---
title: "T/F: power transfer capability vs line length"
subject: "Transmission and Distribution System Design"
chapter: 2
key: "semester_8/transmission_and_distribution_system_design:tf-power-transfer-line-length"
source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/tf-power-transfer-line-length
site: BCT Engineering
---

# T/F: power transfer capability vs line length

Transmission and Distribution System Design · Chapter 2: Transmission Voltage Level and Number of Circuits

- Asked in: 2079 Chaitra, 1+3 marks; 2074 Magh, 1+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-power-transfer-line-length
- Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-2

## Question

State and justify whether the following statement is TRUE or FALSE: Power transfer capability of a transmission line increases with increase in line length.

## Short answer

FALSE. Power transfer capability decreases as line length increases because line reactance grows with length, so maximum power falls roughly as 1/L. Voltage drop also increases, further limiting capability.

## Answer

**FALSE.** Power transfer capability **decreases** as line length increases.

- Maximum power across a line: $P_{max} = \frac{V_s V_r}{X}$, and $X = xL$ grows with length, so $P_{max}$ falls roughly as $1/L$ (stability limit).
- Voltage drop also grows with length, so the voltage-regulation limit is reached at lower power.
- Only short lines (below about 80 km) are limited by the thermal rating, which does not depend on length.

The line capability curve shows this: power capability in multiples of SIL is about $2.75 \times SIL$ at 80 km, $1.75 \times SIL$ at 240 km and only $0.75 \times SIL$ at 640 km.

## Check yourself

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

1. Power transfer capability of a line increases with length. This is:
   A. True
   B. False, it decreases
   C. True for stability only
   D. True for thermal only
   Answer: B. Capability decreases as length increases. (Key point: Verdict)

2. Why does P_max fall with length?
   A. X = xL grows so P_max = VsVr/X falls
   B. Voltage rises
   C. SIL rises
   D. Resistance falls
   Answer: A. Reactance grows with length, lowering the stability limit roughly as 1/L. (Key point: Reason)

3. Which lines are limited by thermal rating?
   A. Very long lines over 600 km
   B. Short lines under about 80 km
   C. All lines
   D. None
   Answer: B. Thermal limit is independent of length and governs short lines. (Key point: Thermal limit)

## Related in Transmission and Distribution System Design

- [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
- [Power transfer limit vs SIL ratio and length](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Apower-transfer-sil-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
- [Power loss change on doubling line length](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Apower-loss-doubling-line-length) — Transmission and Distribution System Design, chapter 7
- [T/F: economic voltage vs number of circuits](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-economic-voltage-number-circuits) — Transmission and Distribution System Design, chapter 2
- [Justify higher voltage/more circuits for more power](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Ajustify-higher-voltage-more-circuits) — Transmission and Distribution System Design, chapter 2

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