---
title: "Ruling span and surveying requirements"
subject: "Transmission and Distribution System Design"
chapter: 4
key: "semester_8/transmission_and_distribution_system_design:ruling-span-surveying"
source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/ruling-span-surveying
site: BCT Engineering
---

# Ruling span and surveying requirements

Transmission and Distribution System Design · Chapter 4: Conductor and Support Selection

- Asked in: 2074 Bhadra, 1+3 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/ruling-span-surveying
- Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-4

## Question

What is ruling span? Explain the surveying requirement for transmission and distribution line design.

## Short answer

Ruling span is the equivalent span in a tension section that represents the effect of multiple unequal spans on sag-tension changes, calculated as sqrt(∑L³/∑L). Surveying involves reconnaissance, walk-over survey, detailed profile and tower spotting using sag templates, soil investigation, and right-of-way assessment.

## Answer

#### Ruling span
In a line section between two tension (anchor) towers, the suspension strings let the tension equalise across spans of unequal length. The **ruling (equivalent) span** is the single hypothetical span whose tension changes with temperature and loading in the same way as the whole section. It is used to prepare the stringing (sag–tension) charts:

$$
L_r = \sqrt{\frac{L_1^3 + L_2^3 + \dots + L_n^3}{L_1 + L_2 + \dots + L_n}}
$$

A simpler approximation is $L_r \approx L_{avg} + \frac{2}{3}(L_{max} - L_{avg})$.

#### Surveying requirements for T&D line design
1. **Reconnaissance survey:** study topographic maps or satellite images and visit the site. Identify alternative routes, avoiding forests, settlements, protected areas, landslide zones and airports.
2. **Preliminary (walk-over) survey:** choose the best route for length, access, crossings and right-of-way cost. Fix the angle points.
3. **Detailed survey:**
   - Traverse along the route with total station or GPS. Record angle points and deviation angles.
   - Take the longitudinal profile along the centre line and cross-sections on slopes.
   - Plot the profile and use a sag template to place towers.
4. **Tower spotting:** place towers on the profile using the hot-curve (maximum sag) and cold-curve (uplift) templates. Ground clearance must be kept everywhere. This also gives the ruling span of each section.
5. **Check survey:** stake out the tower locations and confirm the clearances to roads, power and telecom lines, rivers and buildings.
6. **Soil investigation:** test the soil at the tower locations to classify it for foundation design.
7. **Right-of-way and land records:** identify land owners and the trees to cut, for compensation and permits.
8. **Distribution lines:** survey the road-side routes, the load locations for transformer siting, and the pole spans (about 50–80 m).

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the ruling span?
   A. The longest span in a section
   B. The single equivalent span whose tension varies with temperature and load like the whole section between tension towers
   C. The span between the first two towers
   D. The shortest span in a section
   Answer: B. Suspension strings equalise tension, so the section behaves like one equivalent span used for sag-tension charts. (Key point: Ruling span meaning)

2. Which formula gives the ruling span?
   A. sqrt(sum L^3 / sum L)
   B. sum L / n
   C. sqrt(sum L^2)
   D. L_max/2
   Answer: A. L_r = sqrt((L1^3 + ... + Ln^3)/(L1 + ... + Ln)). (Key point: Ruling span formula)

3. Which is the correct order of the survey stages?
   A. Detailed, reconnaissance, check
   B. Reconnaissance, preliminary walk-over, detailed survey, tower spotting, check survey
   C. Check survey, reconnaissance, detailed
   D. Tower spotting, then reconnaissance
   Answer: B. Route is narrowed progressively, then towers are spotted on the profile and finally staked and checked. (Key point: Survey sequence)

4. What is used on the profile to place towers while keeping ground clearance?
   A. Contour map only
   B. Hot (maximum sag) and cold (uplift) curve templates
   C. A level staff
   D. Soil boring logs
   Answer: B. Sag templates ensure clearance at maximum sag and no uplift at the cold condition. (Key point: Tower spotting)

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