Voltage Unbalance Calculator (NEMA Method)
By Santaji GadepatilinUpdated October 27, 2026
When the three phase voltages are not equal, motors draw unbalanced currents and run hotter — even a small voltage unbalance causes a much larger current unbalance. This guide to voltage unbalance calculation for motors explains the NEMA method and its effect on motor rating.
How to use the voltage unbalance calculator
Measure the three line-to-line voltages with a true-RMS meter and enter them. You get the average, the maximum deviation, the percentage unbalance and the approximate increase in motor winding temperature rise.
For more background, read Motor Protection Relay Types and Power Quality Issues with VFDs and PLCs.
The formula (NEMA MG 1)
% unbalance = 100 × max. deviation from average ÷ average voltage
Extra temperature rise ≈ 2 × (% unbalance)² percent.
Example
412, 405 and 418 V:
- Average = 411.7 V
- Largest deviation = 411.7 − 405 = 6.7 V
- Unbalance = 6.7 ÷ 411.7 = 1.62%
- Temperature rise increases by about 5.2%
Motor derating (NEMA MG 1, approximate)
| Voltage unbalance | Derating factor |
|---|---|
| 0–1% | 1.00 |
| 2% | 0.95 |
| 3% | 0.88 |
| 4% | 0.82 |
| 5% | 0.75 |
Do not operate motors above 5% unbalance.
Current unbalance
A small voltage unbalance causes a much larger current unbalance — typically 6 to 10 times larger. A 2% voltage unbalance can produce 15–20% current unbalance, which may trip motor protection relays set for phase unbalance.
Other definitions
IEC 61000 uses the ratio of negative-sequence to positive-sequence voltage. For small unbalances it gives results close to the NEMA method above.
Frequently asked questions
How do I calculate voltage unbalance?
Average the three line voltages, find the largest deviation of any one from the average, and divide by the average. 412, 405 and 418 V give 1.6%.
What voltage unbalance is acceptable?
NEMA MG 1 advises derating motors above 1% unbalance and not operating them above 5%. Many utilities aim to keep unbalance below 2%.
Why is unbalance bad for motors?
It creates negative-sequence current that opposes rotation and heats the rotor. Temperature rise increases by about twice the square of the percentage unbalance.
What causes voltage unbalance?
Unevenly distributed single-phase loads, a blown capacitor fuse, high-resistance connections, open-delta transformers and long feeders.