1. Applied voltage must be within 5% of nameplate rating.

2. You must be able to disconnect the motor from the load. (By removing V-belts or disconnecting a coupling).

3. Motor must be 7-1/2 HP or larger, 3450, 1725, or 1140 RPM.

4. The indicated line amperage must be below the full load nameplate rating.

PROCEDURE

1. Measure and record line amperage with load connected and running.

2. Disconnect motor from load. Measure and record the line amperage when the motor is running without load.

3. Read and record the motorâs nameplate amperage for the voltage being used.

4. Insert the recorded values in the following formula and solve.

% Rated HP = (((2 x LLA) ö NLA)/((2 x NPA) ö NLA)) X 100

Where:

LLA = Loaded Line Amps

NLA = No Load Line Amps (Motor disconnected from load)

NPA = Nameplate Amperage (For operating voltage)

Please Note:
This procedure will generally yield reasonably accurate results when motor load is in the 40 to 100% range and deteriorating results at loads below 40%.

Example:

A 20 HP motor driving a pump is operating on 460 volts and has a loaded line amperage of 16.5.

When the coupling is disconnected and the motor is operated at no load the amperage is 9.3.

The motor nameplate amperage for 460 volts is 19.4.

Therefore we have:

Loaded Line Amps LLA = 16.5

No Load Amps NLA = 9.3

Nameplate Amps NPA = 19.4

% Rated HP = (((2 X 16.5) ö 9.3 )/((2 X 19.4) ö 9.3)) X 100 = ((23.7/(29.5)) X 100
% Rated HP = 80.3%

Approximate load on motor slightly over 16 HP.