3 Phase Current and Voltage Unbalance Calculator
Enter three phase currents or voltages to get the percent unbalance using the NEMA MG-1 method: average, maximum deviation, and the phase that deviates most. Voltage mode also returns the MG-1 motor derating factor, so you can see at a glance whether a motor can run at full nameplate load.
Take all three readings simultaneously, or under steady load, with a true-RMS meter. Readings taken minutes apart on a changing load can show unbalance that is not really there. This is a screening tool, not a substitute for a motor or power quality assessment.
Need this validated by a licensed PE?
A free calculator gets you in the ballpark. For permit-stamped, defensible work, ClarkTE delivers a PE-stamped motor and power quality assessment from a working engineer, based on your one-line and load data.
How to calculate phase unbalance with the NEMA MG-1 formula
NEMA MG-1 defines unbalance using only the three measured magnitudes: % unbalance = (max deviation from average ÷ average) × 100. The same formula works for current and for voltage, which is why it is the standard field method for a phase balance calculation.
Worked examples
- Current: 98 A, 102 A, 110 A. Average = 310 ÷ 3 = 103.33 A. Deviations are 5.33 A, 1.33 A, and 6.67 A, so the maximum is 6.67 A on phase C. Unbalance = 6.67 ÷ 103.33 × 100 = 6.45%, in the investigate band.
- Voltage: 470 V, 480 V, 476 V. Average = 1426 ÷ 3 = 475.33 V. Deviations are 5.33 V, 4.67 V, and 0.67 V, so the maximum is 5.33 V on phase A. Unbalance = 5.33 ÷ 475.33 × 100 = 1.12%. Interpolating between 0.98 at 1% and 0.95 at 2% gives a derating factor of 0.976.
Why unbalance heats motors
Any unbalanced set of three-phase voltages can be split into a balanced positive-sequence set, which produces useful torque, and a negative-sequence set, which produces a field rotating backwards. The rotor sees that reverse field at nearly twice line frequency and responds with a low impedance, close to its locked-rotor value. The result is a negative-sequence current that adds heat in the stator and rotor without adding any useful torque. Because heating rises with the square of current, a modest voltage unbalance can raise winding temperature well beyond what the percentage suggests.
Common causes
- Single-phase loads unevenly distributed across the three phases
- A high-resistance connection at a lug, splice, or terminal
- A failing contactor or starter pole
- Unbalance in the utility supply itself
- A blown capacitor fuse in a power factor correction bank
Current and voltage unbalance: frequently asked questions
How do you calculate 3 phase current unbalance?
Use the NEMA MG-1 definition: average the three phase currents, find the phase that deviates furthest from that average, and divide that maximum deviation by the average. Multiply by 100 for percent. For 98 A, 102 A, and 110 A the average is 103.33 A, the largest deviation is 6.67 A on phase C, and the unbalance is 6.67 ÷ 103.33 × 100 = 6.45%.
What is an acceptable voltage unbalance for a motor?
NEMA MG-1 recommends that motors not be operated above 1% voltage unbalance without derating, and it does not recommend operation above 5% at all. Between those points the motor must be derated along the MG-1 curve, for example to roughly 0.95 of nameplate at 2% and 0.88 at 3%.
What is an acceptable current unbalance?
There is no single standard limit for current unbalance. As a rule of thumb, under 5% is typical for a healthy motor or feeder, 5% to 10% is worth investigating, and above 10% points to a problem that should be found before continued operation. Always check the voltage balance first, because a small voltage unbalance drives a much larger current unbalance.
Why is current unbalance so much larger than voltage unbalance on a motor?
An induction motor presents a low impedance to negative-sequence voltage, roughly its locked-rotor impedance. A small negative-sequence voltage therefore drives a large negative-sequence current. Current unbalance is commonly cited at 6 to 10 times the voltage unbalance that causes it, so 2% voltage unbalance can produce 12% to 20% current unbalance.
Is the NEMA unbalance formula the same as the IEC or IEEE definition?
No. IEC and IEEE define voltage unbalance factor as the ratio of negative-sequence to positive-sequence voltage, calculated with symmetrical components. The NEMA MG-1 method uses only magnitudes and is easier to apply in the field. The two methods agree closely at low unbalance but can differ slightly, so note which definition a specification uses.
Want to do this for a living?
If you are running motor and phase balance checks by hand, you already have the instincts. ClarkTE trains technicians and test engineers on the equipment these numbers describe — taught by working engineers, not a content mill.
See the training trackRelated ClarkTE calculators
kW, kVA, kVAR, and full load current, plus an unbalanced-load tab for per-phase loading.
Line current in amps from kW, kVA, or motor horsepower at any voltage.
Positive, negative, and zero sequence from phasor readings (the IEC unbalance method).
Recorded voltage and current unbalance, harmonics, and sags from a site study.