line conductor

A line conductor is the wire in an electrical circuit that carries current directly from the power supply to the load. In three-phase industrial systems, this is one of the three active phases; in single-phase domestic wiring, it is the one energized wire paired with the neutral return path. The line conductor remains at supply voltage relative to ground and presents an active electrical hazard whenever the circuit is live.

Color coding varies by jurisdiction and voltage class. In Europe and most IEC territories, line conductors are brown for the first phase, black for the second, and gray for the third in three-phase installations; older installations may show red, yellow, and blue. In North America, the convention is black for single-phase residential circuits and red, black, or blue in polyphase industrial work. Low-voltage systems typically use PVC or polyethylene insulation rated to 600 V or 1000 V; medium-voltage line conductors (above 1 kV) use XLPE or EPR compounds with thicker insulation and often aluminum or copper stranding for cost and weight optimization.

Sizing and current capacity

Line conductor cross-section is chosen to carry the expected load current with acceptable voltage drop and thermal rise. A 2.5 mm² copper conductor can safely carry about 20 A in fixed installation; a 6 mm² conductor approximately 40 A; a 10 mm² conductor approximately 63 A. These ratings depend on ambient temperature, installation method (buried, in conduit, open air), and the applicable standard (IEC 60364, NEC Article 310, or equivalent). Undersizing causes excessive voltage drop, overheating, and fire risk; oversizing increases material cost and installation labor without benefit beyond a certain point.

Common faults in line conductors include insulation breakdown from moisture, mechanical abrasion, aging, or sustained overcurrent; loose connections causing arcing and heat generation at terminations; and phase-to-phase or phase-to-ground contact from physical damage or contamination. Protective devices (circuit breakers, fuses, RCDs) are wired in series with the line conductor to interrupt fault currents before damage spreads. Cable glands, conduit systems, and segregation from other utilities help prevent mechanical and environmental harm.

The term "line" originates from early power distribution terminology, when single wires literally formed lines strung overhead between poles. "Line conductor" distinguishes it from neutral and earth conductors, which serve return and protection functions respectively. In power factor correction work, line conductors deliver both real and reactive power; in variable frequency drive installations, they carry the modulated waveform generated by the inverter, introducing harmonics that may require derating of the conductor or increased neutral capacity.