Calculation sequence
Step-by-step method
- Confirm the calculation boundary, units, and source of every input.
- Apply the displayed equation to the entered values.
- Review the result against governing criteria and documented uncertainty.
PlumbingIntermediateDrainage Pumps
Duplex Drainage-Pump Drawdown Calculator
Compare working-volume drawdown time with the lead pump and with both pumps operating.
Why it matters: A transparent formula keeps the user-entered hydraulic, gas, or storage basis available for technical review.
Calculated result
Engineering evaluation
Lead-pump drawdown time (t_1)6.25 min
- Dual-pump drawdown time (t_12)
- 2.778 min
Engineering interpretation
The entered working volume drains in 6.25 minutes with the lead pump or 2.777778 minutes with both pumps.
Important limitations
- Constant-volume-flow screen only. Pump/system curves, check-valve return, variable inflow, minimum submergence, control delays, motor starts, redundancy, and emergency storage are omitted.
Motor-driven equipment
Estimate electrical demand and energy use
Use nameplate information and an operating schedule to screen motor input power, current, annual energy, and operating cost. Results update automatically.
Choose the motor nameplate output-power convention.
Rated shaft output, not electrical input.
Used to estimate current; it does not change the hp-based energy result.
Use efficiency at the evaluated load point when available.
Use measured or defensible operating load rather than motor nameplate size alone.
Used only for the estimated current calculation.
Assumes all selected motors operate concurrently at the entered load.
Replace the profile with the actual occupied, process, or control schedule.
$/kWh
Use the applicable marginal utility energy charge; demand charges are not included.Estimated result
Total motor input power (Pin,total)6.21 kW
- Current per motor (I)
- 9.18 A
- Total coincident current (Itotal)
- 9.18 A
- Daily energy (Ed)
- 50 kWh/day
- Annual operating hours (Hy)
- 2,080 h/year
- Annual energy (Ey)
- 12,925 kWh/year
- Annual energy cost (Cy)
- $1,551/year
Equations and assumptions
Nameplate-based screening method
Horsepower is converted using 1 hp = 0.7457 kW. This estimate assumes steady average load and the entered operating-point efficiency. Voltage and power factor affect estimated current, but hp, load, efficiency, quantity, and schedule establish the energy estimate. VFD speed profiles, cycling, standby power, demand charges, and process variation require separate evaluation.
Technical basis: U.S. Department of Energy — Determining Electric Motor Load and EfficiencyEngineering knowledge center
Understand the calculation—not just the answer
Governing relationships
t_1=V/(Q_1−Q_i)
t_12=V/(Q_1+Q_2−Q_i)
Current substitution
t_1=6.25 min; t_12=2.777778 min.
Equation legend
Inputs and calculated quantities
- V
- Working drawdown volume — gal or L
- Q_i
- Continuing inflow — gpm or L/s
- Q_1
- Lead-pump flow — gpm or L/s
- Q_2
- Lag-pump flow — gpm or L/s
- t_1
- Lead-pump drawdown time — min
- t_12
- Dual-pump drawdown time — min
Applications
How this calculation is used
Building design
Screen a documented system relationship without embedding a protected design table.
Construction
Check submittal and field values using the same visible inputs.
Commissioning
Reconcile measured and scheduled conditions before professional disposition.
Quality controls
Assumptions and limitations
Assumptions
- Inputs describe one documented system boundary and compatible operating condition.
- All values use the displayed symbols and one consistent unit system.
Limitations
- This screen does not establish code compliance, final design, equipment selection, or acceptance.
- Adopted codes, local criteria, manufacturer data, uncertainty, and professional judgment remain controlling.
Field use
Verification procedure
- Identify the calculation boundary and source of each input.
- Record representative values with compatible pressure, temperature, flow, and time bases.
- Review the result against the adopted criteria, manufacturer data, and uncertainty.
Common engineering mistakes
- Mixing standard and actual gas volumes or gauge and absolute pressures.
- Treating an arithmetic screen as a code-sizing table or hidden acceptance criterion.
Technical basis
References and source standards
- International Plumbing CodeAdopted edition and local amendments govern; protected sizing tables are not reproduced.
- American Society of Plumbing EngineersPlumbing engineering-method context; verify licensed references and project criteria.
- NIST SI and Unit GuidanceUnit and dimensional-conversion basis.
Engineering calculation disclaimer
These engineering tools are provided for educational, preliminary evaluation, field verification, and commissioning support. Results depend on the accuracy of user-entered information and the assumptions stated for each calculation. Every colored status and comparison is a screening indicator only; project criteria, contract requirements, applicable codes, manufacturer instructions, measurement uncertainty, and authorized engineering judgment govern. The tools do not replace project-specific engineering analysis or the judgment of a licensed professional engineer.
Read the full use and limitation statement