Service voltage

The voltage configuration that is provided by the utility at the customer meter is called the service voltage. In many cases, two voltages are provided based on the phase-to-neutral and the phase-to-phase voltages available. When two voltages are provided, the service voltage is typically stated in the format 120/240 V, where the first number refers to the phase-to-neutral voltage and the second number refers to the phase-to-phase voltage.

Diagram showing how two voltages are provided in a secondary service


Secondary service voltage

Services for most customers are provided at what is called secondary voltage, meaning that the voltage from the primary distribution line has been reduced in a step-down service transformer owned by the utility. Service may be either single-phase or three-phase. Typical single-phase secondary services provided in the U.S. include 120 V, 240 V, 480 V, and a combined 120/240 V service. Three-phase service may be either wye or delta. Typical secondary wye services include 120/208 V and 277/480 V, while typical secondary delta services include 120/240 V and 240/480 V. In much of the rest of the world typical secondary voltages are 220 or 230 V for single-phase service and 380 or 400 V for three-phase service. 


Primary/transmission service voltage

Customers may also take service at the voltage provided by the utility substation. Such service can be at either primary voltage, which the utility has stepped down from the transmission line voltage, or at transmission voltage provided directly from a transmission line. Customers who take primary or transmission voltage must provide their own transformers to reduce the voltage to a level usable by their equipment. In the case of a customer taking service at a transmission voltage, the customer will own and maintain a substation similar to those used by the utility on its own system. Customers who take service at primary or transmission voltages receive a reduced rate from the utility since it is avoiding the expense of buying and maintaining the service transformer. 

Most primary and transmission services provided are three-phase services, but these services do not provide two voltages since the utility typically does not supply a neutral, depending instead on the customer to configure their own transformers and conductors to provide the necessary grounded neutral. Typical primary service voltages include 12 kV, 24 kV, and 36 kV, while typical transmission service voltages include 138 kV and 230 kV.