Power and energy

NEMA voltage unbalance calculator

Calculate percent voltage unbalance from three line-to-line RMS voltage readings using the NEMA maximum-deviation-from-average definition.

Calculate

Simultaneous line-to-line RMS readings
V RMS
V RMS
V RMS

Worked example

NEMA voltage unbalance

 %

Average line-to-line voltage
460 V RMS
Maximum deviation from average (BC)
7 V
Highest reading (BC)
467 V RMS
Lowest reading (CA)
453 V RMS
Highest-to-lowest spread
14 V

NEMA maximum-deviation method only. No pass/fail threshold, motor derating or equivalence to the IEC sequence-component method is inferred.

Underlined figures are rounded. Select one to copy its full value.

Method and assumptions

Formula

% voltage unbalance = 100 × maximum |Vline − Vaverage| ÷ Vaverage

Enter simultaneous line-to-line RMS readings. This is the NEMA arithmetic definition used for motors; IEC voltage-unbalance assessment based on negative- and positive-sequence components is a different calculation.

Limits of this result

  • The result is not the IEC negative-sequence to positive-sequence voltage ratio and the two values must not be substituted for one another.
  • Do not infer current unbalance, motor temperature rise, derating or safe operation from this voltage percentage alone.
  • Measurements must be simultaneous, at the same point and on the same RMS basis; instrument and transformer errors are not included.
  • No pass/fail threshold is applied because the equipment, operating state and governing requirement are not known.

Technical sources

  1. ANSI/NEMA MG 1-2016 (Revised 2018), Part 7National Electrical Manufacturers Association · Part 7 publication carrying 2021 NEMA copyright

Put the value to work

Read the related engineering guidance, or carry this task into System Builder to identify the signal path and EpiSensor products.

Continue in System Builder Read: Intelligent Energy Management Systems

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