Calculation sequence
Step-by-step method
- Average the three line-to-line voltage readings.
- Find the largest absolute deviation from the average.
- Divide the maximum deviation by the average and compare with the entered limit.
CommissioningBeginnerElectrical
Electrical Phase-Imbalance Calculator
Calculate line-voltage unbalance using maximum deviation from the three-reading average.
Why it matters: Voltage unbalance can produce much larger motor-current unbalance and should be documented with phase-specific readings.
Calculated result
Engineering evaluation
Maximum line-voltage unbalance (U)0.625 %
- Average line voltage (Vavg)
- 480 V
- Maximum deviation from average (Dmax)
- 3 V
Engineering interpretation
Maximum line-voltage unbalance is 0.625% at an average of 480 V. Review per-phase currents, source conditions, connections, load distribution, and the applicable equipment requirements.
Motor-driven equipment
Estimate electrical demand and energy use
Use nameplate information and an operating schedule to screen motor input power, current, annual energy, and operating cost. Results update automatically.
Choose the motor nameplate output-power convention.
Rated shaft output, not electrical input.
Used to estimate current; it does not change the hp-based energy result.
Use efficiency at the evaluated load point when available.
Use measured or defensible operating load rather than motor nameplate size alone.
Used only for the estimated current calculation.
Assumes all selected motors operate concurrently at the entered load.
Replace the profile with the actual occupied, process, or control schedule.
$/kWh
Use the applicable marginal utility energy charge; demand charges are not included.Estimated result
Total motor input power (Pin,total)6.21 kW
- Current per motor (I)
- 9.18 A
- Total coincident current (Itotal)
- 9.18 A
- Daily energy (Ed)
- 50 kWh/day
- Annual operating hours (Hy)
- 2,080 h/year
- Annual energy (Ey)
- 12,925 kWh/year
- Annual energy cost (Cy)
- $1,551/year
Equations and assumptions
Nameplate-based screening method
Horsepower is converted using 1 hp = 0.7457 kW. This estimate assumes steady average load and the entered operating-point efficiency. Voltage and power factor affect estimated current, but hp, load, efficiency, quantity, and schedule establish the energy estimate. VFD speed profiles, cycling, standby power, demand charges, and process variation require separate evaluation.
Technical basis: U.S. Department of Energy — Determining Electric Motor Load and EfficiencyEngineering knowledge center
Understand the calculation—not just the answer
Governing relationships
Vavg = (Vab+Vbc+Vca)/3
Dmax = max(|Vab−Vavg|,|Vbc−Vavg|,|Vca−Vavg|)
U = Dmax/Vavg×100
within entered criterion when U ≤ L
Current substitution
Vavg = (480+477+483)/3 = 480 V; Dmax = 3 V; U = 0.625%.
Equation legend
Inputs and calculated quantities
- Vab
- Line voltage A–B — V
- Vbc
- Line voltage B–C — V
- Vca
- Line voltage C–A — V
- L
- Entered unbalance limit — %
- U
- Maximum line-voltage unbalance — %
- Vavg
- Average line voltage — V
- Dmax
- Maximum deviation from average — V
Applications
How this calculation is used
Building design
Screen expected supply quality requirements.
Construction
Evaluate distribution readings during startup.
Commissioning
Calculate measured voltage unbalance against an entered project or manufacturer limit.
Quality controls
Assumptions and limitations
Assumptions
- Entered values represent the same stable operating condition.
- Measurements and reference values use consistent units and boundaries.
Limitations
- This is a screening and field-verification tool, not an automatic acceptance decision.
- Project criteria, manufacturer data, governing standards, and measurement uncertainty remain controlling.
Field use
Verification procedure
- Confirm the approved reference values and test boundary.
- Stabilize the system and record calibrated field measurements.
- Review calculation assumptions, uncertainty, and supporting evidence before disposition.
Common engineering mistakes
- Comparing readings taken at different loads, modes, or reference conditions.
- Treating a calculated status as proof of code, contract, or manufacturer compliance.
Technical basis
References and source standards
- NEMA StandardsElectrical equipment terminology, ratings, motor, transformer, and generator performance context; the current applicable standard requires professional review.
- ASHRAE Commissioning ResourcesCommissioning-process context for defining test methods, acceptance criteria, evidence, and owner requirements.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement