How to size a sump drainage pump
A sump pump has to clear the peak inflow reliably without being oversized. An oversized pump empties the basin in seconds and then short cycles, which is the single most common cause of burnt motors and welded contactor tips. This calculator starts from your design inflow, applies a safety factor to set the pumping rate, and adds static lift to pipe friction loss to get the total dynamic head the pump has to work against.
Power follows the standard hydraulic relationships used throughout US practice: water horsepower = GPM × head in feet × specific gravity ÷ 3,960, and brake horsepower is that figure divided by pump efficiency. Small submersible sump pumps typically run 50 to 70% efficient. Adding a motor sizing allowance of 10 to 20% and rounding up to the next standard NEMA motor rating gives the nameplate horsepower to specify.
Effective sump volume is the design flow multiplied by the retention time. Around 10 minutes of storage keeps the number of starts per hour inside the range most motor manufacturers allow, though check the specific pump curve since many submersibles limit starts to roughly 10 per hour. For stormwater or fire drainage, where inrush can be sudden and flooding damage severe, plan on duplex pumps that alternate and can run together in an emergency, and have the final selection reviewed by a mechanical engineer.
Frequently asked questions
Usually not. An oversized pump empties the basin almost instantly and short cycles, overheating the motor and destroying contacts. Matching the pump to the inflow with adequate sump volume matters more.
For parking garages, electrical rooms and anywhere flooding is costly, two identical pumps that alternate and can run simultaneously in an emergency is the normal arrangement.