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Voltage Imbalance Calculator

Calculate the percent voltage or current unbalance on a three-phase supply from three readings, and see whether it is within the DOE recommendation.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Readings are

Result

Above 1 %

Voltage unbalance
1.09%

1.09 % voltage unbalance.

  • DOE recommends keeping voltage unbalance at the motor terminals at or below 1 %; above that, motors need derating per the motor manufacturer. Find the cause (single-phase loads, connections, transformer taps).
  • A small voltage unbalance causes a much larger current unbalance — DOE notes it can be 6 to 10 times as large.
Average
460
Largest deviation from average
5
Show the working
  1. Average of the three readings
    (462 + 463 + 455) ÷ 3 = 460
  2. Unbalance = largest deviation ÷ average × 100
    5 ÷ 460 × 100 = 1.09 %

How it is calculated

Average the three readings, find the one furthest from the average, and divide that deviation by the average. For example, 462, 463 and 455 V average 460 V; the largest deviation is 5 V; the unbalance is 5 ÷ 460 = 1.1 %.

DOE’s motor tip sheet recommends keeping voltage unbalance at the motor terminals at or below 1 %. Above that, motors run hotter, lose efficiency and need derating according to the motor manufacturer.

Why a little matters

A small voltage unbalance produces a much larger current unbalance — DOE notes it can be six to ten times as large — and the extra current heats the windings. Common causes are unevenly distributed single-phase loads, poor connections and transformer issues.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Readings are
Line-to-line voltages
Reading 1
462
Reading 2
463
Reading 3
455

Result

Voltage unbalance
1.09 %
Average
460
Largest deviation from average
5

1.09 % voltage unbalance.

Assumptions

  • Percent unbalance = 100 × maximum deviation from the average ÷ the average, as defined in the DOE tip sheet (after NEMA).

Limitations

  • Motor derating for unbalance follows the motor manufacturer’s data; it is not calculated here.

Frequently asked questions

Should I measure line-to-line or line-to-neutral?

Use line-to-line voltages at the motor terminals for a three-phase motor.

Can I use the same formula for current?

Yes. The same calculation gives current unbalance; switch the readings type to currents.

What should I do if it is above 1 %?

Find the cause — load distribution, loose or corroded connections, supply problems — and consult the motor manufacturer about derating until it is fixed.

Sources

  1. Eliminate Voltage Unbalance (Motor Systems Tip Sheet #7) — U.S. DOE. Percent unbalance = 100 × maximum deviation from average ÷ average (example 462/463/455 V → 1.1 %); keep unbalance at the motor terminals at or below 1 %; current unbalance may be 6–10 times the voltage unbalance.

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