The calculation, without hidden assumptions
Check a three-phase supply from three consistent line-to-line RMS readings. MAXScanner shows the average, each deviation, the worst phase pair, percent unbalance and an optional nominal-voltage comparison without confusing NEMA unbalance with voltage regulation or IEC negative-sequence unbalance.
How to use this calculator
Measure Vab, Vbc and Vca at the same location and operating condition using consistent line-to-line RMS readings.
Enter all three measured voltages; optionally enter the system nominal voltage for a separate average-voltage comparison.
Use the maximum-deviation result for NEMA-style motor voltage-unbalance screening.
Investigate uneven single-phase loading, supply issues or high-resistance connections when unbalance is elevated, then re-measure under representative load.
Use the equipment manufacturer or specified power-quality standard when a different unbalance definition or derating curve is required.
Where people use it
- •Screening motor-terminal voltage unbalance during preventive maintenance.
- •Comparing before/after readings when balancing single-phase loads.
- •Finding which line-to-line reading is furthest from the three-reading average.
- •Separating voltage unbalance from simple deviation of average voltage from nominal.
Example: 460 V, 467 V and 450 V
Average = 459 V. Deviations are 1 V, 8 V and 9 V, so maximum deviation = 9 V. NEMA voltage unbalance = 9 ÷ 459 × 100 ≈ 1.961%.
What the result does not assume
- •Use three consistent line-to-line readings for this NEMA maximum-deviation method; do not mix line-to-line and line-to-neutral measurements.
- •This is not the IEC negative-sequence voltage-unbalance-factor calculation. Instruments using symmetrical components can report a different value.
- •The commonly referenced 1% motor threshold is a screening boundary, not permission to infer a specific derating factor. Use the current motor manufacturer/NEMA guidance for derating.
- •A single spot reading may miss load-dependent problems; trend or repeat measurements under representative operating conditions when diagnosis matters.
- •This calculator does not diagnose phase sequence, harmonics, loose connections or insulation condition by itself.
Frequently asked questions
What is the NEMA voltage unbalance formula?+
Calculate the average of the three line-to-line voltages, find the largest absolute deviation from that average, divide it by the average and multiply by 100.
Is voltage unbalance the same as voltage variation from nominal?+
No. Unbalance compares the three phase-to-phase readings with their own average. Nominal deviation compares the average operating voltage with a rated or nominal voltage.
Should I use line-to-line voltages?+
Yes for this calculator. Enter Vab, Vbc and Vca measured consistently at the same location and condition.
Why does a small voltage unbalance matter to motors?+
Three-phase motors can develop current unbalance much larger than the voltage unbalance, increasing losses, heating, vibration and stress.
Is 1% a hard trip setting?+
No. It is a commonly referenced NEMA motor operating threshold. Protection settings and derating must follow the actual motor, relay and applicable standard.
Is the max-min spread divided by average the NEMA formula?+
No. This owner uses maximum single deviation from the average, not the full highest-to-lowest spread.
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