In electrical engineering, single-phase electric power refers to the distribution of electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors. A single-phase supply connected to an alternating current electric motor does not produce a revolving magnetic field; single-phase motors need additional circuits for starting, and such motors are uncommon above 10 or 20 kW in rating.
The generation of AC electric power is commonly three phase, in which the waveforms of three supply conductors are offset from one another by 120. Standard frequencies of single-phase power systems are either 50 or 60 Hz. Special single-phase traction power networks may operate at 16.67 Hz or other frequencies to power electric railways.No arrangement of transformers can convert a single-phase load into a balanced load on a polyphase system. A single-phase load may be powered from a three-phase distribution system either by connection between a phase and neutral or by connecting the load between two phases. The load device must be designed for the voltage in each case. The neutral point in a three phase system exists at the mathematical center of an equilateral triangle formed by the three phase points, and the phase-to-phase voltage is accordingly times the phase-to-neutral voltage. For example, in places using a 415 volt 3 phase system, the phase-to-neutral voltage is 240 volts, allowing single-phase lighting to be connected phase-to-neutral and three-phase motors to be connected to all three phases.
In North America, a typical three-phase system will have 208 volts between the phases and 120 volts between phase and neutral. If heating equipment designed for the 240-volt three-wire single phase system is connected to two phases of a 208 volt supply, it will only produce 75% of its rated heating effect. Single-phase motors may have taps to allow their use on either 208 V or 240 V supplies.
On higher voltage systems (kilovolts) where a single phase transformer is in use to supply a low voltage system the method of splitting varies. In North America utility distribution practice the primary of the step-down transformer is wired across a single high voltage feed wire and neutral, at least for smaller supplies (see photo of transformer on right). Rural distribution may be a single phase at a medium voltage; in some areas single wire earth return distribution is used when customers are very far apart. In Britain the step-down primary is wired phase-phase.