---
title: "T/F: ampacity vs ambient temperature"
subject: "Transmission and Distribution System Design"
chapter: 4
key: "semester_8/transmission_and_distribution_system_design:tf-ampacity-ambient-temperature"
source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-ampacity-ambient-temperature
site: BCT Engineering
---

# T/F: ampacity vs ambient temperature

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

- Asked in: 2080 Chaitra, 4 marks; 2073 Bhadra, 4 marks (asked 2 times)
- Video: [Calculate Conductor Ampacity with Temperature Correction](https://www.youtube.com/watch?v=yOUp88WD2P8) — Raiya Academy
- 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-ambient-temperature
- 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 and justify your answer with brief explanation: Ampacity of a conductor decreases with an increase in ambient temperature.

## Short answer

TRUE. With a fixed maximum conductor temperature, a higher ambient temperature reduces the temperature difference driving convective and radiative heat loss. Allowable current I ∝ √(θ_c – θ_a) thus decreases as ambient temperature rises.

## Answer

**TRUE.**

In steady state, the heat produced in the conductor equals the heat it loses:

$$
I^2R + q_{solar} = q_{conv} + q_{rad}
$$

where $q_{conv} \propto (\theta_c - \theta_a)$ and $q_{rad} \propto (T_c^4 - T_a^4)$.

The conductor temperature $\theta_c$ is limited to a fixed maximum (for example 75–80 °C for ACSR). When the ambient temperature $\theta_a$ rises, the difference $(\theta_c - \theta_a)$ falls. So less heat can be lost by convection and radiation, and a smaller $I^2R$ is allowed. Neglecting the solar term:

$$
I \propto \sqrt{\theta_c - \theta_a}
$$

Example: a conductor rated 400 A at 35 °C ambient with an 80 °C limit can carry only $400\sqrt{(80-40)/45} = 377$ A at 40 °C ambient. Hence ampacity decreases as ambient temperature increases. Line ratings are therefore given for summer conditions.

## Check yourself

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

1. Ampacity of a conductor decreases as ambient temperature increases. Is this true?
   A. False, ampacity is independent of ambient temperature
   B. False, ampacity increases because the air is thinner
   C. True, because less heat can be dissipated
   D. True, but only for copper
   Answer: C. A higher ambient temperature reduces the allowed temperature difference, so less heat is lost and less current is permitted. (Key point: Verdict)

2. In steady state the conductor heat balance (neglecting solar) is:
   A. I^2R = convection + radiation losses
   B. I^2R = 0
   C. I^2R = solar gain only
   D. I^2R = convection loss times ambient temperature
   Answer: A. Heat generated equals heat lost by convection and radiation. (Key point: Heat balance)

3. Neglecting solar heating, how does ampacity I relate to ambient temperature θa for a fixed maximum conductor temperature θc?
   A. I is proportional to (θc - θa)
   B. I is proportional to sqrt(θc - θa)
   C. I is proportional to (θc - θa)^2
   D. I is proportional to 1/(θc - θa)
   Answer: B. Since I^2R is proportional to (θc - θa), I varies as the square root of the temperature difference. (Key point: Current relation)

## Related in Transmission and Distribution System Design

- [Ampacity at new ambient and max temperature](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aampacity-change-new-temperatures) — Transmission and Distribution System Design, chapter 4
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