Sag Formula and Calculations in Transmission Lines

What is Sag

Sag is defined as the difference in level between points of support and the lowest point on the conductor.

Sag and Tension Calculation When Supports are at an equal level

Consider a conductor that is suspended between two equal-level supports A and B. The lowest point is at O.


l = Length of span;

w = Weight per unit length of conductor

and T = Tension in conductor

The formula for Sag = wl2/8T

Also see: 13 Most Important Formulas on Variable Loads Every Power Engineer Should Know

Sag and Tension Calculation When Supports are at an unequal level

In mountains and hilly areas, the transmission line conductors are suspended at unequal levels.

l = Length of span

T = Tension in the conductor

h = Difference in levels between two supports

x1 = Distance of support at lower level (A) from O

x2 = Distance of support at higher level (B) from O

Also read: 5 Types of conductors used in Transmission lines

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