Enter the three line-to-line voltages from your meter. The tool returns the NEMA MG-1 unbalance percentage, the motor derating factor from the MG-1 curve, and the estimated extra winding heating.
The NEMA definition is simple — maximum deviation from the average, divided by the average — but its effect on motors is vicious: a small voltage unbalance drives a current unbalance roughly 6–10× larger, and extra winding heating scales near 2 × (unbalance %)². A 3% unbalance means ~18% more heat in the hottest phase, which halves insulation life by the 10 °C rule. NEMA MG-1 assumes ≤1% in motor design, publishes a derating curve from 1–5%, and advises against operation beyond 5%.
Unevenly distributed single-phase loads on a three-phase service (the usual suspect), a blown fuse in a power-factor capacitor bank, loose or corroded connections heating one phase, single-phase welders or large 277 V lighting banks, and upstream utility issues. Measure at the motor terminals under load — unbalance at the service can grow or shrink along the feeder depending on where the single-phase loads tap off.
No — drop is all three phases sagging together along a conductor; unbalance is the phases disagreeing with each other. A circuit can pass every drop check and still cook a motor through unbalance.
That computes to about 2.3% — operable only with derating (~93% of nameplate) and worth investigating. Find the single-phase loading or connection problem before it finds your windings.