Another Pump/Generator question


Old 07-31-06, 03:05 PM
Forum Topic Moderator
Thread Starter
Join Date: Sep 2005
Location: USA
Posts: 4,689
Received 28 Votes on 27 Posts
Another Pump/Generator question

I'm trying to determine the load from a well pump. As the well was there when we moved in, I haven't a clue as to the actual size of the pump. I have some clues, and I'm wondering if it's enough information to make an educated guess, or if the safest answer is to splurge and get a clamp-on ammeter and see what it actually draws.

The pump control box (in the house) specifies 1HP, 230v.

I'm figuring it would be incorrect for me to deduce that since the control box is rated at 1HP, the pump is max 1HP, and that 1HP = 746w, the draw of the pump would be max 746w, with a startup load of 4476w (746 * 6). So a generator rated at 7,500 rated watts, 10,750 surge watts should be reasonable to handle the pump and some lights.

How's my logic so far? And of course, this would all go through a transfer switch, etc.

Thanks in advance for the help.

Sponsored Links
Old 07-31-06, 04:57 PM
Join Date: Mar 2006
Location: Wet side of Washington state.
Posts: 18,489
Received 32 Votes on 24 Posts
You're right, figuring 746 Watts for a one-horsepower pump IS incorrect. While 746 Watts does equal one electrical horsepower this would assume that the pump motor was 100% efficient. Most motors take far more power (Watts) than the electrical equivalent. I would add at leat 20% to the 746 figure.

Also, I doubt that your pump motor will take six times the full-load wattage when starting. Starting load is measured in Amperes because the voltage is changing during start up. Since the pump motor is probably a capacitor-start type the starting current is more likely about three times the full-load current.

So yes, a clamp-on ammeter would help in making the decisions. In this particular case I would prefer an analog meter with a peak hold feature. Analog because it will give a better approximation of the peak current that your generator will see whereas a digital meter will (usually) read the absolute highest current draw and that will be a much higher current than the generator will actually experience.
Old 08-01-06, 10:03 AM
Forum Topic Moderator
Thread Starter
Join Date: Sep 2005
Location: USA
Posts: 4,689
Received 28 Votes on 27 Posts
Thanks for the suggestion. I guess I have another tool to add to my collection.

I'll let you know how it goes - and I'm sure I'll have more questions.


Old 08-01-06, 10:57 AM
Join Date: Jul 2006
Posts: 47
Received 0 Votes on 0 Posts
The pump control box is probably rated at 1HP at 230V,, this tells you nothing about whats down that hole,, I.E.,, what the pumps name plate says,, is this fed from a breaker,, if so whats the size,, or a fusible dissconnect,, if so whats the fuse size? are they time delay fuses?,, I ran into a problem similiar to this over the weekend, I was putting a service in that included a well pump,, (old house),, no one living in it,, owner had no clue what size the pump was,, and asked if I could just toss a two pole 20 in the panel,, I said I could but wont,, he told me,, well it has #12 running to it,, this tells me nothing,, unless he finds the documentation for THAT pump,, i;m not running one foot of wire,, I suspect he;ll be pulling the pump,,,and finding out for sure what the name plate says.. Do it right and sleep at night

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Display Modes
Ask a Question
Question Title:
Your question will be posted in: