Electric watt-hour meters are the humble servants of power utilities, dutifully recording energy consumption for residential and industrial customers alike. Nameplate specifications stamped on the face of the meter provides useful technical data to trained meter technicians. Nameplate data applies to both the classic electromechanical meter, identified by its signature revolving metal disc, and the modern solid-state electronic meter equipped with a digital liquid crystal display (LCD).
A meter's form type specifies several physical and electrical characteristics, including whether the meter was designed for single or three-phase service, quantity of meter elements, number of service wires and also if the meter is considered self-contained or transformer rated. Customers with light and medium loads can be served with self-contained meters, whereas large industrial customers usually require transformer-rated meters. Typical form types for self-contained and transformer-rated meters are designated 1S, 2S, 12S, 16S and 3S, 5S, 6S, 9S, respectively.
Watt-hour Constant (Kh)
The watt-hour constant, often referred to as Kh, represents the amount of electrical energy (in watt-hours) required to spin the disc of the classic electromechanical meter one full revolution. By counting the number of disc revolutions, a customer can determine how much energy is being consumed. Although newer solid-state meters have no revolving discs, the legacy Kh notation has carried forward to its modern equivalent. A typical Kh value for a form 2S meter is 7.2 watt-hours per revolution.
Meters are assigned a class rating by the American National Standards Institute (ANSI) in accordance with its power-handling capability. For example, self-contained meters typically have an ANSI rating of 200 (CL 200), meaning that the meter can safely handle 200 continuous amperes of electrical current flowing through it. Other ANSI classes are the CL20 (transformer-rated), CL100 and CL320.
Similar to weights and measures in other industries, electric watt-hour meters are tested for accuracy against a calibrated standard of known accuracy. This is performed for the benefit of both consumers and the utility. The electrical current applied to the meter under test is called the test current, often referred to as test amperes and abbreviated TA.
Test ampere values are considerably less than the ANSI-class rating of the meter. Self-contained meters can have TA values of 15, 30 or 50 amperes, while 2.5 amperes is typical of transformer-rated meters.
Power companies offer customers a variety of service voltages for commercial AC power depending on load their requirements. Residential customers generally have 120/240V single-phase service while industrial customers often require three-phase 120/208V and 277/480V services. Older electro-mechanical meters were often designed to operate with a specific voltage but newer solid-state meters offer the flexibility of multi-voltage ranging functionality.
About the Author
Gerritt Lee began writing in 2007. His work has appeared in the trade publications “Photovoltaic World,” “Utility Automation and Engineering T&D” and “Electrical Business.” He is a registered professional engineer and has over 28 years of combined utility experience in telecom and electric power revenue metering. Lee holds a bachelor's degree in electrical engineering from the University of Hawaii.