Pressure tank size / 7 gpm
Pressure tank size for a 7 gpm pump
A 7 gallon-per-minute pump needs a tank that holds 7 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 27.1 gallons total; on 30/50, 23.7; on 20/40, 20.3. The smallest Goulds HydroPro tank that clears it at 40/60 is the V100, 31.8 gallons.
Seven gallons a minute is Goulds' minimum for a one-bathroom house, meeting a 45-gallon seven-minute peak.
Tank sizes stay modest: the 25.9-gallon class covers a one-minute run on 20/40 and 30/50, but the 40/60 setting tips the requirement to 27.1 gallons and the next size up.
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% | 20.3 gal | V80 (25.9 gal) | 40.6 gal | V140B (45.2 gal) |
| 30/50 psi | 28 psi | 29.5% | 23.7 gal | V80 (25.9 gal) | 47.4 gal | V200B (65.1 gal) |
| 40/60 psi | 38 psi | 25.8% | 27.1 gal | V100 (31.8 gal) | 54.3 gal | V200B (65.1 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 V100 holds 31.8 gallons in all. Precharged to 38 psi, it is 25.8% usable between 40 and 60 psi: 8.2 gallons, enough for 1.2 minutes of pump run at 7 gpm. Goulds' own table lists this tank at 9.4 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 — 6.8 gallons more at 7 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 · 10 gpm · 14 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.