---
title: "Reason for bundling conductors in HV lines"
subject: "Power System Analysis I"
chapter: 2
key: "semester_4/power_system_analysis_i:reason-bundling-hv-lines"
source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/reason-bundling-hv-lines
site: BCT Engineering
---

# Reason for bundling conductors in HV lines

Power System Analysis I · Chapter 2: Overhead and Underground Transmission

- Asked in: 2076 Bhadra, 3 marks
- Video: [21). Why Bundle Conductor used in Transmission lines?](https://www.youtube.com/watch?v=HDPrly_JVHY) — ToonNagri
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/reason-bundling-hv-lines
- Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-2

## Question

Give reason in brief: bundling of conductors in HV transmission lines.

## Short answer

Bundling increases the effective conductor radius, reducing surface voltage gradient and corona loss. It also increases the geometric mean radius, lowering inductance, raising surge impedance loading, and improves current capacity with less skin effect.

## Answer

In EHV lines, each phase is made of two or more sub-conductors (a bundle) held apart by spacers. This is done mainly to reduce corona and line reactance.

- **Less corona:** A bundle acts like one conductor of much larger effective radius. This lowers the surface voltage gradient, so the corona loss, radio interference and audible noise are reduced.
- **Lower inductance:** The GMR of the bundle, $D_s^b$ (e.g. $\sqrt{D_s d}$ for two sub-conductors), is much larger than that of one conductor. This lowers the series reactance, so more power can be transmitted ($P \propto 1/X$) with less voltage drop.
- **Higher capacitance and SIL:** The capacitance rises, so the surge impedance $\sqrt{L/C}$ falls and the surge impedance loading rises.
- **Better current capacity and cooling:** Several smaller conductors have more surface area than one large conductor and less skin effect.

**Example:** 400 kV lines usually use twin or quad bundles of ACSR "Moose" conductor.

## Check yourself

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

1. Why does bundling reduce corona?
   A. It increases the voltage between phases
   B. The bundle acts like a conductor of larger effective radius, lowering surface voltage gradient
   C. It reduces the number of strands
   D. It lowers line frequency
   Answer: B. A larger effective radius cuts the surface gradient, reducing corona loss, noise and radio interference. (Key point: Corona reduction)

2. How does bundling affect reactance?
   A. It raises the reactance
   B. It has no effect on GMR
   C. Larger GMR reduces inductance and series reactance
   D. It changes only the resistance
   Answer: C. The bundle's GMR is larger, so L and X fall, allowing more power transfer. (Key point: Reactance)

3. Why does the SIL rise with bundling?
   A. Resistance increases
   B. L rises and C falls
   C. L falls and C rises, so surge impedance sqrt(L/C) falls
   D. Corona loss rises
   Answer: C. SIL = V^2/Zc, so a lower Zc gives a higher SIL. (Key point: SIL)

## Related in Power System Analysis I

- [Stranded and bundled conductors; bundling in EHV](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Astranded-bundled-ehv) — Power System Analysis I, chapter 2
- [Requirements of conductors for overhead lines](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aconductor-requirements) — Power System Analysis I, chapter 2, asked 2×
- [Skin effect in transmission lines](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Askin-effect-brief) — Power System Analysis I, chapter 4
- [Reasons for stranded and bundled conductors](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Areasons-stranded-bundled-conductors) — Power System Analysis I, chapter 2, asked 5×
- [Reasons for aluminium conductors](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Aaluminium-conductors-reasons) — Power System Analysis I, chapter 2

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- [Band applications and microwave transmission lines](https://www.bctengineer.com/question.md?key=semester_7%2Frf_and_microwave_engineering%3Aband-applications-transmission-lines) — RF and Microwave Engineering, chapter 1
- [Minimum spacing for 30 mm conductors (m = 0.95)](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Acorona-spacing-30mm-0-95) — High Voltage Engineering, chapter 5, asked 4×
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- [Design: 310 m head plant, HV breaker](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Adesign-2080-310m-hvcb) — Power Plant Design, chapter 4, asked 3×