In a three-phase system, the voltage imbalance between any two legs may not vary by more than which percent?

• A. 10%
• B. 5%
• C. 2%
• D. 1%

Answer: (C)

In a balanced three-phase system, the three line voltages should be equal in magnitude and evenly spaced in time. When there’s any imbalance, the line-to-line voltages (Vab, Vbc, Vca) no longer track each other closely. The practical threshold for how far those line-to-line voltages can drift is about two percent of the nominal voltage. Keeping the imbalance within this range helps ensure motors and power electronics see nearly equal voltages, minimizing circulating currents, additional heating, and torque ripple. If the imbalance grows beyond this level, performance and reliability suffer, with hotter equipment and potential tripping risks. The other options are longer-standing industry tolerances only, but two percent represents the typical acceptable limit for maintaining good balance in many systems.