Residential Well Pump Duty-Point Power Calculator

A residential well pump duty-point calculation converts the hydraulic work required at a known operating point into an estimated electrical load. The primary electrical result is Estimated electrical input in kW, followed by the kVA and current values needed for an early load review, branch-circuit or feeder planning, voltage-drop review, and raceway layout.

The calculation starts with Flow and Total dynamic head, then accounts for losses through the pump and motor or drive. It does not select a pump, establish the actual operating point on a pump curve, or guarantee delivered pressure at a fixture, pressure tank, or irrigation zone.

Duty-Point Inputs

For a pump duty-point review, enter values only after the pump duty point and efficiency information are available from a pump curve, equipment submittal, or documented design assumption.

Input	Electrical and hydraulic use
Flow (gpm)	The water volume delivered at the intended duty point. Higher flow increases hydraulic power in direct proportion.
Total dynamic head (ft)	The total head used for the estimate. It represents the pumping work required against elevation, pressure, friction, and other system losses included in the underlying hydraulic calculation.
Pump efficiency (%)	The percentage of shaft power converted into water power at the duty point. Use an efficiency taken from the applicable pump curve or a documented assumption.
Motor or drive efficiency (%)	The percentage of incoming electrical power converted into mechanical output at the motor shaft or drive output, as applicable.
Supply voltage (V)	The voltage used for the optional current screen. Enter the nominal system voltage being evaluated.
Phase	Selects the phase basis for the current arithmetic.
Power factor (%)	Converts estimated real electrical input in kW to apparent power in kVA for the kVA/current screen.
Pump curve note	Record the pump curve, water conditions, and model revision separately. This preserves the source of the assumed flow, head, and pump efficiency.

A duty point is a single operating condition, not a pump selection. A pump’s real operating point changes when water level, discharge pressure, piping losses, valve position, impeller condition, and system demand change.

Power Calculation

The calculator first determines Hydraulic power—the power transferred to the water.

\(\displaystyle \text{Hydraulic power (hp)} = \frac{\text{Flow (gpm)} \times \text{Total dynamic head (ft)}}{3960}\)

It also presents hydraulic power in kilowatts:

\(\displaystyle \text{Hydraulic power (kW)} = \text{Hydraulic power (hp)} \times 0.7457\)

The motor must draw more electrical power than the hydraulic output because the pump and motor or drive are not 100% efficient:

\(\displaystyle \text{Estimated electrical input (kW)} = \frac{\text{Hydraulic power (kW)}}{ \left(\frac{\text{Pump efficiency}}{100}\right) \times \left(\frac{\text{Motor or drive efficiency}}{100}\right)}\)

The kVA screen applies Power factor:

\(\displaystyle \text{Estimated electrical input (kVA)} = \frac{\text{Estimated electrical input (kW)}}{ \left(\frac{\text{Power factor}}{100}\right)}\)

For the selected voltage and phase basis, the calculator then displays Estimated current screen. For single-phase power:

\(\displaystyle \text{Current (A)} = \frac{\text{kVA} \times 1000}{\text{Supply voltage (V)}}\)

For three-phase power:

\(\displaystyle \text{Current (A)} = \frac{\text{kVA} \times 1000} {\sqrt{3} \times \text{Supply voltage (V)}}\)

The current result is an operating-current estimate derived from the entered duty point. It is not a substitute for the motor nameplate current, marked minimum circuit ampacity, overcurrent-protection instructions, locked-rotor data, or the equipment manufacturer’s installation documentation.

Calculation Example

For a well pump delivering 10 gpm at 100 ft of total dynamic head, with 60% pump efficiency, 85% motor or drive efficiency, 240 V single-phase supply, and 85% power factor:

Field	Entered value
Flow	10 gpm
Total dynamic head	100 ft
Pump efficiency	60%
Motor or drive efficiency	85%
Supply voltage	240 V
Phase	Single-phase
Power factor	85%

Hydraulic horsepower:

\(\displaystyle \frac{10 \times 100}{3960} = 0.2525\text{ hp}\)

Hydraulic kilowatts:

\(\displaystyle 0.2525 \times 0.7457 = 0.1884\text{ kW}\)

Estimated electrical input:

\(\displaystyle \frac{0.1884}{0.60 \times 0.85} = 0.3694\text{ kW}\)

Estimated apparent power:

\(\displaystyle \frac{0.3694}{0.85} = 0.4346\text{ kVA}\)

Estimated single-phase current:

\(\displaystyle \frac{0.4346 \times 1000}{240} = 1.8107\text{ A}\)

The resulting values are:

• Hydraulic power: 0.2525 hp
• Hydraulic power: 0.1884 kW
• Estimated electrical input: 0.3694 kW
• Estimated electrical input: 0.4346 kVA
• Estimated current screen: 1.8107 A

Electrical Application

The estimated kW and kVA help characterize how a proposed well-pump duty point contributes to a residential electrical load. The estimated current can support preliminary conductor and voltage-drop calculations when the actual motor data is not yet finalized.

For a long underground run from service equipment to a well pump, use the estimated operating current with the actual one-way circuit length, conductor material, conductor size in AWG or kcmil, and installation method to evaluate voltage drop. Raceway routing, pull-box locations, and conduit sizing must also account for the final conductor set, equipment grounding conductor, raceway fill, bend limits, burial conditions, and available pull path.

Final branch-circuit or feeder design requires the actual equipment information. Verify conductor ampacity using the applicable terminal rating, insulation temperature rating, correction factor, adjustment factor, ambient conditions, and number of current-carrying conductors. Confirm the motor circuit conductors, disconnecting means, controller or pressure-switch arrangement, grounding and bonding, and overcurrent protection against the equipment documentation and the requirements enforced by the AHJ.

Field Verification

Use Pump curve note to retain the curve source, model revision, water conditions, and assumptions behind the selected duty point. A changed static water level, fouled screen, altered pipe run, pressure-tank setting, or replacement pump can change both head and flow, which changes the calculated hydraulic and electrical demand.

Do not treat the displayed Estimated current screen as a motor nameplate value or as a final ampacity determination. Final electrical design should be based on the installed pump motor or drive nameplate, manufacturer instructions, measured supply conditions where appropriate, and the code requirements adopted by the local AHJ.