Motor Current Unbalance Calculator

A three-phase motor current unbalance calculation converts three measured line currents into a percentage showing how far the most unequal phase is from the three-phase average. The result supports maintenance and commissioning review when a motor is operating under a recorded load condition.

Current unbalance is calculated from Phase A current, Phase B current, and Phase C current. The calculator reports:

• Average current
• Maximum deviation
• Current unbalance
• Margin vs entered reference, when an optional maintenance, project, or manufacturer reference is entered

The result is useful for comparing repeated motor readings, documenting commissioning conditions, and identifying a phase-current pattern that warrants further electrical investigation. It does not diagnose the motor, select a repair, establish motor conductor ampacity, or determine branch-circuit or feeder sizing.

Recorded Phase Currents

Enter the three current readings exactly as measured and identified in the field:

Input	Unit	Electrical use
Phase A current	A	Current recorded on phase A
Phase B current	A	Current recorded on phase B
Phase C current	A	Current recorded on phase C
Entered reference unbalance	%	Optional project, manufacturer, or maintenance reference
Measurement note	—	Record instrument, phase labels, and load condition separately

The phase labels must remain consistent with the motor starter, disconnect, VFD output labeling where applicable, panel schedule, and maintenance records. A reading marked Phase A on one inspection and Phase B on a later inspection cannot be reliably trended as the same phase.

Use measurements taken under a stated operating condition. Motor current changes with mechanical load, supplied voltage, motor speed, process demand, and operating state. A reading at reduced load or during startup is not directly comparable to a stable full-load operating reading unless the condition is documented.

Current Unbalance Formula

The calculator first finds the arithmetic average of the three measured currents:

\(\displaystyle I_{\text{average}} = \frac{I_A + I_B + I_C}{3}\)

It then identifies the largest absolute difference between any phase current and the average:

\(\displaystyle \text{Maximum deviation} = \max \left( |I_A-I_{\text{average}}|, |I_B-I_{\text{average}}|, |I_C-I_{\text{average}}| \right)\)

The motor current unbalance percentage is:

\(\displaystyle \text{Current unbalance} = \frac{\text{Maximum deviation}}{I_{\text{average}}} \times 100\)

This is a current-based comparison. It is not a voltage-unbalance calculation, a power-factor calculation, a load calculation, or a determination of motor efficiency.

Calculation Example

For a recorded three-phase motor current set:

Input	Value
Phase A current	10 A
Phase B current	11 A
Phase C current	12 A
Entered reference unbalance	0%

The average current is:

\(\displaystyle \frac{10 + 11 + 12}{3} = 11\text{ A}\)

The deviations from the 11 A average are:

Phase	Measured current	Deviation from average
Phase A	10 A	1 A
Phase B	11 A	0 A
Phase C	12 A	1 A

The maximum deviation is 1 A. The resulting current unbalance is:

\(\displaystyle \frac{1\text{ A}}{11\text{ A}} \times 100 = 9.0909\%\)

Result:

• Average current: 11 A
• Maximum deviation: 1 A
• Current unbalance: 9.0909%
• Margin vs entered reference: 0%

An Entered reference unbalance of zero leaves the result unassigned. Enter a nonzero reference only when a project requirement, motor manufacturer instruction, or established maintenance criterion is available for the equipment and operating condition being reviewed.

Field Interpretation

A current-unbalance result identifies unequal current distribution; it does not by itself identify the cause. The measured pattern may be associated with supply-voltage conditions, unequal phase impedance, loose or degraded connections, contactor or overload-device issues, VFD output conditions, winding problems, mechanical loading, or a measurement issue. Field evaluation requires supporting readings and equipment-specific inspection.

For useful maintenance records, document:

• Clamp meter or power analyzer used, including its operating range and measurement method
• Phase labels at the measurement point
• Motor nameplate data and controller arrangement
• Supply or output voltage readings, when part of the troubleshooting scope
• Motor operating load, process condition, and speed
• Whether readings were taken at the line side, load side, motor leads, or VFD output
• Ambient conditions and any observed heating, noise, vibration, or protective-device operation

A three-phase motor can show balanced current at one load condition and materially different current under another condition. Trending should therefore compare like-for-like readings rather than isolated percentages.

Installation and Code Limits

The calculation is arithmetic based on three recorded currents. It does not determine conductor AWG or kcmil size, branch-circuit ampacity, feeder ampacity, overload setting, short-circuit and ground-fault protection, raceway fill, voltage drop, terminal rating, insulation temperature rating, adjustment factor, correction factor, or the number of current-carrying conductors.

Any decision involving motor branch-circuit conductors, feeder conductors, disconnects, overload protection, or overcurrent protection must be evaluated separately using the applicable installation requirements, equipment instructions, available fault current, conductor terminations, ambient temperature, raceway conditions, and AHJ requirements.

Where the motor is supplied by a VFD, use an instrument and method suitable for the drive output waveform, and record whether readings were taken on the input or output side of the drive. Current readings from those locations serve different electrical purposes and should not be treated as interchangeable.