Basics of Meter Mounting Equipment

Power Distribution

Power, generated at a power plant and stepped up to a high transmission voltage, is brought to a local substation. Here, it is stepped down to a lower distribution voltage. When it reaches its final destination at a residential customer, it is stepped down to 240 volts.

Only single-phase power is used in a typical residential application.

Power Supply

The most common supply system used in U. S. residential applications today is a single-phase, three-wire supply system. In this system, the voltage between either hot wire and neutral is 120 volts and the voltage between the two hot wires is 240 volts. The 120-volt supply is used for generalpurpose receptacles and lighting.

The 240 volt supply is used for heating, cooling, cooking, and other high-demand loads.

Three-Phase Voltage

While single-phase power is needed for most residential applications, three-phase power is used in many other applications. In a three-phase system, the generator produces three voltages. Each voltage phase rises and falls at the same frequency (60 Hz in the U.S., 50 Hz in many other countries); however, the phases are offset from each other by 120°.

Three-Phase Transformers

Transformers used with three-phase power require three interconnected coils in both the primary and the secondary. These transformers can be connected in either a wye or a delta configuration. The type of transformer and the actual voltage depend on the requirements of the power company and the needs of the customer. The following illustration shows the secondary of a wyeconnected transformer and the secondary of a delta-connected transformer. These are only examples of possible distribution configurations, the specific voltages and configurations vary widely depending upon the application requirements.

Power, purchased from a utility company, enters the house through a metering device and connects to a load center. This is the service entrance. Service can come from an overhead utility transformer or from a lateral service run underground.

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