Introduction
This case study is hypothetical and illustrative — a composite household scenario built to demonstrate the complete pump-sizing workflow, not a report of a real installation. Every number below is worked through the same formulas the Pump Size Calculator uses, so you can follow the arithmetic and then apply the identical procedure to your own system.
The scenario: a family of four in a two-story, 2.5-bath home on a private well. Their 20-year-old jet pump has failed, the well is 150 ft deep, and they need to size its replacement — deciding in the process whether the old jet-pump arrangement still makes sense.
The Starting Facts
Gathered from the well log, a tape measure, and a walk around the property (the procedure in the estimating guide):
- Well depth: 150 ft; driller’s log shows pumping water level at 70 ft below grade at 12 GPM (resting level 45 ft — noted but not used).
- Household: 2.5 baths, kitchen, laundry, two hose bibs. Fixture tally ≈ 25 GPM if everything ran at once; realistic peak 12 GPM (two showers + washing machine + a faucet).
- Pipe run: 180 ft of 1-in PVC from wellhead to house, plus the 140-ft drop pipe; fittings: two 90° elbows, one tee branch, one check valve, one sediment filter.
- Pressure: they want a 40/60 pressure switch (60 psi cut-out).
- Old system: deep-well jet with down-well ejector — functional for 20 years but noisy, and the family wants to evaluate a submersible.
Step 1: Assemble the TDH
Static head: pumping water level 70 ft below grade to the pressure tank at grade = 70 ft. (The resting 45-ft level is ignored — sizing uses the pumping level, per the common mistakes guide.)
Friction loss: equivalent pipe = 180 ft horizontal + 140 ft drop + fittings (2 × 2.7 + 5.4 + 8 = 18.8 ft) ≈ 339 ft. At 12 GPM, 1-in PVC loses ~4.3 ft per 100 ft → 339 × 0.043 ≈ 14.6 ft. Sediment filter at dirty rating ~3 psi ≈ 7 ft. Friction subtotal ≈ 22 ft.
Pressure head: 60 psi cut-out × 2.31 = 138.6 ft.
TDH = 70 + 22 + 138.6 = 230.6 ≈ 231 ft.
Step 2: Convert to Horsepower
HP = 12 × 231 ÷ 3960 ÷ 0.60 = 2772 ÷ 3960 = 0.70 ÷ 0.60 = 1.17 HP → round up to a 1.5 HP pump.
Metric cross-check: 12 GPM = 45.4 L/min; 231 ft = 70.4 m. (45.4 ÷ 60) × 70.4 × 9.81 ÷ 0.60 = 0.7567 × 70.4 × 9.81 = 522.7 ÷ 0.60 = 871 W ≈ 1.17 HP. The unit systems agree.
Step 3: Choose the Pump Type
The duty point — 12 GPM at 231 ft — plus one decisive fact: the pumping water level is 70 ft down, far beyond the ~25 ft practical suction limit. That rules out any surface pump except a deep-well jet with a down-well ejector (what they had). The options:
| Option | Pros | Cons |
|---|---|---|
| 1.5 HP deep-well jet (like-for-like) | Motor stays accessible on surface | Noisy; less efficient; ejector tuning finicky |
| 1.5 HP submersible | Quiet; more efficient; no suction limit; 10–15 yr life typical | Pull requires hoist if it fails; higher upfront cost |
They choose the submersible: the efficiency gain and silence outweigh the pull-day downside, and the well professionals they quote — this is firmly pro territory at 150 ft — recommend it. They verify the duty point sits near the middle of the chosen pump’s curve — not at either extreme.
Step 4: Size the Supporting Components
- Pressure tank: 12 GPM → ~12 gal drawdown minimum → an 80-gallon bladder tank on a 40/60 switch gives comfortable margin. Precharge set to 38 psi.
- Wire: 140 ft down the well plus run to the panel ≈ 200 ft one-way; 1.5 HP at 230V → 10 AWG submersible cable for under 5% drop.
- Drop pipe: 1-1/4-in Sch 80 PVC for the 140-ft string (upsized from 1-in to cut friction — the standing recommendation to upsize pipe rather than the pump).
- Protection: pump protector with dry-run cutoff, whole-house surge protector, new check valve, new pressure switch and gauge.
Step 5: Budget and Outcome
Itemized pro quote: 1.5 HP submersible ($900), 200 ft 10 AWG wire ($350), 140 ft drop pipe + fittings ($250), 80-gal tank ($450), switch/gauge/protector ($150), labor for pull-and-set ($1,200) → ≈ $3,300 installed, squarely inside the cost guide’s $2,500–$5,000 range for a well of this depth. Permit pulled, inspection passed, well shock-chlorinated and tested before use (code compliance workflow).
The illustrative outcome: steady 60 psi at peak demand, ~3-minute pump cycles, and an energy draw of roughly 1.3 kW while running — about $200/year at their electricity rate for typical household use. The system’s documentation (pump model, setting depth, wire gauge, tank precharge, install date) is taped inside the control box for the next service call, per the pro tips.
What This Scenario Demonstrates
- The pumping level decides the pump type before horsepower decides the pump size — 70 ft of lift ended the surface-pump discussion immediately.
- Friction was ~10% of TDH — small enough to estimate, large enough that omitting it would have cost nearly a sixth of a horsepower.
- The system around the pump (tank, wire, pipe size, protection) determined reliability as much as the pump selection did.
- Documentation and permits were part of the job, not afterthoughts.
Related Guides
- Pump Size Complete Guide — the full workflow this study demonstrates.
- Estimating Guide — the field procedure behind the study’s measurements.
- Pump Size Formula Explained — every calculation in the study, derived.
- Cost Guide — the budget ranges the study’s quote fell inside.
- Sizing Checklist — run your own system through the same verification.
- Pro Tips — the practices the study’s outcome depended on.
Frequently Asked Questions
Is this case study based on a real installation?
No — it is explicitly hypothetical and illustrative, constructed to demonstrate the sizing workflow with realistic numbers. Your well depth, water level, pipe run, and pressure needs will differ; run your own measurements.
Why a submersible instead of another jet pump?
At a 70-ft pumping level, a surface jet pump would need a down-well ejector — workable but noisy and less efficient. A submersible pushes from below with no suction limit, runs silently, and is the standard professional recommendation at that depth.
Why 1.5 HP and not 1 HP?
The calculation gave 1.17 HP of shaft power required. Pumps come in standard sizes — 1 HP would be undersized (1.0 < 1.17), so 1.5 HP is the correct round-up. Rounding down is never acceptable in pump sizing.
Could they have used 1-in drop pipe instead of 1-1/4-in?
Physically yes, but the larger pipe cut ~8 ft of friction from the TDH permanently and reduced energy use for the pump’s whole life, for a small one-time cost difference. It’s the classic “upsize the pipe, not the pump” trade.
What would change for a shallower well?
Everything downstream of static head: a 30-ft pumping level would cut TDH roughly in half, likely landing on a 3/4 HP pump — and would reopen the surface-pump option, since 30 ft is within deep-well-jet range but beyond shallow-well suction range.
Get Your Number
The workflow is transferable even though the numbers aren’t: measure your pumping level, count your fixtures, walk your pipe run, assemble TDH, convert to horsepower, check the curve, and spec the system around the pump. Run your own figures through the Pump Size Calculator and verify with the checklist — your installation deserves the same rigor as this hypothetical one got.
Disclaimer: This case study is hypothetical and illustrative, created for educational purposes. It is not a report of a real installation and not a recommendation for any specific equipment. Well conditions, codes, and costs vary by site — verify your own measurements and consult a licensed well contractor or pump professional before purchasing or installing equipment.
