How can Temperature affect Sag in an Overhead Line?

What is Sag?

Sag is defined as the vertical difference in level across points of support & the conductor’s lowest point. Sag is a crucial factor to consider since it influences the

  • Clearance,
  • Tension and
  • Length of an Overhead Line.

Effect of Temperature on Sag

Temperature affects sag since it causes the conductor to expand (or) contract as a result of thermal expansion and elastic elongation.

When the temperature rises, the conductor grows and lengthens, increasing the sag while decreasing the tension.

When the temperature drops, the conductor compresses and shortens, lowering the sag and increasing tension.

Formula to determine Sag

For Equal Support

The formula for estimating sag is determined on whether the supports are equal or unequal.

To compute sag for equal-level supports,

S = WL^2/8T


S - Sag,

w - Weight per unit length of the conductor,

L - Span length, and

T - Tension in the conductor.

For Unequal Support

To determine sag for unequal level supports

S = (WL^2/8T) + (h^2/2L)


h - Height difference between the supports.