Why does my calculated horsepower not match the motor nameplate?

Q: How do I calculate three-phase kW from volts and amps?

Use kW = √3 × V × I × PF ÷ 1000. This calculator performs the same relation and also reports apparent power in VA so you can see how power factor affects usable output.

Q: What is the difference between apparent power and true power?

Apparent power (VA) is volts times amps and includes reactive power. True power (W) is VA multiplied by power factor, representing the real work done by the load.

Q: Can I use this calculator to size motor branch-circuit wire?

Not by itself. NEC motor branch-circuit sizing uses table FLA values in Article 430, not only measured volts and amps. Use this tool for quick capacity checks and use the motor circuit calculator for code sizing.

Q: Do I enter line-to-line voltage or line-to-neutral voltage?

For typical three-phase equipment, enter line-to-line voltage (208 V, 480 V, etc.). Single-phase circuits are entered at their nominal circuit voltage such as 120 V or 240 V.

Q: Can I estimate generator loading with this?

Yes as a first pass. Use apparent power and true power outputs to compare against generator kVA and kW limits, then verify starting inrush and transient performance separately.

Question

Why does my calculated horsepower not match the motor nameplate?

Accepted Answer

Nameplate horsepower is rated mechanical output at specified conditions, while this tool estimates theoretical horsepower from measured electrical input and efficiency assumptions. Differences are normal when PF, efficiency, or load conditions vary.

Why does my calculated horsepower not match the motor nameplate?

Answers