Voltage Unbalance Calculator
Check how balanced a three phase supply is. A few percent of voltage unbalance causes much larger current unbalance and overheating in induction motors.
- Average = (415 + 408 + 421) ÷ 3 = 414.67 V
- Unbalance = max deviation ÷ average × 100 = 1.61 %
NEMA Voltage Unbalance Formula
Worked example
Line voltages 415, 408 and 421 V average 414.7 V. The largest deviation is 6.7 V, so the unbalance is 1.6 %.
Frequently Asked Questions
What is acceptable voltage unbalance?
Below 1 % is good practice for motors. Above 1 % they should be derated, and NEMA MG1 advises not to run them above 5 %.
What causes voltage unbalance?
Unevenly distributed single phase loads, a blown capacitor fuse, a faulty tap changer or a high resistance joint on one phase.
Is this the same as the IEC definition?
IEC uses the ratio of negative to positive sequence voltage. The NEMA method here gives a very close result for small unbalances and needs only three meter readings.
Learn more on the blog
How an AC induction motor works
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
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