phase
In a polyphase electrical power system, one of the power-carrying conductors, or the alternating current carried by it.
phase: one current path in a multi-wire power system
A phase is a single conductor carrying alternating current as part of a polyphase power distribution system. In three-phase systems, the most common industrial standard, three separate conductors each carry current that peaks at different moments in the AC cycle, spaced 120 degrees apart. This staggered timing is the defining feature: the three currents do not reach their maximum values simultaneously, which is what makes the system efficient and stable.
The voltage difference between any two phase conductors in a three-phase system is typically 400V, 480V, or 600V depending on the installation country and equipment class. The voltage from any phase to ground (the neutral or earth reference) is lower, usually 230V or 277V. Single-phase loads can be connected between any phase and neutral; three-phase motors and heavy industrial equipment connect across all three phases to exploit the continuous power delivery that polyphase systems provide.
Phase imbalance and loss of phase
Unequal loading on the three phases creates imbalance, where the currents no longer peak equally. This causes voltage imbalance, overheating in motors, and reduced efficiency. Loss of phase, where one conductor fails or disconnects, is more serious: three-phase motors will stall or run backward depending on which phase fails, and protective relays should trip the system to prevent damage.
The term 'phase' also describes the timing relationship itself. Phase angle, measured in degrees, quantifies how far apart two AC signals are in their cycles. A phase shift of 90 degrees means one signal reaches its peak while the other crosses zero. In power systems, this relationship affects power factor and the reactive power drawn by inductive loads like motors and transformers.
Three-phase distribution dominates industrial supply because it delivers constant power without the pulsations of single-phase systems, requires lighter conductors for the same power capacity, and allows efficient operation of rotating machinery. Single-phase systems persist for light commercial and residential use, where the lower installation cost outweighs the efficiency penalty.