---
title: "Stranded and bundled conductors; bundling in EHV"
subject: "Power System Analysis I"
chapter: 2
key: "semester_4/power_system_analysis_i:stranded-bundled-ehv"
source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/stranded-bundled-ehv
site: BCT Engineering
---

# Stranded and bundled conductors; bundling in EHV

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

- Asked in: 2073 Magh, 5 marks
- Video: [What is Bundle Conductor| Why Bundle Conductor use in Transmission Lines| Electrical Interview](https://www.youtube.com/watch?v=53gLKdrYtdY) — Electrical With Abhishek Joshi
- 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/stranded-bundled-ehv
- Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-2

## Question

What is meant by stranded conductor and bundled conductor in power transmission system? Explain the reason for using bundled conductors in Extra High Voltage (EHV) transmission lines.

## Short answer

A stranded conductor is made of many thin wires twisted for flexibility. Bundled conductors use multiple sub-conductors per phase to increase effective radius, reducing corona and inductive reactance, which raises surge impedance loading and power capacity in EHV lines.

## Answer

#### Stranded conductor
A stranded conductor is made of many thin wires twisted together in concentric layers. Each layer is laid in the opposite direction to the one below it. For $n$ layers around a central wire, the number of strands is $3n^2+3n+1$ (7, 19, 37, ...). ACSR is the common example: aluminium strands around a steel core for strength.

- It is flexible and easy to wind on drums, string and sag.
- It does not break easily from vibration.
- A broken strand does not cause the whole conductor to fail.

#### Bundled conductor
A bundled conductor uses two, three or four sub-conductors per phase, held a short distance apart (30–45 cm) by spacers. Together they act as one phase conductor.

```text
 twin       triple       quad
 o   o        o         o   o
             o o        o   o
```

#### Reasons for bundling in EHV lines
- **Reduced corona:** The effective radius is larger, so the surface voltage gradient falls. Corona loss, radio interference and audible noise are lower.
- **Lower reactance:** The GMR of the bundle is large ($D_s^b = \sqrt{D_s d}$ for twin, $\sqrt[3]{D_s d^2}$ for triple, $1.09\sqrt[4]{D_s d^3}$ for quad), so the inductive reactance is reduced.
- **Higher power capacity:** Lower $X$ and higher $C$ reduce the surge impedance, so the SIL and the steady-state stability limit $P_{max} = V_SV_R/X$ increase.
- **Better current rating:** Several conductors have a larger surface area for cooling and less skin effect.
- **Less voltage drop and better regulation,** because of the lower reactance.

Typical practice is a twin bundle at 220–400 kV and a quad bundle at 400–765 kV.

## Check yourself

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

1. For n layers around a central wire, how many strands are there?
   A. 3n + 1
   B. n^2 + 1
   C. 3n^2 + 3n + 1
   D. 6n + 1
   Answer: C. For n = 1, 2, 3 this gives 7, 19, 37. (Key point: Number of strands)

2. What is the main reason for bundling at EHV?
   A. Higher resistance
   B. Lower capacitance
   C. Lower surface gradient (less corona) and lower reactance
   D. Easier stranding
   Answer: C. A bundle has a larger effective radius and larger GMR. (Key point: Why bundle)

3. How far apart are sub-conductors held?
   A. 3-5 cm
   B. 30-45 cm by spacers
   C. 1-2 m
   D. No spacers are used
   Answer: B. Spacers hold sub-conductors 30-45 cm apart. (Key point: Spacing)

4. How does bundling raise power capacity?
   A. It raises X
   B. Lower X and higher C reduce Zc, raising SIL and P_max = VsVr/X
   C. It lowers voltage
   D. It increases Zc
   Answer: B. Reduced reactance raises the stability limit. (Key point: Power capacity)

## Related in Power System Analysis I

- [Reason for bundling conductors in HV lines](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Areason-bundling-hv-lines) — Power System Analysis I, chapter 2
- [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×
- [Stranded vs bundled conductors](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Adifferentiate-stranded-bundled-conductors) — Power System Analysis I, chapter 2
- [Inductance of 3-conductor bundled line, 6.1 m spacing](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Ainductance-three-subconductor-bundle-6-1m) — Power System Analysis I, chapter 4
- [Effect of bundled conductors on L and C](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Abundled-conductor-effect-l-c) — Power System Analysis I, chapter 4
- [GMR of stranded vs solid conductor](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Agmr-stranded-lower-than-solid) — Power System Analysis I, chapter 4

## Related in other subjects

- [Issues of EHV-AC power transmission](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Aehv-ac-transmission-issues) — High Voltage Engineering, chapter 1
- [Need for EHV and UHV lines](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Aehv-uhv-lines-justify) — High Voltage Engineering, chapter 1
- [Aerial bundled conductor in LT distribution](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Aabc-lt-distribution) — Transmission and Distribution System Design, chapter 7, asked 5×
- [How corona occurs in EHV lines](https://www.bctengineer.com/question.md?key=semester_8%2Fhigh_voltage_engineering%3Acorona-occurrence-ehv) — High Voltage Engineering, chapter 5