Troubleshooting guide

Why Are My Three-Phase Motor Currents Unequal?

Unequal motor currents come from voltage unbalance, connections or motor faults. Includes the percentage-unbalance calculation.

Electrical & Power · Last reviewed October 17, 2026

Even a healthy motor on a healthy supply draws slightly different currents in its three phases. A large difference points to the supply, a connection or the motor itself, and the readings give a way to tell which.

Current and voltage measurements on energized motor circuits carry a shock and arc-flash risk. They belong with qualified personnel using the right protective equipment and following site procedures. Never measure inside an energized enclosure without the training and equipment.

What you observe	Likely cause	What to check
Voltage unbalance at the motor, the high-current phase changes if the supply connections are rotated	The supply is unbalanced	Voltage on all three phases. Single-phase loads on the same transformer or bus
Same phase always high, even after rotating the leads	Motor winding problem (shorted turns or another winding fault)	Winding resistance and insulation tests with the motor isolated
One phase high and one low, voltages equal	A high-resistance connection (loose lug, worn contactor contacts)	Thermal imaging under load, then torque and contact inspection de-energized
Current very low in one phase	An open or nearly open phase (blown fuse, bad contact)	Fuses, contactor and overload relay on that phase
Unbalance varies with the plant load	Single-phase loads taking more from one phase of the supply	The load distribution on the upstream panel and transformer
Unbalance after a drive or capacitor change	A drive input problem or a power-factor capacitor fault	Drive input currents and the capacitor bank condition

The percentage-unbalance calculation

The usual definition (used by NEMA) is the largest deviation from the average, divided by the average. Take line-to-line voltages of 470 V, 462 V and 448 V:

Average = (470 + 462 + 448) ÷ 3 = 460 V     Largest deviation = 12 V (448 V)     Unbalance = 12 ÷ 460 = 2.6%

The line currents at the motor are 54 A, 50 A and 40 A:

Average = 48 A     Largest deviation = 8 A     Current unbalance = 8 ÷ 48 = 16.7%

The current unbalance is about 6.4 times the voltage unbalance. That is a typical range: a small voltage unbalance produces a much larger current unbalance, because the motor has a low impedance to the negative-sequence component of the supply. The extra current heats the windings.

Why it matters

NEMA MG 1 advises limiting voltage unbalance to 1% where possible, and derating the motor above that. At about 2.6% the allowed load is typically reduced to roughly 90% of rated (check the derating curve for the motor).
The heating from unbalance is in the winding and does not show up on a simple average current reading.
Rotating the supply leads one position and repeating the measurements helps to separate a supply effect from a motor effect: the high-current phase follows the supply if the cause is voltage.

Write a useful problem statement

Motor rating, nameplate current and the load it carries.
Line-to-line voltages and line currents on all three phases, with the time and load.
What happens to the readings after the leads are rotated.
Single-phase loads on the same source, and recent changes.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

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Questions

What is an acceptable current unbalance?

There is no single number. Many sites investigate above 5 to 10% current unbalance, and they check the voltage unbalance first, because voltage is what the motor responds to.

Does a drive fix an unbalanced supply?

A drive supplies the motor with balanced voltages at its output, but the drive input still sees the unbalance and can trip or overheat.

Can I balance the supply myself?

Rebalancing loads on a panel or transformer is a design and safety task for qualified personnel, and the utility may need to be involved.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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