Voltage Unbalance Calculator

Calculate 3-phase voltage unbalance percentage, average voltage, maximum deviation and motor risk warning from measured phase-to-phase or phase-to-neutral readings.

⚡ Field Rule: Even small voltage unbalance can cause high current unbalance and motor heating. Use this with the future Motor Starting Current Calculator, future Contactor Size Calculator, Motor Current Calculator, Power Factor Calculator and Voltage Drop Calculator.

⚡ 3-Phase Voltage Balance Check

Voltage AB / L1-L2

Voltage BC / L2-L3

Voltage CA / L3-L1

Nominal Voltage Optional

Warning Limit

High Risk Limit

NEMA-style voltage unbalance = maximum deviation from average voltage ÷ average voltage × 100.

Presets:good supplywarningbad unbalance

Voltage L1-N

Voltage L2-N

Voltage L3-N

Nominal Phase Voltage

Warning Limit

High Risk Limit

Line-to-neutral readings are useful for checking phase balance and neutral shift in 4-wire systems.

Voltage Unbalance

Motor Rated Current

Measured Highest Line Current Optional

Service Factor / Margin

Voltage unbalance can produce much higher current unbalance. This is a practical warning check, not a full motor derating standard.

📐 Formula Reference

Average Voltage

Vavg = (V1 + V2 + V3) ÷ 3

Maximum Deviation

Max deviation = largest |Vphase - Vavg|

Voltage Unbalance

Unbalance % = max deviation ÷ average voltage × 100

Current Risk Rule

Current unbalance may be many times voltage unbalance

📋 Quick Reference

Practical Guide

Below 1%good

1–3%watch

Above 3%high risk

Possible Causes

Uneven loadscommon

Loose terminaldanger

Weak phasecheck

Motor Symptoms

Overheatinglikely

High currentphase

Tripoverload

📚 Engineering Notes

Small voltage unbalance can be seriousMotors may heat badly because current unbalance can be much higher than voltage unbalance.

Measure under loadVoltage may look normal without load. Measure at motor terminals while the motor is running.

Check current and protectionUse the Motor Current Calculator, future Motor Starting Current Calculator and Breaker Size Calculator.

Also check voltage dropIf one feeder/phase has poor connection or high impedance, use the Voltage Drop Calculator and inspect terminals/cables.

What is a Voltage Unbalance Calculator?

A voltage unbalance calculator checks how uneven three phase voltages are. It calculates average voltage, maximum deviation and voltage unbalance percentage for motor, panel and power distribution troubleshooting.

How voltage unbalance is calculated

The common method is to find the average of the three measured voltages, find the largest deviation from that average, then divide by the average voltage and multiply by 100.

Why voltage unbalance matters for motors

Three-phase motors are sensitive to unbalanced voltage. Even a small voltage imbalance can cause higher current imbalance, overheating, reduced torque and overload tripping.

Important limitation

This calculator is a troubleshooting aid. Final decisions should consider motor manufacturer limits, NEMA/IEC guidance, measured current, load condition, cable voltage drop, loose terminals, harmonics and supply quality.

❓ Frequently Asked Questions

Below 1% is generally good. Between 1% and 3% should be investigated, especially for motors. Above 3% can be risky and may require corrective action.

For three-phase motors, line-to-line voltage at motor terminals is usually the most useful. Line-to-neutral is useful in 4-wire systems and panel diagnostics.

Common causes include uneven single-phase loads, loose terminals, damaged cables, weak supply phase, bad contactor contacts, transformer issues or poor connections.