1 Answer

To solve for the horsepower in an accurate fashion you need several pieces of data.

For single phase motors use this method:

(volts x amps x motor efficiency x power factor) divided by 746

For three phase motors use this method:

(volts x amps x motor efficiency x power factor x 1.73) divided by 746

The motor efficiency and power factor are somewhat unknown unless you can obtain the motor efficiency from the manufacturer, and the power factor (for the electrical distribution) from the electric utility.

Since you may have to guess at these two variables you still may not get an entirely accurate answer. However, it might provide an idea of what you have.

Another method is to use a rule-of-thumb. For three phase motors on 460 volts the amperage will be approximately 1.5 amps per horsepower.

For three phase motors on 230 volts the amperage will be approximately 3.0 amps per horsepower.

If my memory is correct you can use this for single phase motors. You might want to check this out since it's been a LONG time for me to have worked on these small systems.

For single phase motors on 230 volts the amperage will be approximately 6 amps per horsepower.

For single phase motors on 115 volts the amperage will be approximately 12 amps per horsepower. 

Source > http://www.refrigeration-engineer.com/forums/showthread.php?5050-compressor-horse-power