir-mil area.
    2. With pulling eye attached to aluminum conductors, the maximum pulling tension in pounds should not exceed 0.006 times cir-mil area.

      TM=0.008 x n x CM

      where

      TM = max. tension, lb.
      n = number of conductors
      CM = cir-mil area of each conductor

    3. With cable grip over a lead sheath, the maximum pulling tension in pounds should not exceed 1,500 lbs./sq. inch of lead sheath cross-sectional area for commercial lead.

      TM=4712t(D-t)

      where

      t = sheath thickness, inches
      D = overall diameter of cable, inches

    4. With cable grip over a non-leaded cable, the maximum pulling tension should not exceed 1,000 lbs. and may not exceed the maximum tension based on 0.008 or 0.006 x total conductor area.
    5. When more than three conductors are pulled together, reduce the pulling tension 20%.
       
  2. Maximum Permissible Pulling Lengths

    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)

  3. Pulling Tension Requirements in Ducts of Conduits
    1. For straight sections, the pulling tension in pounds equals the length of duct multiplied by the weight per foot of cable and the coefficient of friction (paragraph B, above).
    2. For curved sections, the following formula applies:

      where

      T2 = tension for straight section at pulling end, lb.
      T1 = tension for straight section at feeding end, lb.
      TT = total tension
      a = angle of bend in radians (1 radian = 57.3 deg.)
      F = coefficient of friction (usually 0.5)
      = log
      = Naperian logarithm base = 2.718

    3. The maximum pulling tension in pounds shall not exceed 300 times the radius of curvature of the duct expressed in feet.