---
title: "Effect of parameters on conductor and insulator cost"
subject: "Transmission and Distribution System Design"
chapter: 2
key: "semester_8/transmission_and_distribution_system_design:parameters-effect-conductor-insulator-cost"
source: https://www.bctengineer.com/q/semester_8/transmission_and_distribution_system_design/parameters-effect-conductor-insulator-cost
site: BCT Engineering
---

# Effect of parameters on conductor and insulator cost

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

- Asked in: 2073 Bhadra, 8 marks
- 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/parameters-effect-conductor-insulator-cost
- Whole chapter: https://www.bctengineer.com/md/semester_8/transmission_and_distribution_system_design/chapter-2

## Question

Explain the effect of varying following parameter on conductor and insulator cost per unit length of transmission line. (i) Power to be transmitted and length of line (ii) Voltage level (iii) Number of circuit

## Short answer

Increasing power or line length raises conductor cost per km (larger conductor for current/drop) but does not directly change insulator cost. Raising voltage lowers conductor cost (current decreases) but increases insulator cost (more insulation). More circuits keep total conductor cost about the same while insulator cost grows roughly proportionally to circuit number.

## Answer

Line current is $I = \dfrac{P}{\sqrt{3}\,V\cos\phi\,N_c}$ per circuit. Conductor size depends mainly on current, while insulator size depends only on voltage.

#### (i) Power to be transmitted and length of line
- **Conductor cost:** larger $P$ means larger current, so a larger conductor area (for thermal limit and to keep losses low); conductor cost per km rises roughly in proportion to $P$. Longer $L$ increases voltage drop and loss, so for long lines a larger conductor (or higher voltage) is needed, again raising conductor cost per km.
- **Insulator cost:** per km it does not depend directly on $P$ or $L$, because insulation depends only on voltage. Indirectly, large $P$ and long $L$ push the design to a higher voltage, which increases insulator cost per km.

#### (ii) Voltage level
- **Conductor cost:** current $\propto 1/V$, so conductor area and conductor cost per km fall as voltage rises (loss $\propto 1/V^2$ for the same conductor). At EHV, however, minimum size or bundling may be fixed by corona, so the fall stops.
- **Insulator cost:** number of discs, string length, cross-arm length, tower height and clearances all increase with voltage; insulator cost per km rises roughly in proportion to $V$ (or faster at EHV due to switching-surge design).

#### (iii) Number of circuits
- **Conductor cost:** for the same total power, each circuit carries $1/N_c$ of the current, so each conductor is smaller, but there are $N_c$ times as many. Total conductor cost per km is roughly the same or a little higher (smaller conductors cost more per unit current), while losses for the same total aluminium are similar.
- **Insulator cost:** number of strings is proportional to $N_c$, so insulator cost per km roughly doubles from single to double circuit. Double-circuit towers are taller and heavier, but cheaper than two separate S/C lines.

| Parameter increased | Conductor cost/km | Insulator cost/km |
|---|---|---|
| Power $P$ | Increases | No direct change |
| Length $L$ | Increases (to limit drop/loss) | No direct change |
| Voltage $V$ | Decreases | Increases |
| Number of circuits $N_c$ | About same / slight increase | Increases (≈ proportional) |

These opposite trends give an optimum (most economical) voltage and number of circuits for each $P$ and $L$.

## Check yourself

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

1. As voltage increases, conductor cost per km:
   A. Increases
   B. Decreases, since current is proportional to 1/V
   C. Unchanged
   D. Doubles
   Answer: B. Current falls so conductor area and cost fall. (Key point: Voltage on conductor)

2. As voltage increases, insulator cost per km:
   A. Falls
   B. Rises (more discs, longer string, taller towers)
   C. Is unaffected
   D. Is zero
   Answer: B. Insulation grows with voltage. (Key point: Voltage on insulator)

3. Increasing P affects insulator cost per km:
   A. Directly in proportion
   B. No direct change; only indirectly via higher voltage
   C. By halving
   D. By eliminating it
   Answer: B. Insulation depends only on voltage. (Key point: Power effect)

4. Going from single to double circuit, insulator cost per km:
   A. Halves
   B. Roughly doubles as strings are proportional to Nc
   C. Unchanged
   D. Falls to zero
   Answer: B. Number of strings is proportional to Nc. (Key point: Number of circuits)

## Related in Transmission and Distribution System Design

- [Number of insulator discs for 220 kV line](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Ainsulator-discs-220kv-appendix) — Transmission and Distribution System Design, chapter 3
- [T/F: pin insulator cost vs voltage](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atf-pin-insulator-cost-linear) — Transmission and Distribution System Design, chapter 3
- [Effect of voltage level on losses and economy](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Avoltage-level-effect-loss-economy) — Transmission and Distribution System Design, chapter 2
- [Insulator discs per string, 180 MW over 200 km](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Ainsulator-discs-180mw-200km) — Transmission and Distribution System Design, chapter 3
- [Conductor tension under varying conditions](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atension-varying-conditions-equation) — Transmission and Distribution System Design, chapter 4
- [Tower dimensions for 220 kV single circuit line](https://www.bctengineer.com/question.md?key=semester_8%2Ftransmission_and_distribution_system_design%3Atower-dimensions-220kv) — Transmission and Distribution System Design, chapter 4, asked 2×

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