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Thread: Generator sizing for submersible pump

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    DIY Member JAR8832's Avatar
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    Default Generator sizing for submersible pump

    I'm in the process of installing a manual transfer switch for use with a portable generator (which we have yet to buy). It will be served by a 30 amp inlet. I thought I had this all figured out, but now am having some uneasiness about the adequacy for use with our deep well submersible pump. It is a Sta-rite 1/2 hp on a 15 amp double pole (240 volt) circuit. The depth is 175 feet. The instruction manual for the transfer switch indicates that a shallow well 1/2 hp pump requires 1000 running watts plus 2350 starting watts. What would be the smallest generator capable of running my deep well pump if it is the only load on the generator?
    Last edited by JAR8832; 12-24-2009 at 12:12 PM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    watts/volts = amps. So, peak = about 9.8A...should be fine on a 30A circuit.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Electrical Contractor/Instructor jwelectric's Avatar
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    The locked rotor amps for a half horse power 240 volt motor are 29.4 amps or just over 7000 watts.

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    DIY Member JAR8832's Avatar
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    Thanks for the replies. I made a mistake in the specs for my pump. It is actually 3/4 hp. It is Sta-Rite model 10P4D20J-02 3-wire on a 15amp 2 pole breaker.

    I was not familiar with the term locked rotor amps, so that helps a lot.

    I cannot find specifications for my particular pump, but have found a different 3/4 hp submersible showing a LRA of 33.1 amps at 230 volts, or 7613 watts. Does this sound reasonable?

    The 5500 watt generator I'm looking at has a surge rating of 6875 watts. Is this close enough to be adequate or am I required to move up to a 6500 watt (8000 surge watt) model?

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    Electrical Contractor/Instructor jwelectric's Avatar
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    The 5500 will work fine as long as it has nothing but the pump

    Table 430.251(A) list a 3/4 hp motot as 41.4 amps locked rotor.

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    DIY Member JAR8832's Avatar
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    I opened the control box for the pump and found this information:

    HP: 3/4
    Voltage: 230
    S.F. Max Amp: 8.0
    KVA Code: M

    Googling "KVA Code" found:

    Code Letter M value: Range of 10.00 - 11.19 (10.6 midrange)

    and the equation:

    INRUSH AMPERES(SINGLE PHASE MOTORS)
    = ((CODE LETTER VALUE) X HP X 1000)/(RATED VOLTAGE)

    Using the high end code letter value of 11.19, yields a maximum LRA of 37.83



    Is there any way to determining if a particular generator can handle this magnitude of LRA?

    I appologize in advance if this is too anal, but I really don't want an undersized generator! Thanks!

  7. #7
    Electrical Contractor/Instructor jwelectric's Avatar
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    At locked rotor amps the windings of the motor start to fail should the current continue for any amount of time (more than a couple of seconds).

    If the motor did lock up then the generator would simply shut down and no harm would come to the generator.

    I have horses and a portable generator (5500) that I connect to the well via a twist lock straight to the generator. This way I donít have the issues of grounding and bonding that come into play when using a transfer switch.

    Anyone that has owned a horse knows that you can lead the horse to water but you canít make him drink. In icy weather the horse will always wait until you are about back to the barn then decide that he wants to drink and back you go to the pond either being dragged along or to fetch the horse so it is easier to just fill a bucket in his stall.

    The math you found on Google is good but will not fly when sizing things such as conductors, overcurrent and generators. The tables found at the end of Article 430 is what must be used in order to be code compliant as well as insure that everything will carry the current imposed.
    What you found on Google is lab type numbers such as 746 watts equal one horse power but you will never find a one horse power motor that pulls 746 watts. The best I have found has been just over 1200 watts with no load.

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    DIY Member JAR8832's Avatar
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    Thanks for your help, it is greatly appreciated.

    One more question: The owner's manual for the generator I'm looking at has a wattage reference guide, which indicates that a 1 hp submersible pump needs only 6000 watts to start. While that contradicts the math and code, is that just their way (wink, wink, nudge, nudge) of indicating that the generator should be able to start the load even though it technically isn't rated to do so?

    It is a Generac GP6500, see page 7 if so inclined: http://www.electricgeneratorsdirect....5623manual.pdf

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    Homeowner Thatguy's Avatar
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    ***************xx

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    DIY Senior Member Rich B's Avatar
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    I work in the generator field and work on them all the time. From small units to very large units.......RV, commercial you name it I have worked on them.

    When it comes to pumps and compressors and motors on small units we usually advise people they need 3 times the running amp requirement for starting the motor/pump. There is a huge variance in motor types and in generators themselves. The rough formula we used to use was that an engine for a generator needs to produce 1.5 hp for every KW of output. That is not always true with many units out there today. Generacs are not real high quality units. If the generators engine and voltage regualtion circuitry is poor it may very well stall when trying to start a motor that runs a pump that on paper should work. The locked rotor amp draw ratings is a good guide but with small units there is no substitute for an actual test of your equipment to see how it responds.

    I have a Camper with an AC unit on it....it is a 9200 BTU unit and runs easily with a 3500 watt Honda portable. It has a soft start blower motor. BUT if you shut the AC off and then back on without waiting at least a couple minutes.....the generator cannot restart the compressor in the AC unit. The generator bogs down badly even though it is at wide open throttle..That is the LOCKED ROTOR condition mentioned. This is very common with AC units. Another example, I have been on jobs with big sewage pumps that have "hard start" pump motors and they really hit hard when they start.......they also make soft start pump motors that ramp up slowly to full speed. That is an example of the difference in motors.....A much smaller generator can operate the soft start pump motors.....

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    DIY Member JAR8832's Avatar
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    I received my generator today. I decided to go with the 6500 watt (8000 watt surge) model. Thankfully, the well pump ran just fine on it. The watt meter on my transfer switch showed a surge of about 7000 watts, which lasted maybe a few tenths of a second, then a running wattage of about 2200 watts. So the 3x rule is pretty accurate. Thanks for the help!

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