Pressure tank size / 14 gpm
Pressure tank size for a 14 gpm pump
A 14 gallon-per-minute pump needs a tank that holds 14 gallons of drawdown between cut-out and cut-in, so the motor runs at least one minute each time it starts. On a 40/60 switch that means a tank of at least 54.3 gallons total; on 30/50, 47.4; on 20/40, 40.6. The smallest Goulds HydroPro tank that clears it at 40/60 is the V200B, 65.1 gallons.
Fourteen gallons a minute is Goulds' minimum for two to two and a half bathrooms, the most common layout in newer homes, meeting a 98-gallon seven-minute peak.
At this flow the one-minute rule needs 40.6 gallons of tank on 20/40 but 54.3 on 40/60, which moves a house from the 45-gallon class to the 65-gallon class just by turning up the pressure switch.
Tank size by switch setting
| Switch | Precharge | Usable share | Tank, 1-min run | Smallest Goulds HydroPro | Tank, 2-min run | Smallest Goulds HydroPro |
|---|---|---|---|---|---|---|
| 20/40 psi | 18 psi | 34.5% | 40.6 gal | V140B (45.2 gal) | 81.3 gal | V250 (83.5 gal) |
| 30/50 psi | 28 psi | 29.5% | 47.4 gal | V200B (65.1 gal) | 94.8 gal | V350 (115.9 gal) |
| 40/60 psi | 38 psi | 25.8% | 54.3 gal | V200B (65.1 gal) | 108.5 gal | V350 (115.9 gal) |
Tank volumes: Goulds Water Technology (Xylem), Technical Manual TTECHWP R7, Table 1. Run time: Franklin Electric, Application • Installation • Maintenance (AIM) Manual, Submersible Systems, 60 Hz (2026) — copy published by the Nebraska Department of Water, Environment and Energy. Precharge: AMTROL, "Using an air gauge, adjust the precharge to 2 psig below the cut-in (LO) setting."
What the tank does at 40/60
The V200B holds 65.1 gallons in all. Precharged to 38 psi, it is 25.8% usable between 40 and 60 psi: 16.8 gallons, enough for 1.2 minutes of pump run at 14 gpm. Goulds' own table lists this tank at 19.2 gallons of drawdown "at 38/60" — the larger figure counts water drawn below the 40 psi cut-in, down to the precharge, which the pump never lets happen in use. That is why this page sizes from the switch points instead.
Why the setting changes the tank
Air in the tank is compressed as water comes in, so the share of the tank you can actually use depends on the pressures. Between 20 and 40 psi it is 34.5%; at 30/50 it is 29.5%; at 40/60 only 25.8%. Raising the switch for stronger showers therefore needs a bigger tank for the same pump — 13.6 gallons more at 14 gpm when moving from 20/40 to 40/60.
Longer cycles are easier on the motor. Franklin Electric's minimum is one minute; its starts-per-day limits (300 for single-phase motors up to 0.75 hp, 100 for single-phase motors 1 to 5.5 hp, 50 for single-phase motors 7.5 to 30 hp) are the other half of the picture. Sizing for two minutes doubles the tank and roughly halves the starts.
Other pump flows
5 gpm · 7 gpm · 10 gpm · 17 gpm · 20 gpm. Not sure of your pump's flow? The pump calculator works it out from bathrooms and depth, and the pump-by-depth pages list rated flows.