Three-phase • RMS • Line-to-line

Three-Phase Voltage Unbalance Calculator

Enter simultaneous RMS line-to-line voltages. The calculator applies the NEMA maximum-deviation method and shows the complete calculation.

Step 1

Enter line-to-line voltages

All three values must be simultaneous RMS readings from the same system condition.

Line R to line Y.

Line Y to line B.

Line B to line R.

Step 2

Calculation result

Enter all three voltages to see the unbalance calculation.

Method

1. Vavg = (R-Y + Y-B + B-R) ÷ 3.

2. Find each absolute deviation from Vavg.

3. Voltage unbalance % = 100 × maximum deviation ÷ Vavg.

Band selection uses the full calculated value, not the displayed rounded percentage.

Reference bands

At or below 1%: NEMA reference for successful rated-load operation of AC polyphase motors, while performance may differ from balanced-voltage operation.

Above 1% to 5%: investigate and follow manufacturer guidance.

Above 5%: DOE guidance reports that NEMA recommends against motor use.

Limits

This result uses only three simultaneous RMS line-to-line voltage values. It does not assess other electrical quantities, harmonics, transient conditions, cause, equipment condition, protection settings, or product suitability.

Use the equipment manufacturer's requirements and applicable electrical standards for decisions. The calculator is not a substitute for qualified electrical assessment.

Sources
NEMA MG 1 Part 7 — percent voltage-unbalance definitionNEMA MG 1 Part 12 — 1% rated-load referenceU.S. Department of Energy — Energy Management for Motor-Driven Systems

Calculate three-phase voltage unbalance from R-Y, Y-B and B-R readings and identify the highest deviation from average voltage.

What to enter

Use current, project-specific values for R-Y, Y-B and B-R voltage readings. Check every unit before calculating and compare like-for-like inputs. The template controls the actual fields and calculations; this supporting copy does not replace its labels, formulas or result logic.

How to use the result

Use the unbalance and highest-deviation result as a measurement screen. It does not diagnose the source or authorize live electrical work; verify readings and investigate safely.

Before making a decision

Verify source measurements, equipment specifications and operating conditions. Have a qualified professional review the final selection, safety and installation requirements before purchase, connection or construction.