Three phase is a common method of electric power transmission in industrialised countries. It is a type of a polyphase system.
At the power station an electrical generator converts mechanical power into a set of alternating electric currents, one from each electromagnetic coil or winding of the generator. The currents are sinusoidal functions of time, all at the same frequency but with different phases. In a three-phase system the phases are spaced equally, separated from each other by 120°. The frequency is typically 50 Hz in Europe and 60 Hz in the US.
At the destination, a substation or transformer supplies the power stepped down from the high-voltage transmission line to three sinusoidally varying electric currents of 120 V (in the US) or 230 V (in Europe) alternating current (VAC). This is then delivered to the customer's circuits at a master breaker panel through four conductors. One conductor is the neutral or ground at the power source, the other three lines or phases carrying electrical power to point destinations or supply transformers. Connecting an electrical circuit from one phase to the neutral supplies 120 VAC (or 230 VAC) to the circuit.
The power transmission grid is organised so that each phase carries the same magnitude of current out of the power station; the currents returning from the customers' premises to the power station all share the neutral wire, but the three-phase system ensures that the sum of the returning currents is approximately zero.
Connecting between two phases provides √3 or 173% of the single-phase voltage (208 VAC in US; 400 VAC in Europe) because the out-of-phase waveforms add to provide a higher peak voltage in the resulting waveform. A common 60 A service in older homes in the United States typically uses only two phases. Standard domestic installations in the UK have only one phase. Small load circuits are connected to breakers across the neutral and one phase (pole or live). Large load items such as ovens, water heaters and electric furnaces are often connected across two phases (208 VAC in US; 400 VAC in Europe) for greater efficiency.
All three phases are typically used in large industrial motors as this is the most efficient way to transmit large amounts of electrical power. The greatest power demand is when starting the motor.
For further information on three phase circuits see:
single phase electric power, alternating-current electric power, polyphase systems