---
title: "T/F: tower height independent of span"
subject: "Transmission and Distribution System Design"
chapter: 4
key: "semester_8/transmission_and_distribution_system_design:tf-tower-height-independent-span"
source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-tower-height-independent-span
site: BCT Engineering
---

# T/F: tower height independent of span

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

- Asked in: 2074 Magh, 1+3 marks; 2072 Magh, 3 marks (asked 2 times)
- 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/tf-tower-height-independent-span
- Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-4

## Question

State whether the following statement is TRUE or FALSE. Justify your answer with a brief explanation: The height of a tower is independent of span length of the transmission line.

## Short answer

FALSE. Maximum sag grows with the square of span length, requiring taller towers for longer spans to maintain ground clearance. Tower height thus directly depends on span.

## Answer

**FALSE.**

The tower height is

$$
H_t = h_g + S_{max} + \text{(vertical spacing between conductors)} + h_{ew}
$$

The maximum sag at mid-span between supports at equal level is

$$
S_{max} = \frac{w\,L^2}{8T}
$$

It grows with the **square of the span** $L$. A longer span therefore needs a taller tower to keep the same minimum ground clearance.

For example, if the sag is 3.1 m at a 250 m span, then at the same tension a 350 m span gives $3.1 \times (350/250)^2 \approx 6.1$ m. The tower must then be about 3 m taller.

This is why fewer but taller and heavier towers are traded against more, shorter towers when finding the most economical span.

## Check yourself

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

1. The height of a tower is independent of span length. This is:
   A. True
   B. False, longer span means more sag and a taller tower
   C. True for suspension towers
   D. False, but only for double circuits
   Answer: B. Sag increases with span, so more height is needed for ground clearance. (Key point: Verdict)

2. How does maximum sag vary with span L (equal supports)?
   A. Directly with L
   B. With L^2: S = wL^2/(8T)
   C. Inversely with L
   D. Independent of L
   Answer: B. Smax = wL^2/(8T). (Key point: Sag relation)

3. What trade-off is made when finding the economical span?
   A. Fewer, taller and heavier towers versus more, shorter towers
   B. Higher voltage versus lower voltage
   C. Copper versus aluminium
   D. Single versus double earth wire
   Answer: A. Longer spans reduce tower count but raise tower height and weight. (Key point: Trade-off)

## Related in Transmission and Distribution System Design

- [T/F: tower height function of voltage only](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-tower-height-voltage-only) — Transmission and Distribution System Design, chapter 4, asked 2×
- [T/F: tower height same for single/double circuit](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-tower-height-single-double-circuit) — Transmission and Distribution System Design, chapter 4
- [T/F: minimum wind speed for tower strength](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-tower-strength-min-wind) — Transmission and Distribution System Design, chapter 4
- [T/F: span choice depends on voltage only](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-span-depends-voltage-not-conductor) — Transmission and Distribution System Design, chapter 4
- [T/F: capability curve inversely proportional to length](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-capability-curve-inverse-length) — Transmission and Distribution System Design, chapter 2
- [T/F: X/R ratio, transmission vs distribution](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-xr-ratio-transmission-distribution) — Transmission and Distribution System Design, chapter 1

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