---
title: "Reasons for stranded and bundled conductors"
subject: "Power System Analysis I"
chapter: 2
key: "semester_4/power_system_analysis_i:reasons-stranded-bundled-conductors"
source: https://www.bctengineer.com/q/semester_4/power_system_analysis_i/reasons-stranded-bundled-conductors
site: BCT Engineering
---

# Reasons for stranded and bundled conductors

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

- Asked in: 2079 Chaitra, 4 marks; 2078 Chaitra, 2+2 marks; 2072 Magh, 3 marks; 2071 Magh, 5 marks; 2070 Magh, 5 marks (asked 5 times)
- Video: [Solid vs Stranded vs Bundled Conductors | Which is Best for Power Transmission?](https://www.youtube.com/watch?v=JmtKNjKOQ38) — Electrology
- 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/reasons-stranded-bundled-conductors
- Whole chapter: https://www.bctengineer.com/md/semester_4/power_system_analysis_i/chapter-2

## Question

What are the reasons for making stranded and bundled conductors for transmission lines?

## Short answer

Stranded conductors offer flexibility, resist breakage from vibration, and allow composite strength (e.g., ACSR). Bundled conductors (2, 3, or 4 sub-conductors per phase) increase effective radius to reduce corona, lower inductive reactance, raise surge impedance loading, and improve stability and current capacity.

## Answer

#### Stranded conductors
A stranded conductor is made of many thin wires twisted together in layers (e.g. 1, 7, 19, 37 ... strands), each layer twisted in the opposite direction.

Reasons:
1. **Flexibility:** A solid conductor of large area is stiff and hard to handle, wind on drums and string. Stranding makes it flexible.
2. **Less breakage:** Solid wires can crack due to vibration and bending; in a stranded conductor a broken strand does not break the whole conductor.
3. **Higher strength with composite strands:** Steel core strands can be combined with aluminium strands (ACSR) to give high tensile strength and good conductivity.
4. **Easy manufacture and transport** of long lengths.
5. Slightly larger diameter for the same area, helping to reduce surface stress.

```text
   Stranded (7 strands)    ACSR
       o o                ooo
      o O o              o###o   ### = steel core
       o o                ooo
```

#### Bundled conductors
In EHV lines (220 kV and above), each phase uses two or more sub-conductors spaced 30–45 cm apart by spacers.

Reasons:
1. **Reduced corona:** The bundle behaves like a conductor of larger equivalent radius, so the surface electric field falls, raising the critical disruptive voltage and reducing corona loss, radio and audible noise.
2. **Lower inductive reactance:** The geometric mean radius (self-GMD) increases, so $L = 2\times10^{-7}\ln(D_m/D_s)$ falls.
3. **Higher capacitance and surge impedance loading:** Lower $Z_c=\sqrt{L/C}$ gives higher SIL and power capacity.
4. **Better stability:** Lower reactance raises $P_{max}=V_sV_r/X$.
5. **Better cooling and current capacity:** More surface area dissipates heat.

```text
 2-bundle    3-bundle      4-bundle
  o---o        o          o----o
              / \         |    |
             o---o        o----o
```

## Check yourself

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

1. Stranding is used mainly because
   A. It raises resistance
   B. It reduces strength
   C. It makes the conductor flexible and a broken strand does not break it
   D. It reduces diameter
   Answer: C. Solid large conductors are stiff and crack. (Key point: Stranding flexibility)

2. ACSR combines
   A. Steel core for strength with aluminium for conductivity
   B. Copper and silver
   C. Steel and rubber
   D. Aluminium only
   Answer: A. Steel gives tensile strength. (Key point: ACSR)

3. Bundling reduces corona because
   A. It raises voltage
   B. It lowers spacing
   C. It increases current
   D. Larger equivalent radius lowers surface field
   Answer: D. Critical disruptive voltage rises. (Key point: Corona reduction)

4. Bundling lowers inductive reactance because
   A. GMD decreases
   B. Length decreases
   C. GMR increases
   D. Frequency decreases
   Answer: C. L = 2e-7 ln(Dm/Ds). (Key point: Inductance)

## 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
- [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
- [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
- [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
- [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×
- [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

## Related in other subjects

- [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