I have a well pump and hope to buy a generator for power outages soon, so I need to know what horsepower the pump is to size the generator sufficiently. The owner documentation that came with the pump somewhat unhelpfully says that it is 1 or 1.5 or 2 or 2.5 horsepower. I put a clamp ammeter on the pump wiring and see that it runs at 5.6 +/- 0.1 amps, but this says how many horsepower is it developing, not what the size of the motor is. The challenge, of course, is when the pump starts, not while it runs. And the electrical demand for starting the pump depends on what horsepower motor it is, not what horsepower it will run at in use (which of course will depend on how high the water level in the well is).
I did try watching the digital clamp ammeter through three start cycles, to see the highest reading. I saw 19.8, 12.7 and 26.6. This was good practice for staring down inanimate objects in poor lighting, but I am not sure there was any other value in the exercise. I guess the digital display updates at about half second intervals, and there was always one big number flanked by much smaller ones. Even if I did my job correctly and the meter reported true amps for instants a half second apart, I don’t know that I caught a peak.
My well is 240 feet deep and the pump is (I think) about 200 feet down. The pump thrashes around on the end of a hanging plastic semirigid pipe without any spacing means. It was installed about 5 years ago to replace the original. My voltage runs around 220 or 230.
So, can I deduce a motor design horsepower from the available information, or other information it’s easy to get?
Was it professionally installed or do you know where it was purchased? In 2007, I’m sure that those records would still be available. I know enough about well pumps to electrocute myself, though I do have running water with great pressure.
The controller box should have a sticker placed there when the pump was installed. Most likely it will be Franklin Electric. It will have the HP and serial number. I just had my pump and well pipe replaced and I had to tell him what size so he could bring the correct equipment.
if you can’t find the specific model, or the motor doesn’t have it listed on its nameplate, I’m not sure what else you can do. Measuring the current can be misleading because motors typically have a poor power factor. You might be able to take your current measurement, multiply it by the voltage (assuming you’re certain about that) to get its apparent power, then multiply by the power factor to get real power.
What’s the water output and output pressure of the pump at the wellhead? Put a 5 gal bucket under the faucet and time it. If the well’s only 230 feet deep and it’s not pumping over 5 gals/min (and it’s not spiked to the well casing) it’s probably a 1.5 or 2 hp. I would think any half-assed 2.5 or 3.5 K generator will do the work. I run a 650’ well at 5 HP at 15gpm on a 6.5K.
Two ways to go with generators. Harbor Freight with a long warranty and replace them as you go … or buy a nice Honda 2000 or 3000 and be done with it. Cheap off brand generators (Generac, etc.) make a hell of a racket and die early.
On second thought, shouldn’t you size the generator to the voltage and amps? What size circuit breaker powers the pump? My pump is on a 30 amp 230 volt circuit.
The rule of thumb I’ve always used is inrush current is 6-7 times the running current of the motor. It typically lasts a fraction of a second. Your generator needs to be able to keep voltage above about 75% of rated until the motor gets up to speed otherwise your motor will stall your generator.
