Sump Pump Cycle Time and Level Switch Calculator
Set the start and stop levels for a sump or wet well pump. The calculator finds the cycle time and starts per hour for your inflow, and the working volume needed to stay within the motor start limit.
- Working volume V = A × Δh = 1 m³
- Cycle = V ÷ Qin + V ÷ (Qp − Qin) = 8 min
- Shortest cycle occurs at Qin = Qp ÷ 2: tmin = 4V ÷ Qp
- V needed = Qp × tmin ÷ 4 = 0.75 m³
Pump Cycle Formulas
Shortest cycle (Qin = Qp/2): tmin = 4V ÷ Qp
Working volume V = Qp × tmin ÷ 4
V is the working volume between start and stop levels, Qp the pump capacity, Qin the inflow and tmin the minimum cycle time allowed by the motor (3600 ÷ maximum starts per hour). This is the standard method for sizing wet well working volume.
Worked example
A 30 m³/h pump in a 2 m² sump with a 0.5 m start to stop band and 15 m³/h inflow cycles every 8 min (7.5 starts per hour). To keep under 10 starts per hour at any inflow the working volume must be 0.75 m³, a 375 mm band.
Frequently Asked Questions
How many starts per hour can a motor take?
It depends on motor size and type; small submersible pumps often allow 10 to 15 starts per hour, large motors fewer. Check the motor or pump data.
Why is half the pump capacity the worst case?
Fill time and run time are then equal and their sum, the cycle time, is shortest.
How do duty and standby pumps change this?
Alternating two pumps halves the starts per pump. Many controllers alternate automatically.
Learn more on the blog
Basics of level measurement
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