Determining Present Load, Power Factor, & Horsepower
Purpose
It is often necessary to determine how much load is passing through a watt-hour meter at any given point and time. For example, knowledge of the present load might be useful when settling a high bill complaint. This information can also prove useful when determining if equipment has been sized properly and when calculating power factor. A stopwatch may be used to time each revolution of the meter disk. By timing the meter disk and taking a few basic electrical measurements, a variety of useful information can be gathered.
The following formula may be used to determine present watts by timing the meter disk. Timing a meter disk for several disk revolutions will provide more accurate information. If a meter is heavily loaded, time the meter for 5 or 10 revolutions. If a meter is very lightly loaded, a single revolution will suffice.
3600 X Kh X Revolutions X CT Ratio X VT Ratio
Watts = -----------------------------------------------------------------------
Total Seconds
The Kh (listed on the meter nameplate) represents the watt-hours per disk revolution. We are going to time the meter disk in seconds for a number of revolutions. A conversion will be necessary. Since there are 3600 seconds in an hour, the Kh should be multiplied by 3600 to provide common units of time. The number of revolutions is of course based upon when you start and stop the stopwatch. Revolutions are the number of revolutions the meter disk makes while the stopwatch is running. CT and VT ratio are only required if the meter is an instrument rated meter. Total seconds are of course the time on the stopwatch.
Determining Present Load & Power Factor
(Self-contained meter)
Example 1:
Suppose a self-contained form 2S house meter with a Kh of 7.2 makes 10 disk
revolutions in 13.5 seconds. Since the meter is self-contained, the CT and VT
ratio components of the formula may be ignored.
Kh Revolutions
3600 X 7.2 X 10
19,200
Watts = ------------------------------------ = 19,200 watts & Kilowatts = --------- = 19.2 kW
13.5
Seconds 1,000
Suppose you measure 240 volts and a balanced current of 100 amps. The following
assumption may also be made.
Volts X Amps 240 X 100
kVA = --------------------- = ------------------- = 24 kVA
1,000 1,000
Power factor may now be determined with the following formula.
kW 19.2 kW
Power Factor (in percent) = -------- X 100 = ----------- X 100 = 80%
kVA 24 kVA
Let us review. By timing the meter disk, the present load in kilowatts was determined. By measuring the voltage and current in the circuit, the kVA was determined. And finally, by knowing both kW and kVA, the power factor was easily calculated.
Determining Present Load, Power Factor, & Horsepower
(Self-Contained Meter)
Example 2:
Suppose a self-contained form 12S three phase meter with a Kh of 28.8 makes
10 disk revolution in 46.33 seconds. Since the meter is self-contained, the CT
and VT ratio components of the formula may be ignored.
Kh Revolutions
3600 X 28.8 X 5
11,190
Watts = ------------------------------------ = 11,190 watts & Kilowatts = ----------- = 11.2 kW
46.33 Seconds 1,000
Suppose you find the line to line voltage to be 240 volts and you measure a balanced current of 36 amps. (This three-phase load calculation will require the addition of the square root of 3 component in the formula). The following assumption may be made.
Volts X Amps X 1.732 240 X 36 X 1.732
kVA = --------------------------------- = --------------------------- = 15 kVA (approx.)
1,000 1,000
Power factor may now be determined with the following formula.
kW 11.2 kW
Power Factor (in percent) = -------- X 100 = ----------- X 100 = 75% (approx.)
kVA 15 kVA
One horsepower is equal to 746 watts. Horsepower may be determined by the following formula.
Watts 11,190 Watts
Horsepower (HP) = ----------- = ------------------ = 15 HP
746 746
Let us review. By timing the meter disk, the present load in kilowatts was determined. By measuring the voltage and current in the circuit, the kVA was determined. By knowing both kW and kVA, the power factor was easily calculated. Finally, the horsepower for the load was determined by using a formula, which was based upon the assumption, that one horsepower is equal to 746 watts.
Determining Present Load, Power Factor, & Horsepower
(Instrument Rated Meter)
Example 3:
Suppose an instrument rated form 9S three-phase meter with a Kh of 1.8 makes
1 disk revolution in 25.93 seconds. The installation has 400/5 CT’s with 1 turn
with a transformer factor (TF) of 80. There are no VT’s in this example.
CT
Kh Rev. Ratio
3600 X 1.8 X 10 X 80 199,923
Watts = ----------------------------------- = 199,923 watts & Kilowatts = ------------- = 199.92 kW
25.93 Seconds 1,000
Suppose you find the line to line voltage to be 208 volts and you measure a balanced current of 653 amps. (This three-phase load calculation will require the addition of the square root of 3 component in the formula). The following assumption may be made.
Volts X Amps X 1.732 208 X 653 X 1.732
kVA = ------------------------------------ = ------------------------------ = 235.25 kVA (approx.)
1,000 1,000
Power factor may now be determined with the following formula.
kW 199.92 kW
Power Factor (in percent) = -------- X 100 = ---------------- X 100 = 85% (approx.)
kVA 235.25 kVA
One horsepower is equal to 746 watts. Horsepower may be determined by the following formula.
Watts 199,923 Watts
Horsepower (HP) = ----------- = -------------------- = 268 HP (approx.)
746 746
Let us review. By timing the meter disk, the present load in kilowatts was determined. By measuring the voltage and current in the circuit, the kVA was determined. By knowing both kW and kVA, the power factor was easily calculated. Finally, the horsepower for the load was determined by using a formula, which was based upon the assumption, that one horsepower is equal to 746 watts.