Work out the power a pump needs: hydraulic power, brake horsepower (shaft power), electrical input and the next standard motor size in kW or HP.
SI: Phydraulic (kW) = ρ × g × Q × H ÷ 1000 = 9.81 × SG × Q (m³/s) × H (m)
US: WHP = gpm × ft × SG ÷ 3,960; BHP = WHP ÷ pump efficiency
Shaft (brake) power is the hydraulic power divided by the pump efficiency; electrical input divides again by the motor efficiency. Example: 10 L/s at 40 m, 70% efficient: hydraulic 3.92 kW, shaft 5.61 kW; with a 15% margin a 7.5 kW motor.
Pick the next standard motor above the shaft power plus margin. For centrifugal pumps that may run out on their curve (open discharge, low system resistance), check the shaft power at maximum flow and size for that. Get the head from the Pump Head Calculator and check suction conditions with the Pump NPSH Calculator.
Water horsepower = gpm × head in feet × specific gravity ÷ 3,960. Divide by the pump efficiency for brake horsepower. In SI, kW = 9.81 × Q (m³/s) × H (m) × SG ÷ efficiency.
BHP is the power the pump shaft needs at the duty point. The motor is chosen from standard sizes above BHP, with a margin, so it is usually larger.
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