Electrical Metering for Linemen and Techicians


Nameplate Meter Information


Modern generating facilities in the United States generate alternating current at a frequency of 60 cycles per second. Since a mechanical watt-hour meter is actually a small induction motor, it is designed to operate at a frequency of "60 Hz" (hertz). (Hertz is the modern term for cycles per second).

Number of Wires

The number of wires ("W"), as stated on the nameplate, refers to total number of wires in the circuit (including the neutral) for which the meter was designed. In practice, meters may be applied to circuits that use more or less wires than the number stated on the nameplate.

Voltage Rating

The potential coil is designed to operate at a specific "Voltage Rating". Voltage levels should be within plus or minus 20% of this rated voltage for mechanical watt-hour meters. An exception is the common 240-volt house meter, which may be used to meter a 120 volt 2 wire circuit.

Test Amp (TA) Rating

Meters are given a full load accuracy test at the rated voltage with the applied test amp ("TA") current. A light load test is also given at 10% of the stated test amp rating. These two basic tests provide assurance of accuracy over a wide range of load conditions.

The test amp rating for most modern self-contained meters is either 15 or 30 amps. Instrument-rated meters have a test amp rating of 2.5 amps.


The load placed on a modern self-contained meter can exceed the nameplate test amp (TA) rating by 666 2/3% with no loss of accuracy.


A test amp rating of 15 amps yields a maximum amp rating of 100 amps.
A test amp rating of 30 amps yields a maximum amp rating of 200 amps.
These maximum amp ratings are referred to as the class (CL) rating and will be clearly stamped on the nameplate of the meter.
As stated earlier, the standard test amp (TA) rating on instrument-rated meters is 2.5 amps. Class ratings will be either 10 amps (400%) or 20 amps (800%).


Meters are calibrated to record a precise quantity of electrical energy for each revolution of the meter disk. This quantity is stated in watt-hours and is known as the disk constant or "Kh" of the meter.


The "Type" designation on the meter nameplate is unique to each manufacturer. Prior to the early 1960’s, cross-reference charts had to be used to find equivalent meters made the various manufacturers.


In the early 1960’s, ANSI standards were established for form (FM) numbers. These form numbers eliminated the need for cross-reference charts for different brands of meters.

For example:

A form 9S meter from one manufacturer with the appropriate voltage rating may be substituted for any other manufacturer’s form 9S meter. (All form 9S meters have 13 terminals and the same basic 3 stator internal design).

Note: Remember, when selecting the appropriate meter form number for any metering application, it is also necessary to select the appropriate voltage rating.

Serial Number

"Serial Numbers" are important for record keeping for both the utility and the meter manufacturer.

(Year of Manufacture List for Antique Meters)

Catalog Number

"Catalog Numbers" are important for re-ordering.