Single-phase power refers to an electrical system that has only one voltage or current curve. While we are used to thinking of electric voltage as being constant (for instance, receiving 120V service) in reality the voltage of an electric line is continually fluctuating from positive to negative values. The 120V in this example is really a calculated value called the root mean square (RMS) voltage. Voltage on an electric line can be represented by a curve as shown below:
The alternative to a single-phase system is a three-phase system. Three-phase power has three different voltage curves, which start from zero at different times:
Single-phase distribution lines require only two wires — a phase wire and a neutral wire. This makes a single-phase system cheaper to install than a three-phase system, which requires three or four wires.
Typically, single-phase lines are tapped off three-phase lines, but single-phase power can also be generated in generators that are configured with a single coil or winding. This is most often found in distributed generation but not in utility-scale generators.
Single-phase power is used by most residential and small business customers because it is the most cost-effective way to serve their limited loads. For customers with larger loads, a three-phase system is more practical. In a three-phase system, each electrical phase carries one third of the power generated. To serve a large customer with a single-phase electrical system, the wires or conductors would have to be very large. But if the same customer is served with a three-phase system, three separate circuits can be used. Each circuit or phase carries a third the amount of power so each of the wires or conductors can be substantially smaller than the one set of large conductors that would be required in a single-phase system. Less power per phase means smaller wires, which significantly reduces the cost of wiring.