Application Note

2 Fluke Corporation Determining load horsepower, wiring, and breaker size for safe and efficient installations
Determining actual motor
horsepower
Motor running current and volt-
age values should be measured
and recorded on a routine basis
as part of a preventive main-
tenance program. The actual
horsepower produced by a motor
should be closely estimated in
the field, as well. Energy losses,
inefficiencies created by over-
sizing motors, or maintenance
issues created by under-sizing
motors are all good reasons to
know how much horsepower the
motor is attempting to produce.
The quickest method to closely
estimate motor horsepower is
to use a digital clamp meter to
measure current and voltage to
the motor, and then perform a
simple calculation. (See the side-
bar “Use this formula to estimate
motor horsepower”). Be sure
to follow the safe work prac-
tices appropriate to the specific
application. With the availability
of remote display digital multi-
meters, such as the Fluke 381
Remote Display True-rms Clamp
Meter, workers can reduce their
exposure to lethal voltages and
the arc-flash hazard zone.
It is important to use a true-
rms clamp meter to get accurate
readings. While motor currents
can usually be read directly off
the face of an adjustable speed
drive supplying an associ-
ated motor, other equipment
will require the use of a meter
providing accurate readings in
the presence of harmonics and
sine-wave distortion.
Measuring loads other
than motors
You also need to record operat-
ing values of loads other than
motors. Since horsepower is not
determined for loads other than
motors, simply use the proce-
dure outlined in the sidebar
“Use this formula to estimate
motor horsepower” to measure
and record the current value
to the load. Examples of such
loads might be hermetic refrig-
erant motor-compressors found
in HVAC equipment, lighting
loads, and heating elements.
Rated-load current on hermetic
refrigerant compressors and
current ratings on other types of
equipment need to be compared
to measured values when you
are dealing with breaker trips or
overheating of equipment.
To determine the size of
breaker and conductors needed
to supply your load, refer to
the National Electrical Code®
(NEC®), manufacturer’s instruc-
tions, drawings, and any local
code requirements. Though the
NEC has specific rules for vari-
ous types of equipment, such as
motors and HVAC equipment,
The iFlex™ probe surrounds a single conductor in this drive
cabinet for an air handler unit (AHU). The Fluke 381 Clamp
Meter is used to record the amperage reading to troubleshoot
a suspected drive problem. The same clamp meter is used to
estimate the horsepower produced by a motor.
Use this formula to
estimate motor horsepower
Horsepower (hp) =
Voltage x Amperage x % EFF x power factor x 1.73/746
Where:
Voltage is the average of the three voltages measured: (A-B + A-C + B-C)/3
Amperage is the average current of the three-phase measured: (A+B+C)/3
% EFF is motor efficiency per motor nameplate
Power factor is the ratio of true power (kW) to apparent power (kVA). In the
absence of power factor measurement tools, a rule of thumb is to estimate power
factor at 0.85
1.73 is a constant used when calculating three-phase power
746 is the constant for converting watts to horsepower (746 watts = 1 Hp)
Example:
How much horsepower is a 25 hp motor producing at 472 volts and draw-
ing an average of 20 amps per phase with a motor nameplate marking of 90 %
efficiency?
Horsepower (hp) = Voltage x Amperage x % EFF x power factor x 1.73/746
= 472 V x 20 A x 0.90 x 0.85 x 1.73/746 = 17 Hp