What Are Conductors?

Conductors are materials or physical structures through which electric charge flows with negligible resistance under an applied electric potential. In electrical engineering, the term refers both to the class of materials that permit current flow, principally metals, and to the physical elements, such as wires, cables, and bus bars, that carry current between the source and load in a circuit or power system. Copper and aluminum are the two dominant conductor materials in commercial use: copper, with a resistivity of approximately 1.72 × 10⁻⁸ ohm-meters, is preferred for indoor wiring, motor windings, and electronic interconnects; aluminum, at 2.65 × 10⁻⁸ ohm-meters, is preferred for overhead transmission and distribution lines where lower weight offsets its higher resistivity.

The physical behavior of conductors is described by Ohm's law, which relates voltage, current, and resistance, and by the more fundamental relationship between resistivity, cross-sectional area, and length. Resistivity itself is a temperature-dependent property: for most metals it increases linearly with temperature, governed by the temperature coefficient of resistance, a factor that must be accounted for in transmission loss calculations. Thermal noise, a broadband voltage fluctuation proportional to temperature and resistance first described by John Johnson and explained by Harry Nyquist in 1928, is an intrinsic electrical property of any resistive conductor and sets the noise floor for low-level signal circuits.

Material Properties and Conductor Forms

Conductors in power systems take the form of stranded cables, solid rods, tubes, and bus bar sections depending on the current rating, voltage class, and mechanical requirements. Stranded conductors, formed by twisting multiple smaller wires into a helical bundle, provide greater flexibility and resistance to fatigue compared to solid rods of equal cross-section. Aluminum conductor steel-reinforced (ACSR) cable uses a galvanized steel core for mechanical strength and concentric aluminum strands for current carrying, a construction that balances the weight and conductivity requirements of long overhead spans. The Engineering Toolbox tables of aluminum and copper conductor resistance provide reference resistance values per unit length at standard temperature for a wide range of cross-sectional areas.

AC Effects: Skin Effect and Proximity Effect

Under alternating current, current distribution within a conductor is not uniform. The skin effect causes current density to be highest at the conductor surface and to decay exponentially toward the center with a characteristic depth called the skin depth, which decreases as frequency and conductivity increase. At 50 or 60 Hz, the skin depth in copper is approximately 9 millimeters, meaning that for conductors much thicker than this, the interior carries little current and the effective resistance is higher than the DC value. Bundled conductors and hollow tubular sections exploit this by placing conductor material where the current actually flows. The proximity effect is a related phenomenon: when two parallel conductors carry current simultaneously, the magnetic field of each conductor distorts the current distribution in its neighbor, further increasing effective resistance and conductor loss. Both effects are significant in the design of high-current bus work, power cables, and high-frequency induction heating coils. The Michigan State University technical note on conductor properties covers skin and proximity effect calculations for common conductor configurations.

Power Transmission and Distribution

In three-phase electric power systems, conductors are arranged in groups of three, each carrying a current 120 degrees out of phase with the others. Overhead transmission lines at 115 kV to 765 kV use bare ACSR or all-aluminum-alloy conductors strung between lattice towers or monopoles. Underground distribution cables enclose conductors in insulation and metallic shielding layers to contain the electric field. The NDTE resource on electrical conductivity and resistivity provides context for how resistivity, temperature, and geometry interact in transmission system loss calculations.

Applications

Conductors have applications across every sector of electrical power and electronics, including:

• Overhead and underground power transmission and distribution lines
• Building and industrial wiring systems for lighting, motors, and process equipment
• Transformer and motor windings using enameled magnet wire
• Coaxial and twisted-pair cable assemblies for signal and data communications
• Inductive wireless power transfer coils in consumer electronics and electric vehicle charging pads