Calculation sequence
Step-by-step method
- Measure true three-phase input power at the operating point.
- Estimate shaft output using efficiency appropriate to the actual load.
- Divide estimated shaft output by nameplate rated output.
ElectricalIntermediateMotors and Drives
Motor Loading Calculator
Estimate shaft loading from measured true input power, estimated operating efficiency, and motor nameplate output rating.
Why it matters: Current alone can misrepresent load. Measured real power combined with a defensible efficiency estimate provides a stronger field screening method.
Calculated result
Engineering evaluation
Estimated motor loading (Load)68.4 %
- Estimated shaft output (Pout)
- 7.65 kW
- Estimated shaft output (Pout)
- 10.3 hp
- Entered efficiency (η)
- 90 %
Engineering interpretation
Estimated motor load is 68.4% of nameplate output. Efficiency varies with load, voltage quality, rewind history, and motor design; use manufacturer data or testing for higher confidence.
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
Pout = Pin × η
Load% = [Pout / (Prated × 0.7457)] × 100
Current substitution
Load% = [Pin × η / (Prated × 0.7457)] × 100 = [8.5 × 0.9 / (15 × 0.7457)] × 100 = 68.4%.
Equation legend
Inputs and calculated quantities
- Pin
- Measured input real power — kW
- Prated
- Nameplate rated output — hp
- η
- Estimated efficiency at load — %
- Load
- Estimated motor loading — %
- Pout
- Estimated shaft output — kW
- Pout
- Estimated shaft output — hp
- η
- Entered efficiency — %
Applications
How this calculation is used
Building design
Screen motor utilization and replacement opportunities.
Construction
Evaluate startup power relative to nameplate rating.
Commissioning
Estimate motor loading while documenting the efficiency assumption and measurement boundary.
Quality controls
Assumptions and limitations
Assumptions
- Measured kW represents motor input at the stated boundary.
- Entered efficiency is appropriate to actual load and condition.
Limitations
- Efficiency is load dependent.
- VFD losses must be treated consistently with the measurement point.
- Does not determine service-factor compliance or thermal condition.
Field use
Verification procedure
- Record nameplate data and operating mode.
- Measure true three-phase kW with a rated analyzer.
- Use manufacturer efficiency at estimated load when available.
- Compare with driven-equipment performance and process demand.
Common engineering mistakes
- Estimating load from current alone.
- Using full-load efficiency at very light load without review.
- Mixing drive input power with motor-only efficiency.
Technical basis
References and source standards
- U.S. Department of Energy—Energy Management for Motor-Driven SystemsChapter 5, motor load and efficiency estimation techniques.
- NEMA MG 1Motor ratings and performance; current edition requires professional review.
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