What is the maximum allowable current imbalance rating in a three-phase supply?

• A. None
• B. 2%
• C. 5%
• D. 10%

Answer: (D)

Current imbalance in a three-phase system is how far the three phase currents are from being equal. It’s measured by comparing the highest and lowest phase currents to the average, often written as I_unb = (I_max − I_min) / I_avg × 100%. This imbalance causes uneven heating of equipment, extra stress on transformers and motors, and can lead to voltage and efficiency issues. Because of these effects, a practical upper limit is set so equipment isn’t overstressed; for many three-phase feeders and motors, 10% is the commonly accepted maximum. Keeping within this limit helps maintain safer temperatures and reliable operation, while tighter limits (like 2% or 5%) would be chosen only for highly sensitive or precision equipment.