What Size Well Pump Do I Need?

Why horsepower is the last number to look at

A pump's horsepower is the motor size needed to push a certain flow up a certain height. Change either one and the horsepower changes.

Franklin Electric's FPS 3200 line shows this well. It's sold in 5, 7 and 10 gallon-per-minute flow ratings, with complete pumps from 1/2 to 1.5 hp. Within one flow rating, the bigger motors are there to lift water from deeper, which is why shut-off heads for complete pumps in that line run from 324 feet up to 899 feet. Two houses with the same family size can need the same flow rating and very different horsepower.

So start with three things: how much water you need, how far it has to be lifted, and what pressure you want at the tap.

The numbers that matter

Flow rate

Flow is gallons per minute at your busiest moment, not your daily total. More people and more fixtures mean a bigger flow rating. HomeGuide notes that larger families use more water and need larger pumps. A well contractor works this out from fixture count, with a margin for irrigation or livestock if you have them.

Your well limits flow too. A pump that pulls water faster than the well recovers draws the level down until it runs dry, and Boshart names an oversized pump as one of the common causes of pumping sand. The well's yield test, on the driller's log, sets the ceiling. See well yield and recovery rate.

Lift and pressure

The pump has to lift water from the pumping level (where the water stands while the pump is running, not the static level) to the surface, then push it up to the pressure switch's cut-out. Pipe friction adds a bit more. Your contractor adds those together as total dynamic head and picks a pump curve that delivers your flow at that head.

Pump type by depth

WAYNE's depth ranges are a good first cut:

Boshart puts submersibles anywhere from 50 to 1,200 feet down in the casing. Once the water sits deeper than 25 feet, a shallow jet pump is out, a convertible jet needs an ejector down the well, and a submersible is usually the better buy. HomeGuide agrees that submersibles are best for deeper wells and jet pumps for shallow ones.

Casing size

WAYNE says a 2-inch casing needs special accessories for a convertible jet pump, while a 4-inch or larger casing can take either a jet pump or a submersible. Most residential submersibles, including Franklin's FPS 3200, are 4-inch pumps.

Don't oversize it

A pump that's too big causes its own problems:

• It empties the pressure tank fast and short cycles. WAYNE notes larger pumps may need larger pressure tanks to prevent rapid cycling.
• It can overdraw a low-yield well and pull sand.
• It costs more to buy and run.

A step up in horsepower makes sense if you've added people or a major water user, which is the case WAYNE calls out. Otherwise, a bigger pump rarely fixes low pressure. Our low water pressure from a well page covers the usual real causes.

Match the tank to the pump

The pressure tank sets how long the pump runs each time it starts. Boshart says the tank should be sized so each cycle lasts at least 1 minute from start to stop. A bigger pump fills the same tank faster, which is why tank size and pump size get chosen together. Our pressure tank sizing guide walks through drawdown.

Wire size and voltage

Wire size depends on the motor, the voltage and the length of the run down the well. Goulds builds its wire charts around a 5% voltage drop, and says systems at 200 to 207 volts need the next larger cable size than its 230-volt charts show. It also warns that voltage more than 10% off the motor's rating will damage motors and controls. Use the wire table in your pump's manual or let the contractor size it.

Larger motors have one more requirement: Goulds says 4-inch motors of 2 hp and up need water moving past the motor at a minimum of 0.25 feet per second to cool it, which its table lists as 1.2 gallons per minute in a 4-inch well.

What to do next

Find what you have now. The pump label, the control box (on three-wire pumps) and the driller's log usually give you horsepower, voltage, depth and flow. If your needs haven't changed, replace like for like. If they have, or if the old pump kept running the well dry, get a well contractor to size it from the yield test and your fixtures. Our well pump replacement cost page shows what that job costs.