Single-phase service

Single-phase is a service configuration offered by electric utilities and is typically provided to residential and small commercial customers. It is most often provided as 120/240V service, 120 volts for the lights and the outlets and 240 volts for the large energy-using appliances. The two voltages are supplied by providing connections to three wires — two “hot” or phase conductors that are carrying current and a neutral wire that is attached to the center bushing in the service transformer

Notice the three bushings on the transformer shown above with the wires connected to them. The neutral is the center wire. Connecting the center bushing and either outside bushing provides 120 volts. This is called a phase-to-neutral connection. Connecting the two outside wires provides 240 volts. This type of connection is called phase-to-phase. The utility provides three wires to the premises, each connected to one of the transformer bushings.

When the electrician wires separate circuits in the facility he can make that circuit carry either 120V or 240V by connecting the circuit to the proper combination of wires. For circuits serving lights and wall outlets, the electrician will provide 120V. And for circuits powering large appliances such as electric ranges, ovens, freezers, refrigerators, and dryers he will provide 240V.

It used to be common for utilities to provide two-wire single-phase service in residential areas. This is done by connecting the neutral and one-phase conductor, usually resulting in a voltage of 120V. The downside to this service configuration is that only one voltage is available. This type of service is not typically offered by utilities in the U.S. for new residential connections, although older existing two-wire connections and new two-wire service to billboards are still in use in some areas.