14
Sag and Tension of
Conductor
D.A. Douglass
Power Delivery Consultants, Inc.
Ridley Thrash
Southwire Company
14.1 Catenary Cables............................................................... 14 -2
Level Spans.Conductor Length.Conductor Slack.
Inclined Spans.Ice and Wind Conductor Loads.
Conductor Tension Limits
14.2 Approximate Sag-Tension Calculations......................... 14 -9
Sag Change with Thermal Elongation.Sag Change
Due to Combined Thermal and Elastic Effects.Sag
Change Due to Ice Loading
14.3 Numerical Sag-Tension Calculations........................... 14 -14
Stress-Strain Curves.Sag-Tension Tables
14.4 Ruling Span Concept .................................................... 14 -22
Tension Differences for Adjacent Dead-End Spans.
Tension Equalization by Suspension Insulators.Ruling
Span Calculation.Stringing Sag Tables
14.5 Line Design Sag-Tension Parameters........................... 14 -25
Catenary Constants.Wind Span.Weight Span.
Uplift at Suspension Structures.Tower Spotting
14.6 Conductor Installation.................................................. 14 -28
Conductor Stringing Methods.Tension
Stringing Equipment and Setup.Sagging Procedure
14.7 Defining Terms .............................................................. 14 -39The energized conductors of transmission and distribution lines must be placed to totally eliminate the
possibility of injury to people. Overhead conductors, however, elongate with time, temperature, and
tension, thereby changing their original positions after installation. Despite the effects of weather
and loading on a line, the conductors must remain at safe distances from buildings, objects, and people
or vehicles passing beneath the line at all times. To ensure this safety, the shape of the terrain along
the right-of-way, the height and lateral position of the conductor support points, and the position of the
conductor between support points under all wind, ice, and temperature conditions must be known.
Bare overhead transmission or distribution conductors are typically quite flexible and uniform in
weight along their length. Because of these characteristics, they take the form of a catenary (Ehrenberg,
1935; Winkelmann, 1959) between support points. The shape of the catenary changes with conductor
temperature, ice and wind loading, and time. To ensure adequate vertical and horizontal clearance under
all weather and electrical loadings, and to ensure that the breaking strength of the conductor is not
exceeded, the behavior of the conductor catenary under all conditions must be known before the line is
designed. The future behavior of the conductor is determined through calculations commonly referred
to as sag-tension calculations.
Sag-tension calculations predict the behavior of conductors based on recommended tension limits
under varying loading conditions. These tension limits specify certain percentages of the conductor’s