TRANSMISSION LINE SAG CALCULATION | ELECTRICAL & POWER SYSTEMS

 

TRANSMISSION LINE SAG CALCULATION | ELECTRICAL & POWER SYSTEMS

What Is Transmission Line Sag?

Sag is the vertical distance between the conductor’s lowest point and the straight line joining its supports.

Correct sag is essential for maintaining safe ground clearance, preventing excessive conductor tension, and ensuring reliable power transmission.

Sag Calculation Formula

For a level span with approximately uniform conductor loading and small sag:

Sag (d) ≈ wL² ÷ 8T

Where:

d = Sag (m)

w = Conductor load per unit length (N/m)

L = Horizontal span length (m)

T = Horizontal conductor tension

Worked Example

Given:

- Conductor load, w = 10 N/m

- Span length, L = 200 m

- Horizontal tension, T = 20,000 N

Step 1: Apply the formula

d ≈ wL² ÷ 8T

d ≈ (10 × 200²) ÷ (8 × 20,000)

d ≈ 400,000 ÷ 160,000

Calculated sag ≈ 2.5 m

Factors Affecting Sag

Conductor temperature

Wind loading

Ice loading, where applicable

Span length

Conductor tension and weight

Difference in support elevations

Electrical loading and resulting conductor heating

Why Is Sag Important?

Maintains required ground and crossing clearances

Helps prevent contact with trees and nearby structures

Avoids excessive mechanical tension

Supports safe and reliable overhead line operation

Helps engineers assess conductor performance under design conditions

Design Note: This is a simplified calculation for a level span. Actual sag-and-tension design must consider conductor properties, temperature, wind and ice loads where applicable, support geometry, creep, and statutory clearance requirements. In India, follow current CEA regulations and the relevant utility specifications.

Never approach an overhead line to measure sag or clearance