What Size Generator for a Well Pump: The Start Decides It

A well pump is easy to run and hard to start. For a moment as it starts, the motor draws several times its running current, and the generator has to supply that surge while holding its voltage up. A generator that would run the pump all day can still fail to start it, so the start is what sets the size.

Rules of thumb for this are everywhere, and they disagree because they are guesses. Two published sources do better. Franklin Electric, which makes submersible well motors, publishes the smallest generator for each size of its motors. And a motor’s nameplate can carry a NEMA code letter that says how much its start asks for.

Three things to find out about your pump

• Horsepower. It is on the motor’s nameplate. For a submersible, the control box has to match the motor’s horsepower and voltage, so its label gives both, and the well’s paperwork should too.
• The voltage it is wired for. A pump rated 230 volts runs on a 240 volt supply, and a generator with only 120 volt outlets cannot run it at any size. Some nameplates list two voltages because the motor can be wired either way. What matters is how yours is wired.
• What kind of pump it is. A jet pump sits above ground, where you can read its motor’s nameplate. A submersible sits down in the well, and it is either two-wire or three-wire. A three-wire motor has a separate control box that holds its starting parts. A two-wire motor has no box, because its starting parts are inside the motor. The difference changes the generator it needs by half again.

Submersible pumps: Franklin’s minimum generator

Franklin Electric’s installation manual lists the smallest generator for each of its submersible motors. Up to 5 HP, in kilowatts:

Compare these with the generator’s running watts, which some labels call rated watts, and not with its starting or peak figure. Franklin’s figures are generator ratings, and a generator’s rating is what it delivers continuously.

Will a 5,000 watt generator run it?

For a 1 HP submersible, a generator with 5,000 running watts clears Franklin’s minimum for a three-wire motor, 4 kW, and falls short of the 6 kW a two-wire motor needs. Check which figure the 5,000 is before relying on it.

The conditions that come with the table

Franklin states them beside it:

• The generator has to hold at least 65% of the motor’s nameplate voltage while the motor starts. That is the test the minimums are sized to pass.
• A generator at the minimum already acts as a soft start. Adding a reduced voltage starter to it drops the voltage further and risks damaging the motor.
• Start the generator before turning the pump on, and turn the pump off before stopping the generator. Letting the generator coast down or run out of fuel with the motor running can damage the motor’s thrust bearing.

Jet pumps and other motors: the code letter

Franklin’s table covers its own submersible motors. For a jet pump, or any motor whose nameplate you can read, look for a NEMA code letter. It gives the motor’s starting draw in kilovolt-amperes (kVA) per horsepower, as a range:

Multiply the range by the horsepower to get the starting draw in kVA. Multiply that by 1,000 and divide by the voltage to get it in amps.

My pump, worked through

My pump is a 1 HP Goulds J10S jet pump, wired for 230 volts, and its nameplate carries code K. Code K is 8.0 to 8.99 kVA per horsepower, so at 1 HP its start asks for 8.0 to 8.99 kVA. At 230 volts that is 35 to 39 amps, against the 6.5 amps the plate gives for running: five to six times what it runs on.

It starts cleanly on my Pulsar G12KBN, converted to run on natural gas, with the rest of the house running on it at the same time. I bought it large on purpose, knowing natural gas would take some of its output and wanting plenty left over. How much it gives up I have no published figure for, so this says little about how a smaller generator would do.

What the code letter figure means for a generator

The code letter gives the draw at full voltage, with the motor not yet turning: the most a start can ask for. A generator does not have to supply all of it. Its voltage dips as the motor starts, and a motor fed less voltage draws less current.

Franklin’s own motors show how much less. Its 1 HP three-wire motor draws 41.8 amps at 230 volts as it starts, about 9.6 kVA, and Franklin’s minimum generator for it is 4 kW, or 5.0 kVA. Its 1 HP two-wire motor draws 48.7 amps, about 11.2 kVA, against a minimum of 6 kW, or 7.5 kVA. In both, the minimum generator is about half to two thirds of the motor’s full-voltage start. The gap is what the 65% rule allows for: the generator is not expected to hold full voltage through the start.

So treat a code letter figure as a ceiling for the generator, and not as a target. A generator rated at or above it is sized for the start with no help from the voltage dip. One below it may well start the pump, as Franklin’s minimums do for its motors, but how far below depends on the motor and the generator, and the nameplate cannot tell you.

What else changes the answer

• Anything else running. If the refrigerator, the lights or anything else is on the generator when the pump starts, the start lands on top of them. Size for what is running plus the pump’s start.
• Fuel and altitude. Generators give up output on propane and natural gas and at elevation. Franklin says to follow the generator maker’s derating, and the running watts for each fuel are on the maker’s spec sheet. Generators for off-grid systems covers why the fuels differ.
• Frequency. A pump’s speed follows the generator’s frequency. By Franklin’s figures, 1 to 2 Hz low and the pump falls short of its performance curve, and 1 to 2 Hz high and it may trip its overloads.

If it hums and will not start

A pump that hums, will not turn, or trips the generator looks exactly like a generator that is too small. A failing start capacitor does the same, and it is a cheap part, so check the capacitor first before buying a bigger generator.

Connecting it is a separate question

This page covers how big the generator has to be. How it connects to the pump depends on whether the house also has a utility connection, and that is covered in backup power. Franklin’s manual is plain about one part of it: a generator backing up a utility line needs a transfer switch, automatic or manual, to prevent electrocution.

Where these numbers come from

The generator minimums, the conditions on them and the two Franklin motors’ starting figures are from Franklin Electric’s 60 Hz application, installation and maintenance manual, 2026 edition, which also gives each minimum in kVA. The code letter ranges are NEMA’s, as printed in ABB’s guide How to read a NEMA motor nameplate. My pump’s figures are from its nameplate. The amps and the comparisons are arithmetic on those.

This page recommends no generators. The number that decides one is its running watts on the fuel you will burn, read on the maker’s own spec sheet.