The Line Tension & Sag Calculator is an online calculator tool designed for power utility professionals, electrical engineers and transmission line designers.

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It calculates 

• Sag, 
• Tension, 
• Wind & ice loading and 
• Ground clearance 

for overhead conductors in accordance with IEEE and IS 802 standards.

Accurate sag and tension calculations are very important for ensuring 

• Safe conductor clearances, 
• Mechanical integrity of tower structures and 
• Compliance with statutory requirements. 

Calculate the sagging transmission lines and tension accurately to make sure a safe conductor clearance, optimal mechanical strength and reliable power system performance under a varying load & weather conditions.

This online calculator tool that can withstand both level and inclined spans, incorporating thermal effects, wind loading and ice accretion.

What is Sag?

Sag is defined as the distance across the highest point of electric poles (or) towers and the lowest point of a conductor that is connected between 2 poles (or) towers.

Span Length

It is the shortest distance between 2 towers (or) poles.

Sag S = (WL²)/ (8T)

Where

S – Sag of the conductor

W – Weight of the conductor

I – Span length of the conductor

T – Working tension on the conductor

Input Parameters

Span & Tower Parameters

Parameter	Symbol	Unit	Description
Span Length	S	metres (m)	Horizontal distance between towers
Tower 1 Height	h₁	metres (m)	Attachment point height at tower 1
Tower 2 Height	h₂	metres (m)	Attachment point height at tower 2

Conductor Parameters

Supported conductor presets:

• ACSR: Aluminium Conductor Steel Reinforced
• AAC: All Aluminium Conductor
• AAAC: All Aluminium Alloy Conductor
• HDC: Hard Drawn Copper

Parameter	Symbol	Unit	Notes
Weight per metre	w	kg/m	Self-weight of conductor
Cross-section Area	A	mm²	Electrical & mechanical sizing
Outer Diameter	d	mm	Used for wind & ice load calculation
Ultimate Tensile Strength	UTS	N	Breaking load from manufacturer
Safety Factor	FOS	dimensionless	Typically 2.0 for HT lines
Modulus of Elasticity	E	N/mm²	ACSR ≈ 65,000 N/mm²
Thermal Expansion Coeff.	α	×10⁻⁶ /°C	ACSR ≈ 19.3 × 10⁻⁶ /°C

Wind & Ice Loading

Parameter	Symbol	Unit	Notes
Wind Pressure	P	N/m² (Pa)	Per IS 802 wind zone map
Ice Radial Thickness	t	mm	Enter 0 if no ice loading

Temperature Parameters

Parameter	Symbol	Unit	Notes
Erection Temperature	T₀	°C	Temperature at time of stringing
Operating Temperature	T	°C	Maximum design temperature

Calculation Methodology

Maximum Allowable Tension

The maximum horizontal tension H is derived by applying the safety factor to the conductors UTS:

H = UTS ÷ FOS

Loading Calculations

Conductor self-weight (N/m)

wc = w x g = w x 9.81

Ice load per unit length (N/m)

w_ice = 900 x π x t x (d + t) x 9.81

Wind load per unit length (N/m)

Fw = P x (d + 2t)

Total resultant load (N/m)

w_t = √(w_vert² + F_w²)

Sag Calculation

Sag for a level span

Sag = w_t x S² ÷ (8 x H)

For uneven spans (inclined), the point of maximum sag x₁ from tower 1

x₁ = S/2 – (Δh x H) ÷ (w_t x S)

Sag_max = w_t x x₁² ÷ (2 x H)

Catenary Constant

a = H / wt

The catenary constant a (in metres) indicates the “stiffness” of the catenary. A larger value means lower sag relative to span length.

Output Results

Output	Unit	Description
Max allowable tension (H)	N	Horizontal tension at midspan
Sag – level span	m	Sag assuming equal tower heights
Max vertical sag (uneven)	m	Sag at point x₁ for inclined span
Wind load per metre (Fw)	N/m	Transverse wind force per unit length
Total resultant load (wt)	N/m	Combined vertical & wind loading
Slant distance (L)	m	Straight-line distance between supports
Height difference (Δh)	m	Vertical offset between tower attachment points
Catenary constant (a = H/wt)	m	Catenary parameter
Max sag point from tower 1	m	Horizontal position of lowest conductor point
Ground clearance status	–	OK (or) warning based on minimum clearance check

Conductor Preset Data

Conductor Type	Weight (kg/m)	Diameter (mm)	Elasticity E (N/mm²)	α (×10⁻⁶/°C)
ACSR	1.592	18.6	65,000	19.3
AAC	1.35	17.2	58,000	23.0
AAAC	1.28	16.8	69,000	23.0
HDC (Copper)	2.10	17.5	120,000	17.0

Standards & Compliance Reference

IS 802 (Part 1/Sec 1):1995: Indian Standard for overhead transmission line towers, loads and permissible stresses.

IS 802 (Part 1/Sec 2):1992: Material, fabrication and erection of overhead lines.

IEEE Std 524-2003: IEEE Guide to the Installation of Overhead Transmission Line Conductors.

IEC 60826: Design criteria for overhead transmission lines.

IS 398 (Parts 1–5): Aluminium conductors for overhead power transmission.