Well I guess you could calculate your hydraulic horsepower requirement using your flow and the delta P across the pump - (flow (in gpm) x delta P (psi))/1714 = hydraulic horsepower. Assuming a typical efficiency of 70% will give you the brake horsepower. Then Power = Voltage x Current. You should know the supply voltage, you just calculated your power requirements, so then you can calculate the current.
SeanB's answer is correct in methodology but the electrical power calculation is probably incorrect (unless you happen to have a DC motor driving your pump).
The methodology described mentions the pump efficiency (f you have the curves, use them). Motor efficiency also needs to be taken into account.
The formula I have for electric horsepower is:
P= volts x amps x power factor x 1.73 / 746 for 3-phase. You drop the 1.73 for single phase.
how about requesting from motor mfg a copy of motor data sheets &/or performance curves. then determine pump HP; thus determine motor amps from curves/data sheets.
The best thing you could do would be to have one of your Elect/Instrumentation Technicians go out and actually read the pumps instantaneous amp draw. At the same time have pressure gauges installed on the pump suction and discharge and also record the pumps discharge flowrate, (if the pump has one, don't forget to take it's recycle or minimum flow into account as well). Also, get a copy of the pump's curve. You will then have all the data you need. Make sure you get the Technician to tell you the actual voltage going to the motor as well.
