Voltage Unbalance Calculator

Charles LeGeyt Fortescue's 1918 paper "Method of Symmetrical Coordinates" at the AIEE conference gave engineers a way to decompose any unbalanced three-phase system into three balanced components: positive, negative, and zero sequence. The negative-sequence current created by voltage unbalance spins backwards through induction motors, generating opposing torque, extra heating, and harmonic flux — the reason NEMA limits unbalance to 1%.

NEMA MG-1 (Motors and Generators), first published in 1946 and updated continuously since, codified the derate curve still used today: motors must be derated roughly by the square of %VU for operation above 1%. At 5%, the derate reaches ~25%, and above 5% motors should be taken offline. Current unbalance is typically 6–10× the voltage unbalance because motor impedance to negative-sequence current is much smaller than to positive sequence.

Common causes of field unbalance: single-phase loads unevenly distributed across phases (especially lighting and office receptacles), open delta transformer connections, blown fuses on PFC capacitor banks (removes vars from one phase), and high-impedance connections like corroded lugs. A handheld power quality meter at the motor terminals is the fastest diagnostic.

Enter the three line-to-line voltages measured at the motor terminals. The tool computes V_avg = (V_AB + V_BC + V_CA)/3, maximum deviation from average, and %VU = 100 × max_deviation / V_avg — the NEMA definition. The readout also shows the recommended motor derate based on NEMA MG-1 curve.

If your %VU exceeds 1%, check for loose connections, imbalanced single-phase loads on the same panel, failed PFC capacitors, or transformer tap imbalance. Measuring current unbalance on the motor leads often reveals 6–10× the voltage unbalance — enough to double insulation temperature rise. Everything runs client-side; no values leave your browser.