Voltage UnbalanceCalculator
Calculate three-phase voltage unbalance from field readings and flag motor, feeder, and service-quality concerns.
Enter the three measured phase voltages to calculate percent voltage unbalance using the standard maximum-deviation-from-average method. Use line-to-line readings for motor and service checks whenever possible. Line-to-neutral readings are useful for troubleshooting, but final three-phase motor evaluation should normally be based on line-to-line voltage.
Voltage Readings
Use loaded field measurements when available
Unbalance Results
Maximum deviation from average voltage
Within the common 1% target used for sensitive three-phase motor planning.
Continue with normal voltage-drop and wire-size checks.
Motor current unbalance can often be several times voltage unbalance. A planning range for these readings is about 5.00% to 8.33% current unbalance.
Unbalance is not solved by oversizing conductors alone. Check supply voltage, transformer taps, loose terminations, unequal single-phase loading, and long feeder voltage drop.
How To Use This Calculator
Take all three readings at the same operating condition, preferably with the normal load running. For motor feeders and service checks, line-to-line measurements are usually the most useful. If the result is above 1%, investigate balance before treating voltage drop as the only design issue.
Record A-B, B-C, and C-A voltage with the equipment operating, not from disconnected conductors.
The calculator finds the average and the largest deviation, then reports the unbalance percentage.
Once voltage quality is acceptable, use the voltage-drop and wire-size tools for conductor selection.
Related Tools
Continue from voltage-quality troubleshooting into voltage drop, wire sizing, tables, and code review.
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Check feeder and branch-circuit voltage drop after you confirm the phase readings are balanced.
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Use for battery, controls, solar, and other two-wire DC voltage-loss checks.
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Move from acceptable voltage quality into conductor sizing for the connected load.
Wire Tables
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NEC Standards
Review voltage-drop recommendations and code context for electrical design decisions.