---
title: "T/F: maximum wind speed for ampacity"
subject: "Transmission and Distribution System Design"
chapter: 4
key: "semester_8/transmission_and_distribution_system_design:tf-ampacity-max-wind-speed"
source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-ampacity-max-wind-speed
site: BCT Engineering
---

# T/F: maximum wind speed for ampacity

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

- Asked in: 2077 Chaitra, 4 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/tf-ampacity-max-wind-speed
- Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-4

## Question

State and justify whether the following statement is TRUE or FALSE: For conductor ampacity computation, maximum wind speed is taken into account.

## Short answer

FALSE. For safe ampacity, a low wind speed (0.5–0.6 m/s) is assumed to avoid overestimating convective cooling; using maximum wind speed would overrate the conductor, leading to overheating on calm days. Maximum wind speed is applied in mechanical design.

## Answer

**FALSE.**

Ampacity is the largest current the conductor can carry continuously without exceeding its maximum allowable temperature. It comes from the heat balance:

$$
I^2R + q_{solar} = q_{convection} + q_{radiation}
$$

Convective cooling rises strongly with wind speed. If the maximum wind speed were assumed, the cooling would be overestimated. The rating would be too high, and on calm days the conductor would overheat, anneal and sag below the safe ground clearance.

So, for a safe (conservative) rating, a **low wind speed** is assumed, typically 0.5–0.6 m/s (as in the given ACSR table, 0.6 m/s). It is combined with **high ambient temperature and full solar radiation**.

The maximum wind speed is used instead for **mechanical design**: tower loads, conductor tension and insulator swing.

## Check yourself

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

1. Is the statement 'maximum wind speed is used for ampacity' true?
   A. True, because wind cools the conductor most
   B. False, a low wind speed is assumed for a conservative rating
   C. True, only for ACSR
   D. False, wind is ignored entirely
   Answer: B. Ampacity uses low wind (about 0.5-0.6 m/s) so the rating is safe on calm days. (Key point: Verdict)

2. Which balance defines ampacity?
   A. I^2R + solar heating = convection + radiation cooling
   B. I^2R = corona loss
   C. Sag = tension x weight
   D. Solar heating = I^2R only
   Answer: A. The conductor reaches maximum allowable temperature when heat gained equals heat lost. (Key point: Heat balance)

3. What happens if maximum wind speed were assumed in the rating?
   A. The rating becomes too low and wasteful
   B. Cooling is overestimated, rating too high, and the conductor may overheat on calm days
   C. Nothing, convection is independent of wind
   D. Solar gain increases
   Answer: B. Overestimated convection gives an unsafe high ampacity; overheating anneals the conductor and increases sag. (Key point: Effect of assuming high wind)

4. Where is maximum wind speed used in line design?
   A. Thermal rating
   B. Efficiency calculation
   C. Mechanical design such as tower loads and conductor tension
   D. Corona calculation
   Answer: C. Maximum wind governs mechanical loading, not the thermal rating. (Key point: Use of maximum wind)

## Related in Transmission and Distribution System Design

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- [T/F: ampacity vs ambient temperature](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-ampacity-ambient-temperature) — Transmission and Distribution System Design, chapter 4, asked 2×
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