Jason, I wasn't claiming that there is an SI standard for horsepower only one for the conversion as shown
here about 2/3rds of the way down the list.
horsepower (550 ft • lbf/s) watt (W) 7.456 999 E+02
But I think we are in agreement here.
Second, I really think that definitions are important when discussing pump and motors. The definitions below may have been sited before but I wanted to summarize everything in one place. These are terms often used to describe pumps and motors but are sometimes confusing.
Nameplate HP (NHP) - This is the HP rating on the motor nameplate but is pretty much meaningless without the service factor.
Nameplate KW = NHP * 0.7457 - This is the KW rating and is similar to the nameplate HP and is generally used outside the US. Note that this is not the input power to the motor only the rating for the output power of the motor.
Energy Horsepower (EHP) = watts / 745.7 = Volts * Amps * Power Factor / 745.7 - Energy input delivered to the motor
Brake Horsepower (BHP) = EHP * Motor Efficiency - Energy delivered by the motor shaft to the load. As will be explained below this is not the same as SFHP nor SF * NHP in general. BHP is a function of the load on the motor shaft and will change with Head, GPM and RPM.
Pumping HP (PHP) = BHP * Pumping Efficiency = Head (ft) * GPM / 3960 - Energy delivered to the water. Sometimes called water HP (WHP).
Motor Efficiency = BHP / EHP - I2R, magnetic and mechanical losses in the motor only.
Pumping Efficiency = PHP / BHP - Recirculation and internal friction losses in the wet end only.
Total Pump Efficiency = Motor Efficiency * Pumping Efficiency = PHP / EHP (note this is why total pump efficiency approaches 50%).
Service Factor - This is an overload rating for motors which states that the motor can be safely be operated over the NHP by the service factor for short periods of time. However, for pumps, this overload rating is typically used as the maximum load that a motor would need to deliver to the wet end. Because the load on a pump does not rapidly change over time, the service factor load is often used as the maximum design point for the pump.
Service Factor HP (SFHP) = NHP * Service Factor – This is the maximum load that can be safely be driven by the motor. This is not the same as BHP although they are equal at a single point on the head curve, usually the best efficiency point (BEP). BHP is the energy delivered by the motor shaft while SFHP is the motor rating. A motor can be driven above the SFHP but will likely fail in a shorter period of time. For pumps, the motor is usually driven at BHPs that are less the SFHP.
Full Load Amps - This can mean several things depending on the motor manufacture. It is either the amps at the NHP or it can be the amps at the SFHP. I have seen it both ways so unfortunately, there is not a good standard for this one.
Service Factor Amps - The amp draw when the motor is loaded to the service factor. This is not as ambiguous as the above rating. Also, using this with voltage should also give you a pretty good approximation for maximum power draw. However, sometimes the motor is over dimensioned for the pump so it will not always be an accurate measure of input power. This is also true of SFHP.
There are others but I think that the above definitions are the most important.
For further reading on this subject, I strongly recommend a web site put together by Joe Evans, a PHD from Pentair:
http://www.pumped101.com.