In a three-phase system, the maximum voltage imbalance between any two legs should not exceed:

• A. 10%
• B. 5%
• C. 2%
• D. 1%

Answer: (C)

In a three-phase system, everything runs best when the three phase voltages have equal magnitudes and are exactly 120 degrees apart. When one leg sags or leads, the voltages become unbalanced, causing uneven currents in the windings, extra heating, and reduced motor performance. To prevent these issues, a small voltage imbalance is kept within a tight limit. About 2% is a common industry guideline for the maximum allowable imbalance between any two legs, which helps keep motors cooler and operating reliably. Larger imbalances, such as 5% or more, can lead to noticeable heating and efficiency loss, and 10% would be considered excessive for normal operation. A 1% limit is stricter than usual, making 2% the practical standard.