HVAC Procedure and Performance Guide

Test phase balance on three-phase motor

Training for this procedure can be found in these courses:

                         131 – Oil Heat l  and  161 – Boilers l


Voltage in each phase of three-phase motor must be measured. Voltmeter must be calibrated and used according to meter manufacturer’s instructions.  Line voltage must be within manufacturer’s specified limits and individual phase voltages must be within manufacturer’s specified tolerance when compared to the average phase voltage.


  1. Collect tools, equipment and materials
  2. Put on safety and protective equipment and apparel.
  3. Remove access cover(s) and safety guard(s).
  4. Determine nameplate voltage for motor.
  5. Verify that motor is operating satisfactorily under normal load.
  6. Set up voltmeter according to manufacturer’s instructions and measure and record line voltage and voltage reading across each phase of the motor.  (Note:  Disconnect and reconnect electrical power as required for safety reasons during testing).
  7. Bring motor to maximum load conditions, retest and record measurements.
  8. Compare line voltage to manufacturer’s specifications, calculate average phase voltage and compare to individual phase voltages. (Note:  If line voltage and/or phase balance is not to specification, test motor and switch gear for defects.  If none are found, ask power company for assistance).
  9. Remove test equipment.
  10. Replace access cover(s) and safety guard(s).
  11. Leave equipment in required operating status.
  12. Remove safety and protective equipment and apparel.
  13. Gather tools, equipment and leftover materials, and leave work site clean.

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