proximity effect
A condition when a power cable carrying electric current which is non-uniformly distributed due to the presence of other power cable or conductor nearby. This phenomena increases the apparent resistance of the power cable.
proximity effect: current bunching that kills cable efficiency
Proximity effect occurs when alternating current in a cable concentrates unevenly across its cross-section because nearby conductors carrying current create their own magnetic fields. Instead of distributing evenly, the current crowds toward the side of the cable farthest from its neighbors. This redistribution raises the effective resistance of the cable, causing it to heat more than the nominal resistance calculations predict. The effect becomes pronounced at higher frequencies and with larger conductor diameters.
The mechanism is electromagnetic. Each current-carrying conductor generates a magnetic field that forces current in adjacent conductors away from the shared boundary and toward the far edge. In a three-phase power cable with three conductors bundled together, current in each phase conductor is pushed outward, leaving the inner faces underutilized. The effect intensifies when conductors are closer together or when multiple cables run in parallel trays.
Practical impact on power distribution
Proximity effect increases cable heating and reduces the ampacity rating, the maximum continuous current the cable can safely carry. A cable rated for 200 amps in isolation might be derated to 160 amps when bundled with two other similar cables. Standards like IEC 60364 and NEC Article 310 supply correction factors that account for proximity when conductors are grouped. Installation spacing, cable tray design, and routing choices directly affect how much derating is required on a given run.
Separation is the primary control. Running cables in separate conduits, spacing them vertically in a tray rather than laying them side by side, or routing phases through different cable trays reduces proximity coupling. Single-conductor cables are more vulnerable than multiconductor cables because the conductors in a multiconductor assembly are twisted or geometrically balanced to minimize the effect. Shielded cables can also mitigate coupling to some degree, though shielding serves mainly for noise control.
Proximity effect differs from skin effect, which is the concentration of current at the surface of a single conductor at high frequency. Both increase with frequency; proximity effect also increases with the number and closeness of neighbors. In DC circuits, proximity effect does not occur because there is no changing magnetic field. In 50 or 60 Hz power distribution, the effect is real but usually modest; it becomes critical in high-frequency industrial furnace supplies or inverter circuits operating at kilohertz rates.