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Three-Phase Unbalance Calculator

Calculate the percentage of current or voltage unbalance in a three-phase system to prevent motor overheating, ensure efficiency, and comply with NEMA standards.

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Unbalance Calculator

Enter your three-phase readings to determine the unbalance percentage.

Calculation Results

Enter the Phase A, B, and C readings on the left to see the calculation results.

How to Calculate Three-Phase Unbalance

Three-phase unbalance is calculated using the standard NEMA (National Electrical Manufacturers Association) method. It is essential to ensure that voltage and current are distributed evenly across all three phases to prevent motor overheating and efficiency loss.

The NEMA Formula

% Unbalance = (Max Deviation from Average / Average) × 100

Step-by-Step Calculation:

  1. Step 1: Measure the voltage or current for all three phases (L1, L2, L3).
  2. Step 2: Calculate the average: (L1 + L2 + L3) / 3.
  3. Step 3: Calculate the deviation for each phase by subtracting the average from the phase value (use the absolute value).
  4. Step 4: Find the maximum deviation among the three.
  5. Step 5: Divide the maximum deviation by the average, then multiply by 100 to get the percentage.

Voltage Unbalance Derating Chart (NEMA MG-1)

When voltage unbalance exceeds 1%, a three-phase motor must be derated (its usable horsepower decreases) to prevent overheating. Operating a motor with an unbalance greater than 5% is strongly discouraged.

% Voltage Unbalance Derating Factor Recommended Action
≤ 1.0% 1.00 (100%) No action required; ideal operation.
2.0% 0.95 (95%) Derate motor load to 95% of rated HP.
3.0% 0.88 (88%) Derate motor load to 88% of rated HP.
4.0% 0.82 (82%) Derate motor load to 82% of rated HP. Check system.
5.0% 0.75 (75%) Max allowed limit. Correct immediately.
> 5.0% Not Recommended Unacceptable. Severe risk of motor failure.