Calculate hydraulic power, shaft power and motor input power for centrifugal pumps. Includes standard IEC motor size recommendation with service factor.
Always verify that available NPSH (NPSHa) exceeds required NPSH (NPSHr) by at least 0.5 m safety margin.
NPSHa = (P_suction / ρg) + Z_suction − h_friction − P_vapour/ρg
Every centrifugal pump installation requires three power values to be calculated: hydraulic power (theoretical power transferred to the fluid), shaft power (power to the pump shaft accounting for pump efficiency), and motor input power (electrical power consumed accounting for motor efficiency). This tool calculates all three and recommends a standard IEC motor rating.
The recommended motor size includes a 20% service factor — the standard engineering margin ensuring the motor handles density variations, pump wear, and operation away from the best efficiency point without tripping the overload protection.
The minimum theoretical power to move the fluid: P_hyd = rho x g x Q x H. This is independent of pump or motor losses. A pump moving 100 m3/hr of water against 50 m total head requires approximately 13.6 kW of hydraulic power regardless of efficiency.
Power delivered to the pump shaft: P_shaft = P_hyd / pump_efficiency. Typical centrifugal pump efficiencies are 60 to 85% depending on specific speed, flow rate and impeller design. Read efficiency from the vendor pump curve at the design duty point.
Electrical power consumed: P_motor = P_shaft / motor_efficiency. Modern IEC motors achieve 92 to 96% efficiency. The motor is selected from the standard IEC rating series at or above the calculated motor power multiplied by the service factor.
For preliminary calculations: small pumps below 10 kW use 65 to 72%, medium pumps 10 to 100 kW use 72 to 80%, large pumps above 100 kW use 80 to 87%. Motor efficiency 92 to 95% for standard IE3 motors. These are conservative estimates — actual efficiency should come from the vendor curve.
TDH is the total head the pump must develop including: static head (elevation difference between suction and discharge), pressure head difference between vessels, and friction head losses through the piping system. Expressed in metres of fluid — independent of density, which is why it is preferred over pressure difference for pump sizing.
Net Positive Suction Head available (NPSHa) must exceed the pump's NPSHr (required) by at least 0.5 to 1.0 m margin. If NPSHa is insufficient, cavitation occurs — causing noise, vibration, erosion and premature impeller failure. Always verify NPSHa separately using the suction system hydraulics.
Power calculations follow the hydraulic equations per ISO 9906 — Rotodynamic pumps, Hydraulic performance acceptance tests. Motor sizing follows the IEC 60034 standard motor rating series. Always confirm final motor selection with the pump vendor certified performance curve and the site electrical engineering team.