What is conductor sag?

Sag is the vertical distance between the lowest point of a suspended conductor and the straight line connecting its two support points. It results from the conductor's weight under gravity.

What is the difference between parabolic and catenary sag?

The catenary is the exact shape of a hanging cable, described by a hyperbolic cosine. The parabolic approximation is accurate to within 1% when the sag-to-span ratio is less than about 5%, which covers most practical transmission line spans.

What is horizontal tension (H)?

Horizontal tension is the component of conductor tension acting along the horizontal direction. It is constant throughout the span. Maximum tension occurs at the support points and equals H plus the weight of conductor on the sag curve.

How does span length affect sag?

Sag increases with the square of span length (parabolic formula: S = wL²/8H). Doubling the span quadruples the sag, requiring taller towers or higher tension to maintain ground clearance.

What is the catenary constant?

The catenary constant c = H/w (horizontal tension divided by weight per unit length). It equals the length of conductor whose weight equals the horizontal tension and determines the shape of the catenary.

How do I account for ice and wind loading?

Use a combined loading weight w_total = √((w_conductor + w_ice)² + w_wind²) as the effective weight per unit length. Enter this combined value in the conductor weight field to find sag under loaded conditions.

Sag Tension Calculator

Name: Sag Tension Calculator
Availability: InStock
Author: Nham Vu

Calculate sag and tension for overhead transmission line conductors using parabolic or catenary approximations.

Conductor Parameters

Span Length (L)

Conductor Weight per Unit Length (w)

Typical ACSR Drake: 1.628 N/m (0.1116 lb/ft)

Horizontal Tension (H)

Height Difference Between Supports (h) — optional

Set to 0 for level spans (equal tower heights)

Formula

Parabolic Catenary (exact)

Parabolic is accurate when sag/span < 5%

Enter parameters and click Calculate to see results.

Summary

Calculate sag and tension for overhead transmission line conductors using parabolic or catenary approximations.

How it works

Enter the span length between the two support towers.
Input the conductor weight per unit length (linear density).
Provide the horizontal tension (ruling span tension) at the conductor.
Optionally add a vertical height difference between supports for inclined spans.
The calculator applies the parabolic sag formula and reports sag depth, maximum tension, and catenary constant.

Use cases

Design overhead high-voltage transmission lines with correct conductor clearances.
Verify ground clearance meets safety standards for a given span.
Estimate additional tension when ice or wind loads are applied to conductors.
Analyze inclined spans where support towers are at different elevations.
Cross-check field measurements of sag against theoretical values.
Teach electrical engineering students the catenary and parabolic sag relationship.

Frequently Asked Questions

Last updated: 2026-07-24 · Reviewed by Nham Vu