MC4 dual source switch either a or b

I have a 800 watt solar panel array that feeds to my solar water pump.  I would like to add a switch to be able to chose either the solar array to power the water pump or an ac to dc inverter powered with a generator to pump when I don't have enough sun.  Currently I'm manually unplugging the mc4 connectors and plugging into the other source as needed.  I would like to find a switch with to choose either source a or b.  Any ideas if they are available.  I have also thought about just adding it with a y connector but I'm afraid it could back feed the solar array and cause damage or lose of current.


  • PhotowhitPhotowhit Solar Expert Posts: 5,804 ✭✭✭✭✭
    Sounds like you need a 'transfer switch' but one rated for DC.

    Something like a marine battery selector switch. This Blue sea one is rated up to 48 volts;
    m-Series Mini Selector Battery Switch with Removable Knob
    Though in this blow up says 32 volts, the description says 48...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former, 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • BB.BB. Super Moderators, Administrators Posts: 32,601 admin
    You could look at this Midnite transfer switch (available with 30 or 60 amp breakers):

    The problem is that DC current is very difficult to "break" with switches. And you need to look for high current DC rated (and AC?) rated switches or breakers.

    Midnite does have DC Rated breakers available, but I do not know which are in the above transfer switch box (you would have to contact our host Northern Arizona Wind & Sun, or Midnite directly).

    You could possible look into designing/building your own transfer switch panel using Square D QO breakers--Rated to 125 VDC:

    Maybe this (or similar) Square D panel:

    And some sort of (generator) interlock kit:

    Just some idea for the above. I have not gone into details (does genset interlock fit the  6 breaker panel above, etc.).

    The Blue Sea switch is only rated to 32 VDC--And I think you are running much higher voltages to the pump...

    MC4 connectors are not designed for lots of connecting/disconnecting--Something like 50-100 cycles (I think?).

    You are correct, you generally do not want two power sources to connect together--May let magic smoke out of solar array/AC inverter/etc.).

    Another way would be to use two receptacles... One connected to the array, and a second to your alternate power source. And a nice twist lock plug (XX amp rated) to the pump. And just plug into one socket or the other as needed. Using twist lock of some sort to avoid having a standard 15 Amps @ 120 VAC residential outlet that would encourage somebody to plug a hand drill or other non-appropriate genset into the system.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 32,601 admin
    Just to give you an idea of the issues between DC and AC switching--And how DC voltage sustains arcs very well (DC > 12 VDC):

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