Materials Used for Transmission Line Conductor

An overhead Transmission line depends on its bare phase conductor for electrical capacity and on that conductor’s mechanical behaviour for safe spans and clearances. Material choice balances conductivity, strength, mass, sag, creep, operating temperature, corrosion exposure and whole-life cost. Copper conducts better for a given cross-sectional area, but aluminium’s much lower density makes it more economical for most overhead lines.
Modern designs commonly use aluminium 1350, an aluminium alloy or aluminium strands with a reinforcing core. Aluminium has lower conductivity and strength than copper on an equal-area basis, so designers use a larger section, a stronger alloy or a reinforced construction. In an ACSR (Aluminium Conductor Steel Reinforced) conductor, coated-steel core wires raise the rated strength while outer aluminium strands carry most of the current.

ACSR remains one widely standardised overhead-conductor family, but AAC, AAAC, ACAR, ACSS and other high-temperature or composite-core designs serve different requirements. Selection for a Transmission line should account for:

1. Continuous and emergency ampacity, AC resistance, losses and corona performance
2. Rated tensile strength, fatigue performance, vibration and allowable sag
3. Span length, wind, ice, ambient temperature, pollution and marine exposure
4. Installed cost, fittings, maintenance, losses and expected service life

Required Properties of an Overhead Line Conductor

1. Low electrical resistance and adequate current-carrying capacity
2. Sufficient tensile and fatigue strength for the design loads
3. Low mass for manageable support loads and sag
4. Corrosion resistance suited to the local atmosphere and fittings
5. Stable mechanical and electrical properties at operating temperature
6. Acceptable thermal expansion, creep and long-term sag
7. Competitive installed and lifetime cost

Conductor Materials and Constructions

Materials and constructions used for overhead lines include:

1. Hard-drawn copper for some legacy or specialised conductors
2. Electrical-conductor-grade aluminium, commonly aluminium 1350
3. Aluminium alloys such as 6201 for higher-strength all-aluminium alloy conductors
4. Aluminium conductor steel reinforced, known as ACSR
5. Aluminium conductor aluminium-alloy reinforced, known as ACAR
6. Annealed or heat-resistant aluminium on steel or advanced high-strength cores
7. Coated steel or aluminium-clad steel for reinforcement and overhead shield or earth wires

Copper (Cu)

Copper has high electrical and thermal conductivity, good ductility and mature joining methods, so it remains an important conductor material in electrical equipment. Its density is about 3.3 times that of aluminium, and raw-material cost is usually higher. Grade, purity and cold work affect both conductivity and tensile strength.

Representative Properties of Copper

1. Annealed-copper resistivity at 20°C: about 1.72 µΩ·cm.
2. Temperature coefficient of resistance near 20^oC: about 0.0039 per ^oC.
3. Melting point: about 1083^oC.
4. Density at room temperature: about 8.9 g/cm³.

Use of Copper

Hard-drawn copper combines high conductivity with useful tensile strength and was widely used for overhead conductors. On a new Transmission line, aluminium constructions usually provide the required conductance at lower mass and cost. Copper remains relevant in fittings, substations, earthing systems and legacy lines where compatibility or corrosion conditions support its use.

Aluminium (Al)

Aluminium is a low-density, ductile and non-magnetic metal with a protective oxide film. Electrical-conductor-grade aluminium and purpose-made alloys are widely used as an electric conductor for overhead transmission and distribution. Pure aluminium is not the product specification: wire alloy, temper, stranding and reinforcement determine resistance, strength, creep and thermal performance.

Representative Properties of Aluminium

1. Aluminium 1350 conductor-wire resistivity at 20°C: about 2.8 µΩ·cm, depending on temper.
2. Temperature coefficient of resistance near 20^oC: about 0.0040 per ^oC.
3. Melting point of pure aluminium: about 660^oC.
4. Density at room temperature: about 2.70 g/cm³.

Use of Aluminium

Aluminium is the main current-carrying material in many modern overhead conductors. AAC uses aluminium 1350 strands. AAAC uses a stronger aluminium alloy. ACSR combines aluminium strands with coated-steel core wires where higher rated strength is required. The appropriate construction depends on span, sag, ampacity, temperature and corrosion conditions rather than voltage alone.

Cadmium-Copper Alloy

Cadmium-copper adds a small amount of cadmium to copper to increase strength while retaining much of copper’s conductivity. Exact composition and conductivity depend on the specified alloy and temper, so a general 0.6% to 1.2% composition and 90% to 96% conductivity range should not be treated as universal. Cadmium is toxic, and current occupational, environmental and product rules limit or control its manufacture, repair and disposal.

Uses and Limits of Cadmium-Copper Alloy

1. Historical high-strength copper conductors and specialised contact wire
2. Trolley wire in systems whose specification and local rules permit the alloy
3. Not a routine recommendation for heating pads or electrical blankets
4. Processing, welding and recycling require controls for toxic cadmium dust and fumes

Phosphor Bronze

Phosphor bronze is a family of copper-tin alloys containing a controlled phosphorus addition. Composition varies by grade. Phosphorus acts as a deoxidiser during manufacture, while tin and processing provide strength, spring properties, wear resistance and corrosion resistance. Its electrical conductivity is much lower than that of conductor-grade copper, so it is not a usual modern overhead phase-conductor material.

Uses of Phosphor Bronze

1. Not normally used as the phase conductor of a modern transmission line
2. Springs, fasteners and components that need fatigue resistance
3. Marine hardware where high resistance to wear and corrosion is required
4. Electrical contacts and spring terminals
5. Selected cryogenic leads where low thermal conductivity matters more than minimum electrical resistance

Galvanized Steel

Steel provides high tensile strength but corrodes in many outdoor environments. Zinc coating gives galvanized steel both barrier protection and sacrificial protection. During atmospheric weathering, zinc oxide and zinc hydroxide react further to form a relatively stable zinc carbonate, ZnCO₃, patina. Coating class and local moisture, pollution and salinity determine service performance.

Uses of Galvanized Steel

1. Coated-steel cores in reinforced conductors and shield or earth wires on a transmission line
2. Structural hardware, guys, poles and towers when specified with suitable corrosion protection

Copper-Clad Steel

Copper-clad steel bonds a continuous copper layer to a steel core. The steel supplies tensile strength, while the copper surface improves conductivity, corrosion performance and connection compatibility. Conductivity and cladding thickness vary by product standard; copper-clad steel should not be treated as equivalent to solid copper.

Uses of Copper-Clad Steel

1. Grounding and earthing conductors where the specified coating and connections suit the soil
2. Centre conductors of some coaxial cables, including CATV drop wire
3. Telecommunications drop and messenger applications covered by the relevant product standard

Aluminium Conductor Steel Reinforced

ACSR is a concentric-lay stranded overhead conductor made from hard-drawn aluminium wires around one or more coated-steel core wires. The aluminium provides most of the conductance, and the steel raises rated tensile strength. Standards define aluminium temper, core coating, stranding, resistance, mass and rated strength. ACSR is one major conductor family, but its sag, losses and corrosion behaviour must still be checked against AAC, AAAC, ACAR, ACSS and newer core technologies.

Uses of ACSR

1. ACSR is widely specified as the phase conductor for an overhead transmission line when its strength, sag and ampacity meet the design.
2. ACSR is not normally a coaxial-cable centre conductor; that application more often uses copper or copper-clad steel.