Cheap disconnect for two small panels

Steve961Steve961 Solar Expert Posts: 93 ✭✭✭✭
I'm looking for a cheap, and easy, disconnect for two small panels at my off grid cabin. This is not for my main array, which already has a Midnite breaker, but for a backup built with some components I bought before I really knew what I was doing. The two panels are 3.9 amp 12 volt panels run in parallel (7.8 amps total) to a 10 amp SunSaver charge controller. Since the disconnect is going to be indoors, and just for my use, could I possibly use a simple 12 volt automotive switch? I found one at Fleet Farm that's rated for 20 amps at 12 volts for a whopping $1.39.

I was also looking at Morningstars string calculator, and it says that my panel could reach 29.26 volts max voc at the record low temperature in my area. Does this then go beyond the capabilities of an automotive switch?

Thanks.

Steve

Comments

  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Cheap disconnect for two small panels
    Steve961 wrote: »

    I was also looking at Morningstars string calculator, and it says that my panel could reach 29.26 volts max voc at the record low temperature in my area. Does this then go beyond the capabilities of an automotive switch?

    Thanks.

    Steve

    Technically, yes. In practice, it is hard to say without knowing more about the switch. The problem with DC is usually not the insulation but the ability to extinguish the arc which forms when the switch opens under load, without damage to the contacts which will cause problems later. You are correct that in terms of disconnecting under load, the Voc is the important factor.

    One the other hand, if you can disconnect the battery side of the CC first, then there will be little or no current in the panel-side disconnect when you open it.

    Particularly since the switch will be inside, I would be reluctant to use it myself. But I am very conservative where it comes to safety except while climbing on a roof. :-)
    The really serious problems come when dealing with a circuit breaker or fuse, which has to extinguish an arc with the full battery current behind it in case of a circuit failure.
    SMA SB 3000, old BP panels.
  • icarusicarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Cheap disconnect for two small panels

    Square D QO breakers are DC rated for ~48 vdc iirc. Pretty cheap to buy, a small enclosure or load center is also prety cheap. You could also simply use either a automotive blade fuse of the proper rating,, or more simply a simple light switch. Personally, I wouldn't worry too much about 7 amps, when you are more than likely only going to see 5', and probably not going to be throwing the switch very often. I believe there are Dc rated switches out there as well.

    Tony

    PS. Interdogs Battery current not with standing, assuming you have the battery conductors fused, you are only going to be routinely breaking the circuit at under 7 amps,, you 20 amp auto switch should be fine.

    T
  • SlappySlappy Solar Expert Posts: 251 ✭✭✭✭✭✭
    Re: Cheap disconnect for two small panels

    Knife switch. Their cheep and common item.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Cheap disconnect for two small panels

    those cheap dc switches are ok and will suffice, but they aren't high quality and often break sooner rather than later so a spare or 2 may be good to have.
  • Steve961Steve961 Solar Expert Posts: 93 ✭✭✭✭
    Re: Cheap disconnect for two small panels

    Thanks for all the help guys. I'm going to pick up a heavy duty 50 amp DC toggle switch from my local NAPA store just to be on the safe side. And FYI, there's already a 15 amp fuse between the charge controller and battery to deal with any major problems.
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