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Old 11-07-2012, 08:31 PM   #1
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watts needed to run well pump


I have a 1/2 hp , 6 amp pump running single phase 120 volt with 2 15 amp breakers

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Last edited by hankbuck; 11-07-2012 at 08:35 PM.
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Old 11-07-2012, 08:45 PM   #2
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watts needed to run well pump


What is your question?
And WHY do you have two 15A breakers running a 120V load?

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Old 11-07-2012, 08:54 PM   #3
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watts needed to run well pump


I am helping daughter and son-in-law size a generator. I have to do detective work and get little help so I am piecing together info and heres what I have. I looked at circuit breaker and see 2 15 amp switches marked well pump. I called well service (sticker on tank) and he said its a 1/2 hp drawing 6 amps and I can get conversion on line. Thats my story.
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Old 11-07-2012, 09:34 PM   #4
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watts needed to run well pump


You probably have a 240 volt pump, not a 120 volt pump. That would explain why you have two circuit breakers (you said switches, but I am guessing you mean two breakers). So what is your question?
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Old 11-07-2012, 09:46 PM   #5
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watts needed to run well pump


I appreciate sifting through my ignorance so thanks ahead of time.
I want to size a generator. I was told that the well pump is 1/2 hp and draws 6 amps. With the conversion calculators I saw on line I, of meager expertise, figured that to be 720 watts and if its 240 then its 1440. I was suspect of my ability so i wanted to be sure what wattage would be needed to accomodate the well pump. Thanks. What do you think?
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Old 11-07-2012, 09:54 PM   #6
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watts needed to run well pump


One horsepower is 746 watts, so a 1/2 horsepower motor draws approximately 370 watts at full power while it is running. A 240 volt pump drawing 6 amps uses 1440 watts. So there is a mismatch in the values.

Pumps are a bit tricky to compute electric draw on, since they often draw significantly more power on startup than they do when running. The pump power is often reported as continuous run power, not maximum draw. Without some detailed information about the pump, it is difficult to evaluate the electrical requirements of your pump. Your generator typically is rated in several different ways, one of which is maximum continuous power output, and another is peak power output. The peak power is typically used for startup of electric motors, which can draw as much as 5 times running current under startup load.
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Old 11-07-2012, 11:13 PM   #7
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watts needed to run well pump


My Franklin motor book lists several 1/2HP motors; the 115 volt ones draw 10 amps, the 230s draw 5. The SFA (Service Factor Amps) are 12 for 115 volt and 6 for 230.

Most well drillers don't know what a service factor is, so they'll say SFA means starting factor amps. It doesn't. The two are not related.

If the 1/2HP motor is not capacitor run, it'll draw 960 watts. If it is capacitor run, it'll draw 890 watts.

OK, now that the tech talk is done, in real life a 2500 watt generator will most likely start this pump just fine, provided the engine is at operating temperature and there are no other loads.

It might be hard to find a 2500 watt gen that will produce 240 volts, so you'll likely need to get a larger unit. Like 3000 or 3500 watt. Those will start a 1/2HP motor easily, even with a small load already on.

P.S. To further add to the confusion, the locked-rotor amps (also known as the starting surge) is 23 for the 3 wire motors and 32 for the two wire. Regardless of what the nameplate says, a 3000 watt gen that is rated for 12.5 amps can produce 30 amps for a second or so. This is plenty of time for a well pump to start.

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Old 11-07-2012, 11:56 PM   #8
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watts needed to run well pump


Eureka! Thanks! Thats just what i needed/thought. I am set now to go forward. Thanks again one and all. Great feedback.

hankbuck

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