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Thread: Voltage drop

  1. #1
    Join Date
    Jun 2003
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    6

    Voltage drop

    I have a 300' run to a 1/2 horse water well pump to be fed with 240v single phase. With voltage drop I need #6CU conductors. Problem is, #6's will not go under the lugs of the 1/2 HP Franklin motor control which is at the start of the run, nor will they go under the 20A lugs of the 2pole breaker at the panel where the circuit originates. Can I start the circuit at the panel with #12's (20A) for 20' to the Franklin control, go out of the control with 3/#12 +1 ground (for capacitor start), then another 4' to a JB where the #6's start to go underground to the well head? I've wondered before if I can feed larger conductors necessary due to voltage drop on a longer run with smaller conductors starting the circuit, for the same reason each time.

  2. #2
    Join Date
    Feb 2003
    Location
    Indianapolis
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    6,102

    Re: Voltage drop

    No problem at all as long as the OC protection is set for the smallest conductors in the circuit. I am assuming the Vd calcs are correct, I did not check your math.
    Charlie Eldridge, Indianapolis, Utility Power Guy
    Responses based on the 2011 NEC, unless stated otherwise.

  3. #3
    Join Date
    Feb 2003
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    3,172

    Re: Voltage drop

    hconant: Review your calculations, I think you will be surprised.

    Bennie R. Palmer

  4. #4
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    Feb 2003
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    Bremerton, Washington
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    Re: Voltage drop

    hconant
    You also need to consider the VD due to the drop cable feeding the submersible pump. Start at the motor, allow 5% VD from there to the source.
    Its a 230 V rated motor, you want no less than 230-11.5 volts (5%) at the motor. You can use a smaller drop cable and make up the voltage drop on the run from the pump controller to the source.
    The motor is designed to run +/- 10% voltage, so going with -5% gives you a bit of a fudge factor.
    Moderator-Washington State
    Ancora Imparo

  5. #5
    Join Date
    Feb 2003
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    3,172

    Re: Voltage drop

    My calculator indicates that #14 cu will give a terminal voltage of 230 volts from a 240 volt supply, with a load of 5 amps.

    Check out this: http://www.stanselectric.com/vdrop.html

    Bennie R. Palmer

    [ November 28, 2003, 02:18 PM: Message edited by: bennie ]

  6. #6
    Join Date
    Jun 2003
    Posts
    6

    Re: Voltage drop

    Bennie,
    That is interesting. However, according to the plate ratings, I need 15 amps. Even so, your calculator shows #10 is fine for 15A. Here is the link for the calculator I have been using which is where I came up with the #6. What do you think?

    http://www.elec-toolbox.com/calculators/voltdrop.htm

  7. #7
    Join Date
    Feb 2003
    Location
    Massachusetts
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    48,485

    Re: Voltage drop

    15 amps is interesting, Table 430.148 lists a 1/2 hp 240 motor at 4.9 amps, according to the table 15 amps is about a 2 or 3 hp motor.

    Are you sure of the HP?

    Disconnects, conductor size and short circuit ground fault protection are to be selected by use of the tables in 430.

    430.6(A)

  8. #8
    Join Date
    Feb 2003
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    3,172

    Re: Voltage drop

    hconant: I don't understand the 15 amp load.

    3 percent is not required for a pump motor. 10 percent will work. 10 percent drop from 240 volts is 216 volts at the terminals.

    #14 cu is still within the parameters. Of course 10 or 12 would be sensible. #6 cu is overkill.

  9. #9
    Join Date
    Mar 2003
    Location
    Nord'est Wisconsin et Paris France
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    1,026

    Re: Voltage drop

    i have calauator for wire size (made by calcaulated industries)

    and i came up with wire size for 300 feet in lengh then i clicked on 240 volts then clicked on 5 amp ( typical for 240v 1/2 hp motor) and then clicked on wire size it came up 12 gauage wire size and voltage drop is 5.9 vd and at 2.5% volt drop and you can use the 12 or 10 gauage wire it will be plenty fine for this appaction and btw some 240 volts pump motors will be rated for 208 also so check the nameplate too .

    i did read the other post about this and why need a full 15 amp curcuit ???

    is this pump is two or three wire system it can make some diffrence there.
    most pump are 2 wire but few have 3 rd wire for starting only .

    please let us know what is your inted so we can steer you right

    merci , marc
    Marc
    master electrician
    Wisconsin and Paris France

    "Pas de problème, il marche n'est-ce pas?" (No problem, it works doesn't it?)

  10. #10
    Join Date
    Feb 2003
    Location
    Portage, Indiana NEC: 2008
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    9,771

    Re: Voltage drop

    What about startup current stalling the motor. The motor might only pull 4.8 amps running but can pull as much as 28.8 amps starting. this also has be accounted for. or this motor will be very sluggish in starting up if a high power factor cap isn't installed.
    Just a thought.
    Wayne A. From: N.W.Indiana
    Be Fair, Be Safe
    Just don't be fairly safe

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