A kilowatthour meter measures the amount of energy consumed by a load. AC kilowatthour meters often use an induction-disk type of mechanism.
The disk revolves at a speed proportional to the rate at which energy passes through the meter. The metered kilowatthours are indicated on a set of dials driven by the revolving disk through a gear train.
Solid-state kilowatthour meters use a wide variety of electronic methods to integrate energy over time. Many solid-state meters also record other quantities, such as kilovarhours, volts, amperes, and power factor.
The kilowatthour meter may be used to calculate the power being used by a load at the moment of testing. To calculate power, count the seconds for a given number of revolutions of the disk, and then use this formula:
power (kilowatts) = 3.6 x r x Kh x multiplier/ seconds
Kh - is the meter disk constant in watthours per revolution and r - is the number of revolutions.
The Kh will be noted on the kilowatthour meter.
The multiplier is 1 unless a meter is installed with instrument transformers. If current transformers are installed, the multiplier is equal to the ratio of the current transformer.
For example, 400:5 current transformers have a ratio of 80:1, and so the meter multiplier would be 80. If voltage (potential) transformers are also installed, the meter multiplier is the product of the current transformer ratio and the voltage transformer ratio.
A meter connected to 400:5 (80:1 ratio) current transformers and 14 400:120 (120:1 ratio) voltage transformers would have an overall multiplier of 80 times 120, or 9600. Some newer electronic meters allow the user to program the meter with the multiplier.
These meters display actual kilowatthours on the register. Kilowatthour meters come in several classes. Below is a listing of the common classes along with the maximum current each can safely monitor.
Class 10 10 A
Class 20 20 A
Class 100 100 A
Class 200 200 A
Class 320 320 A
High-current services would require a Class 10 or Class 20 meter employed with current transformers. For example, a 1000 A service would use 1000:5 (200:1 ratio) current transformers and a Class 10 (or Class 20) meter.
Kilowatthour meters typically are rated for either 120 or 240 V potential coils. Higher voltage applications require the use of voltage transformers.
The following kilowatthour meter application data can be used only as a general guideline. The number of phases, the number of wires, the amount of phase-to-phase current, and power-factor balance all have an effect on the number of stators (or coils) the kilowatthour meter should have. An unbalanced condition exists if the phase-to-phase differences in load current or load power factor are great.
Other factors used in selecting kilowatthour meters include the following:
Type of mountings: socket, bottom-connected, switchboard
1. Voltage: 120, 240, 240/120, etc.
2. Register: clock, cyclometer (like an odometer), digital
3. Type of load current bypass: automatic, manual
There is a high probability of error in selecting or connecting a kilowatthour meter, especially when using instrument transformers. If there is any doubt, consult a metering specialist. The high probability for error also applies to kilovarhour and demand meters.