Electrical Metering for Linemen and Techicians


Dial Multipliers

Self-Contained Meters

Kilowatt-hour meters accumulate energy use on a register. The readings on this register are recorded each month. The difference between these readings is the monthly energy use. This usage must be multiplied by the dial constant, often referred to as the dial multiplier (Kr). Self-contained mechanical meters usually have a dial multiplier of 1 or 10. The formula for determining the dial multiplier for a meter equipped with a mechanical register is as follows:

           Kh X Register Ratio (Rr) X Shaft Reduction (Rs)

Kr = -----------------------------------------------------------------------



                     Step 1                                   Step 2                         Step 3


             7.2 X 13 8/9 X 100             7.2 X     9     X 100          7.2 X 125 X 100

Kr = ----------------------------------- = ----------------------------- = ------------------------- = 1

                       10,000                               10,000                       9 X 10,000

Step 1:

Kh X Rr X Rs should be written above the line. Remember, Kh is the disk constant representing the number of watt-hours per revolution of the meter disk. The Kh is clearly written on the nameplate of the meter. Rr is the register ratio, which is the relationship of the first takeoff gear on the register to the far right hand dial on the front of the register. The Rr is clearly stamped on the register. Rs is the shaft reduction representing the relationship of the worm wheel on the disk shaft to the first takeoff gear on the register. The Rs value is not generally stamped on the meter or on the register. A list of Rs values is included in the reference data section of this web site.

Step 2:

Next, turn the proper fraction of 13 8/9 into an improper fraction. Since we are working with 9ths, multiply the whole number 13 by 9 to convert everything to 9ths. 9 times 13 is 117. Now add the 8 above the 9 to 117 for a total of 125/9ths.

Step 3:

Finally, since the 9 is below a line we can now drop it below the entire problem and place a multiplication sign between the 9 and the 10,000. This step has not changed the outcome of the problem but has made it much easier to work. Finally, 7.2 X 125 X 100 is 90,000. 9 X 10,000 is also 90,000. Therefore, the dial multiplier is 1.

Instrument Rated Meters

Dial multipliers for instrument rated meters with mechanical registers may be calculated using the same formula as previously stated for self-contained meters. However, after calculating the internal multiplier of the meter, the instrument transformer ratios must be considered. (The term internal multiplier refers to the multiplier based solely on the meters Kh and mechanical gear train). Normally, the internal multiplier of an instrument meter is 1. If this so called internal multiplier is 1, the instrument transformer ratios alone will determine the overall dial multiplier. (Any number multiplied by 1 remains unchanged).

Never assume that instrument transformer ratios alone determine the dial multiplier!

Occasionally instrument rated meters have register ratios such that the internal multiplier does not come out to 1. For example: 0.25, 0.5, 1.25, 1.5, 2.0, 4.0, etc… In this case, the internal multiplier value must be taken into consideration and should be multiplied times the instrument transformer ratios. (See Universal Mechanical Registers)

        Dial Multiplier example 1:

Assume you are using 200:5 CT’s with one turn. Since 200 divided by 5 is 40, the transformer factor (TF) is 40. If the internal multiplier of an instrument meter is 0.5, the overall dial multiplier is 0.5 X 40 = 20.

        Dial Multiplier example 2:

Assume you are using 400:5 CT’s with one turn. Since 400 divided by 5 is 80, the transformer factor (TF) is 80. If the internal multiplier of an instrument meter is 2.0, the overall dial multiplier is 2.0 X 80 = 160.

Universal Mechanical Registers

Most instrument rated meters are equipped with a register that provides for an internal multiplier to be calculated as 1. Therefore, the instrument transformers alone determine the dial multiplier. Why would anyone intentionally use the wrong register ratio? In years past, meter shops had a large variety of register ratios to stock for repairing instrument rated meters even if they used only one brand. Many meter shops used universal register ratios to repair meters so they could cut down on inventory. These universal register ratios were 83 1/3 or 166 2/3. They were called universal because they allowed internal multipliers to be calculated to be 0.25, 0.5, 1.25, 1.5, 2.0, 4.0, etc… This ultimately resulted in dial multipliers with even numbers such as 20, 40, 60, etc…

If you have a car equipped with 14-inch tires and you replace them with 16-inch, the speedometer will no longer be accurate. The same goes for changing registers on a meter. If you replace a given register on a meter with a different register ratio, the internal multiplier must be recalculated. To play it safe and avoid large errors in registration, always calculate the internal multiplier in a meter. Then multiply the transformer factor of the instrument transformers by the internal multiplier.

Hybrid Meters

Hybrid meters can be defined as mechanical induction watt-hour meters with electronic registers. These meters became popular in the early to mid 1980’s. By programming the meter’s nameplate Kh value into the register, the internal multiplier could be assumed to be 1. This greatly simplifies the overall calculation of the dial multiplier. The dial multiplier could now be assumed to be equal to the instrument transformer ratios. Remember, with a purely electronic register, there is no gear train at all.

One common mistake with these electronic registers is to move a register from one meter to another without reprogramming the correct Kh value into the register. This mistake can result in large metering errors for both self-contained and instrument rated hybrid meters.

Solid State Meters

Solid state meters first came into play in the early 1970’s. However, they were not successfully mass-marketed until the late 1980’s. Just as in the hybrid meter, the solid state meter can and should be programmed for an internal multiplier of 1.

Please Note:

Much more information about solid state meters, smart meters, and Advanced Metering Infrastructure (AMI) will be posted to this site in the coming months!