Solar array wire sizing for submersible pump

I have a Grundfos SQ Flex 25-7 submersible pump that is rated 30-300 VDC.  Also I have eight 100 watt panels (5.56 amps & 18 volts) I intend to wire in series to power the pump at a depth of 150'.  My preferred location for the solar array is approximately 120 feet from the water well. I am confused as to how to determine the appropriate wiring size with an voltage loss of 3% or less.  May calculations indicate with a distance of approximately 270' the wire size will be #4 AWG.  Is this correct? 
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  • Dave AngeliniDave Angelini Solar Expert Posts: 3,385 ✭✭✭✭
    Do you have the Grundfos  CU controller?
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail offgridsolar@sti.net

  • BB.BB. Super Moderators Posts: 26,951 admin
    With 4 AWG, 144 volts, 270 feet, 5.56 amps.

    http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=0.8152&voltage=144&phase=dc&noofconductor=1&distance=270&distanceunit=feet&amperes=5.56&x=54&y=12

    Voltage drop: 0.75
    Voltage drop percentage: 0.52%
    Voltage at the end: 143.25

    For ~3% max drop or less, 10 AWG gives:

    http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=144&phase=dc&noofconductor=1&distance=270&distanceunit=feet&amperes=5.56&x=61&y=13

    Voltage drop: 3.00
    Voltage drop percentage: 2.08%
    Voltage at the end: 141

    Check the calculator you are using? Some use 2 way round trip cable length, others (like this one) use one way distance.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • jcwashjcwash Registered Users Posts: 2
    Thanks for the replies - yes I have the CU 200 Controller.  My largest concern was the cost and weight of the approximately 150' of flat submersible wire of which 75' would be submerged.  It doesn't appear submersible is available in that large of a conductor.  So if I install 8/2 with ground I should be fine?
  • Dave AngeliniDave Angelini Solar Expert Posts: 3,385 ✭✭✭✭
    Bill's calc is for a smaller gage than your # 4 Roger?
    Just use the submersable in the well and regular 600 volt wire to a JB at the well head.
    If it were me I would under size the wire at maybe 5% with the plan to add more panels and raise the DC later.
    More solar always lets you run on marginal solar days, none of that generator business if you don't have to.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail offgridsolar@sti.net

  • BB.BB. Super Moderators Posts: 26,951 admin
    As Dave says, you can simply add another panel to make up for >3% losses.

    There is also a good reason that you can use smaller AWG (higher AWG number) on DC well pumps that are designed to work directly from solar panels... These DC pumps have zero surge current (they are "soft start"). Because solar panels cannot surge current either...

    So, you do not have to (over size wiring) and account for 5x surge current (locked rotor current) that a typical AC induction motor pump has.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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