Submersible Well Pump & Pressure Tank Sizing Calculator
Calculate Total Dynamic Head (TDH), required motor horsepower (HP), Hazen-Williams drop pipe friction, electrical wire sizing, and Boyle's Law pressure tank drawdown capacity for deep-well water systems.
Well Depth & Hydraulic Conditions
Pump Hydraulic Sizing & Motor Spec
Pressure Tank Sizing (Boyle's Law)
Complete Wellhead, Aquifer Cone of Depression & House Plumbing Schematic
Live Water Table & Submersible RigFranklin Electric 4-Inch Submersible Motor & Pump Performance Matrix
| Motor HP | Stages (Impellers) | 230V 1-Ph FLA | Max Wire Run (#10 AWG) | Max TDH @ 10 GPM | Optimal Depth Range |
|---|---|---|---|---|---|
| 0.5 HP (1/2 HP) | 5-7 Stages | 5.0 A | 400 ft | 160 ft | Shallow Wells (<100 ft) |
| 0.75 HP (3/4 HP) | 8-10 Stages | 6.8 A | 300 ft | 230 ft | 100 - 180 ft Wells |
| 1.0 HP | 11-14 Stages | 8.8 A | 250 ft | 300 ft | 150 - 250 ft Wells |
| 1.5 HP | 16-19 Stages | 11.5 A | 200 ft | 440 ft | 200 - 350 ft Wells (Standard) |
| 2.0 HP | 21-25 Stages | 13.2 A | 170 ft | 570 ft | 300 - 450 ft Wells |
| 3.0 HP | 30-36 Stages | 17.0 A | 130 ft | 780 ft | 400 - 650 ft Deep Rock |
| 5.0 HP | 45-52 Stages | 27.5 A | 80 ft | 1,150 ft | Ultra-Deep Commercial / Farm |
Engineering Equations & Step-by-Step Derivations
1. Total Dynamic Head (TDH):
The total mechanical work required by the submersible pump to lift water from the drawdown depth and pressurize the indoor distribution manifold:
$$ ext{TDH} = H_{ ext{pumping}} + H_{ ext{elevation}} + ( ext{Cut-out PSI} imes 2.307) + h_f$$
$$ ext{TDH} = 120.0 + 15.0 + (60 imes 2.307) + 2.9 = mathbf{276.3 ext{ Feet}}$$
2. Hazen-Williams Friction Loss ($h_f$):
Head loss through the drop pipe and trench offset (plastic pipe $C=150$):
$$h_f = 10.44 imes L imes rac{Q^{1.852}}{C^{1.852} imes D_i^{4.8655}}$$
$$h_f = 10.44 imes 200 imes rac{(10)^{1.852}}{(150)^{1.852} imes (1.380)^{4.8655}} = mathbf{2.9 ext{ Feet}}$$
3. Water Horsepower (WHP) and Brake Horsepower (BHP):
$$WHP = rac{Q_{ ext{GPM}} imes ext{TDH}_{ ext{ft}}}{3,960} = rac{10 imes 276.3}{3,960} = mathbf{0.70 ext{ WHP}}$$
At 58% typical wet-end pump efficiency: $BHP = rac{0.70}{0.58} = mathbf{1.20 ext{ BHP}}$ → 1.5 HP Motor.
4. Boyle's Law Pre-Charged Tank Sizing ($V_{tank}$):
$$V_{ ext{tank}} = rac{V_{ ext{drawdown}}}{ rac{P_{ ext{cutout}} - P_{ ext{cutin}}}{P_{ ext{cutout}} + 14.7}} = rac{10.0}{ rac{60 - 40}{60 + 14.7}} = rac{10.0}{0.2677} = mathbf{37.0 ext{ Gallons}}$$
5 Fatal Traps & Deep-Well Sizing Pitfalls
⚠️ Trap 1: Submersible Motor Rapid Short-Cycling & Bladder Waterlogging
Submersible motors generate immense starting inrush currents (up to 40 amps) that heat stator windings. Franklin Electric limits 4-inch single-phase motors to a maximum of 25 starts per 24-hour period and mandates at least 1 to 2 minutes of continuous run time per cycle to allow water flow past the motor sleeve to dissipate rotor heat. Installing an undersized 14-gallon bladder tank causes the pump to short-cycle every 20 seconds during dishwashing, burning out start capacitors and melting stator varnish within 6 to 18 months.
⚠️ Trap 2: Sizing Pump Head Against Static Water Level Instead of Drawdown
Static water level is the resting depth of water in an idle well. When a 10 GPM pump engages, water in the surrounding rock aquifer depressurizes into a descending "cone of depression." In low-yield bedrock aquifers (e.g. 3–5 GPM yield), the pumping water level may drop 100 to 150 feet below static level. Sizing TDH using the static depth causes the pump to encounter far higher head than engineered, dropping flow to a trickle or causing dry-run cavitation that melts the thermoplastic Diffusers and Lexan impellers.
⚠️ Trap 3: Choking High Flow Through Undersized 1-Inch Drop Pipe
Pushing 15 to 20 GPM through a 1-inch polyethylene drop pipe creates severe friction head loss exceeding 15 to 25 feet per 100 feet of pipe run! On a 300-foot well run, an undersized 1-inch pipe adds over 60 feet (26 PSI) of parasitic friction head that the motor must fight continuously. Upsizing drop pipe from 1" to 1-1/4" reduces friction resistance by over 65%, lowering motor running amps, reducing electric bills, and boosting household faucet pressure.
⚠️ Trap 4: Improper Tank Pre-Charge Air Pressure (Must Be 2 PSI Below Cut-In!)
A captive air bladder tank must be pre-charged with compressed air to exactly 2 PSI below the pressure switch cut-in setting while the tank is 100% drained of water. For a standard 40/60 PSI switch, air pre-charge must be exactly 38 PSI. If the pre-charge is set equal to or above cut-in (e.g. 42 PSI), the bladder collapses completely against the bottom inlet flange before the switch contacts close, creating an instantaneous complete water pressure dropout at home faucets.
⚠️ Trap 5: Lightning Surges & Omitting Wellhead Surge Arrestors
A steel well casing embedded 200+ feet into wet bedrock acts as an immense earth ground electrode. Nearby lightning strikes induce thousands of volts of electrical back-feed up the submersible cable into the home breaker panel. Omitting a dedicated secondary lightning surge arrestor on the pump control box is the #1 cause of catastrophic winding short circuits in deep well systems. Always install a dedicated MOV/gas-discharge surge arrestor at the pressure switch.
Frequently Asked Well Pump Sizing Questions
What size well pump do I need for a 3-bedroom, 2-bathroom home? +
How does a pressure tank prevent a well pump from burning out? +
What is the difference between 30/50 PSI and 40/60 PSI switch settings? +
What wire gauge is needed for a 230V submersible well pump? +
- 0.5 HP (5.0A): #14 AWG up to 300 ft; #12 AWG up to 500 ft.
- 1.0 HP (8.8A): #12 AWG up to 250 ft; #10 AWG up to 400 ft.
- 1.5 HP (11.5A): #12 AWG up to 190 ft; #10 AWG up to 310 ft; #8 AWG up to 490 ft.