---
title: "High conducting materials: requirements and classification"
subject: "Electric Machine Design"
chapter: 1
key: "semester_5/electric_machine_design:high-conducting-materials"
source: https://www.bctengineer.com/q/semester_5/electric_machine_design/high-conducting-materials
site: BCT Engineering
---

# High conducting materials: requirements and classification

Electric Machine Design · Chapter 1: Materials used in electrical equipment

- Asked in: 2081 Bhadra, 6 marks; 2078 Kartik, 5 marks; 2073 Shrawan, 8 marks (asked 3 times)
- Video: [Electrical Conducting Materials used in Machine - Introduction to Machine Design](https://www.youtube.com/watch?v=brjpOF54rw8) — Ekeeda
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machine_design/high-conducting-materials
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machine_design/chapter-1

## Question

What are the fundamental requirements of high conducting material? Classify and explain the electrical conducting material in brief.

## Short answer

High conductivity materials require low resistivity, low temperature coefficient, high strength, ductility, and corrosion resistance. Low-resistivity materials: copper, aluminium, silver. High-resistivity classification: manganin (standards, near-zero tempco), constantan (rheostats), nichrome (heaters), tungsten (filaments).

## Answer

**Conducting materials** carry current in windings, busbars, commutators and leads. A material of **high conductivity** (low resistivity) is needed so that $I^2R$ loss and voltage drop are small.

#### Fundamental requirements of a high conducting material
1. **High electrical conductivity** (low resistivity), for low copper loss and voltage drop.
2. **Low temperature coefficient of resistance**, so that resistance and loss do not rise much when hot.
3. **High tensile strength** to withstand winding tension, electromagnetic forces and centrifugal forces.
4. **High ductility and malleability**, so it can be drawn into thin wires and strips and bent into coils without cracking.
5. **Good solderability and weldability**, giving low-resistance joints.
6. **Resistance to corrosion and oxidation.**
7. **Good thermal conductivity**, so heat is carried away from hot spots.
8. **Low cost, light weight and easy availability.**

#### Classification of electrical conducting materials
Conducting materials are classified by resistivity into two groups.

**1. High conductivity (low resistivity) materials** carry current with minimum loss.
- **Copper** ($\rho \approx 1.72\times10^{-8}\ \Omega$m at 20 °C). Annealed (soft) copper is used for windings. Hard-drawn copper is used for overhead lines, commutator bars and busbars.
- **Aluminium** ($\rho \approx 2.8\times10^{-8}\ \Omega$m). It is light and cheap, but needs about 1.6 times the copper area. Used in squirrel-cage rotors (die-cast), transformer foil windings, busbars and cables.
- **Silver and gold:** silver has the best conductivity of all metals, and gold does not oxidise. Both are costly, so they are used only for relay and switch contacts.
- **Copper alloys:** brass for terminals; bronze, cadmium-copper and beryllium-copper for springs, contact wires and slip rings.
- **Steel:** used only where strength matters, for example ACSR cores and rail conductors.

**2. High resistivity materials** are used where a resistance is wanted.
- **Manganin, constantan (eureka):** precision resistors, shunts and standards (very low temperature coefficient).
- **Nichrome, kanthal:** heating elements (resist oxidation at high temperature).
- **Tungsten:** lamp filaments. **Carbon:** brushes and resistors.
- **Cast iron and alloy steel grids:** starter and rheostat resistances.

| Property | Copper | Aluminium |
|---|---|---|
| Resistivity (µΩ·cm) | 1.72 | 2.8 |
| Density (kg/m³) | 8900 | 2700 |
| Temp. coefficient (/°C) | 0.00393 | 0.0040 |
| Tensile strength | Higher | Lower |
| Jointing | Easy | Difficult (oxide film) |

## Check yourself

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

1. Which is NOT a requirement of a high conducting material?
   A. Low temperature coefficient of resistance
   B. High ductility
   C. High tensile strength
   D. High resistivity
   Answer: D. A conductor needs low resistivity (high conductivity) to keep I^2R loss small. (Key point: Requirements)

2. Aluminium needs about how much area compared with copper for the same resistance, and why?
   A. About 0.6 times; it is denser
   B. About 1.6 times; its resistivity is higher (2.8 vs 1.72)
   C. The same area; resistivity is equal
   D. About 3 times; it is heavier
   Answer: B. Resistivity ratio 2.8/1.72 is about 1.6, so more area is needed, though it is lighter and cheaper. (Key point: Copper vs aluminium)

3. How are conducting materials classified?
   A. By colour and cost
   B. Into high-conductivity (low resistivity) and high-resistivity materials
   C. By melting point only
   D. Into solid and liquid
   Answer: B. The two groups are low-resistivity carriers (Cu, Al, Ag) and high-resistivity resistance/heating materials. (Key point: Classification)

4. Which material is used for precision resistors because of its very low temperature coefficient?
   A. Nichrome
   B. Manganin
   C. Tungsten
   D. Hard-drawn copper
   Answer: B. Manganin and constantan have very low temperature coefficients; nichrome is for heating. (Key point: Uses of resistivity group)

## Related in Electric Machine Design

- [Insulating materials: classes, requirements and properties](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulating-material-properties) — Electric Machine Design, chapter 1
- [Good insulating material requirements and applications](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulating-requirements-application) — Electric Machine Design, chapter 1, asked 2×
- [Conducting versus insulating material characteristics](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Aconducting-vs-insulating) — Electric Machine Design, chapter 1
- [Insulating material requirements and silicon steel](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulating-requirements-silicon) — Electric Machine Design, chapter 1, asked 2×
- [Insulating materials and their classification](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ainsulating-material-classification) — Electric Machine Design, chapter 1
- [High resistivity materials and their uses](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machine_design%3Ahigh-resistivity-materials) — Electric Machine Design, chapter 1, asked 3×

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