Motor Voltage Unbalance Derating Calculator
Enter the three line-to-line voltages measured at a running motor, then compare the actual load with the NEMA MG 1 voltage-unbalance derating curve. The result shows whether the motor is in the normal range, needs derating, is overloaded after derating, or is outside the encoded curve.
Operating assessmentDerating required: voltage unbalance is above 1%, but the entered load is within the derated limit.
- Average line voltage
- 460
- Maximum voltage deviation
- 8
- Percent voltage unbalance
- 1.74%
- NEMA derating factor
- 96.3%
- Maximum recommended motor load (hp)
- 48.15
- Load margin after derating (hp)
- 3.15
- Derated load utilization
- 9,345.37%
Encoded NEMA MG 1 derating curve
| Voltage unbalance | Derating factor |
|---|---|
| 0% | 100% |
| 1% | 100% |
| 2% | 95% |
| 3% | 88% |
| 4% | 82% |
| 5% | 75% |
Uses the NEMA percent voltage unbalance method on three line-to-line RMS readings.
The derating curve is linearly interpolated and is not extrapolated above 5% unbalance.
How to use this calculator
- Measure A-B, B-C, and C-A RMS line-to-line voltage at the motor terminals while the motor is operating.
- Enter the motor nameplate power and the estimated shaft load in the same unit.
- Choose horsepower or kilowatts for both power entries.
- Read the derating factor, maximum recommended load, load margin, and operating assessment.
How the derating calculation works
This calculator starts with the NEMA percent voltage unbalance method for three-phase motors. Let Vab, Vbc, and Vca be the three measured line-to-line voltages. The average line voltage is A = (Vab + Vbc + Vca) / 3. The maximum deviation is D = max(|Vab - A|, |Vbc - A|, |Vca - A|). Percent voltage unbalance is U = 100D / A.
The motor load factor is then read from an encoded NEMA MG 1 derating curve. The curve points used here are 100% load at 0% and 1% unbalance, 95% at 2%, 88% at 3%, 82% at 4%, and 75% at 5%. Between those points, the calculator uses straight-line interpolation: F = F1 + (U - U1)(F2 - F1) / (U2 - U1).
Nameplate power and actual shaft load must use the same unit. If R is the nameplate power, the maximum recommended load is M = R x F. The load margin is H = M - L, where L is the estimated actual shaft load. Derated load utilization is Q = 100L / M. A negative margin or utilization above 100% means the motor is loaded above the derated limit.
What moves the result most
The largest deviation from the average voltage controls the unbalance value. One low or high phase-to-phase reading can move the result even when the other two readings look close. Because the derating curve stays at 100% through 1% unbalance and then falls to 75% by 5%, small measurement changes around the 1% threshold can change whether a derating warning appears.
The actual-load entry does not change the NEMA factor. It changes the operating assessment by comparing the present shaft load with the derated maximum. A lightly loaded motor may remain within the derated load even with moderate unbalance, while a fully loaded motor can exceed the limit as soon as derating begins.
What this calculator leaves out
The calculation does not diagnose why the voltages are unbalanced. Common causes include unequal single-phase loading, transformer tap differences, supply problems, loose connections, and high-resistance conductors. It also does not model motor-specific temperature rise, harmonics, ambient temperature, altitude, duty cycle, cooling, service factor, or manufacturer-specific limits.
Do not extrapolate the load factor above 5% voltage unbalance. This calculator reports the derating factor and maximum load as outside the NEMA curve when U is greater than 5%, because that condition should be corrected rather than justified with a calculated load allowance.
Worked example
Suppose the motor terminals measure 460 V A-B, 468 V B-C, and 452 V C-A. The average voltage is (460 + 468 + 452) / 3 = 460 V. The largest deviation from average is 8 V, so the voltage unbalance is 100 x 8 / 460 = 1.74%.
For a 50 hp motor carrying an estimated 45 hp shaft load, the calculator interpolates between the 1% and 2% curve points. The derating factor is 96.30%, so the maximum recommended load is 48.15 hp. The load margin is 3.15 hp and derated utilization is 93.45%, so derating is required but the entered load is still within the derated limit.
Common questions
How does NEMA calculate three-phase voltage unbalance?
NEMA percent voltage unbalance uses the three line-to-line RMS voltages. Average the three readings, find the largest absolute deviation from that average, then divide that deviation by the average and multiply by 100. The calculation should use readings taken under the same operating condition.
At what voltage unbalance must a motor be derated?
The encoded NEMA MG 1 curve used here stays at 100% load through 1% voltage unbalance. Above 1%, the load factor decreases along the curve. Above 5%, this calculator does not extrapolate a permissible motor load.
How much motor capacity is available at 2%, 3%, or 5% unbalance?
Using the curve in this calculator, available load is 95% of nameplate at 2% voltage unbalance, 88% at 3%, and 75% at 5%. Values between published curve points are linearly interpolated. The actual safe operating point can be more restrictive if the motor manufacturer gives different instructions.
Why can small voltage unbalance cause larger current unbalance?
Three-phase motors are sensitive to negative-sequence voltage, which creates heating and current effects out of proportion to the voltage difference. A voltage unbalance that looks small on a meter can therefore produce a much larger current unbalance. This is why the derating curve begins just above 1% voltage unbalance.
Where should the three voltages be measured?
Measure A-B, B-C, and C-A line-to-line voltage at the motor terminals with the motor running under representative load. Do not mix line-to-neutral and line-to-line readings. Measurements taken with the motor stopped may miss voltage drop and connection problems that appear under load.
Can motor service factor replace voltage-unbalance derating?
Do not assume service factor cancels voltage-unbalance derating. Service factor, temperature rise, insulation class, ambient conditions, and duty cycle are motor-specific constraints. Use the manufacturer instructions or a qualified engineering review when service factor is part of the decision.