Combined Negative!!!!

BatikikikBatikikik Solar Expert Posts: 141 ✭✭✭✭✭
Lets say we have 2 strings of panels . It means we have 2 positive and 2 negative .
Those string has to go to Sunny Boy DC Disconnect . To reduce number of wires I combined negatives on the roof and run only one negative wire to the negative terminal f the dc disconnect . But positives I run separate to each fused terminal .
Theoritycaly it is right . I just want too know your opinions guys .
Do I lose efficiency ? Does it unsafe? or I dont know , anything wrong with it or no ?
Thank you!

Comments

  • BB.BB. Super Moderators, Administrators Posts: 30,642 admin
    Re: Combined Negative!!!!

    Are you increasing the gauge of the negative wire by at least ~3 sizes?

    Also, for one or two parallel strings, you could combine the positive wires too. You only need fuses/breakers/combiner box for three or more parallel strings.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BatikikikBatikikik Solar Expert Posts: 141 ✭✭✭✭✭
    Re: Combined Negative!!!!

    I know that Bill . But situation is like this .
    I dont use combiner box I use DC disconnect with comes with sunny boy .
    So I run positives separete(because fuse in dc disconnect only 15amps and I can not combine 2 and the run to inverter).
    But negative you dont need overcurrent protection thats why I do combine . It works .I tried in couple jobs it works fine , but I wanna make sure everything is fine with that thats why I share my experience with you
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Combined Negative!!!!

    there's nothing wrong with combining the wire into one and we agree, but you have to increase the size of that wire to accommodate the higher currents presented to it just as bb said or you will have a higher voltage drop and a higher voltage drop %.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Combined Negative!!!!

    Think of it this way:

    The current that goes out the positive side comes back in the negative side; that's why it's called a "circuit".

    So with three positive wires and one negative, the negative has to be large enough to carry the combined current of the three positive wires.

    If each string produces, say, 8 Amps then there is 8 Amps going out each positive wire but 24 Amps coming back in the negative side. Therefor the single, combined negative wire needs to be larger.

    (And someone is going to say DC current flows out the negative and in the positive. I just know it! :p But it doesn't matter; the principle is the same.)
  • BB.BB. Super Moderators, Administrators Posts: 30,642 admin
    Re: Combined Negative!!!!

    I think Batikikik is an installer (out of the US)--More along the lines of a code question rather than basic electrical.

    As long as the return line is not spliced at the panels (safety ground, or to other panel returns to create a parallel return run--for say 4 strings of panels)--I cannot think off hand of any reason that this would be an issue.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Solar GuppySolar Guppy Solar Expert Posts: 1,963 ✭✭✭
    Re: Combined Negative!!!!

    You can't combine any wires on PV side, for isolated inverters you have the code required GFI to deal with, for the new non-isolated inverters, the PV wires don't correspond to what one typically thinks is the "ground" side of the circuit

    Also, if your doing Offgrid, some charger controllers require separate negative returns for proper operation. There is no advantage of combing as you need larger wire size as others have posted
  • BatikikikBatikikik Solar Expert Posts: 141 ✭✭✭✭✭
    Re: Combined Negative!!!!

    BB I am from Los Angeles and me I am not a professional installer I runing my solar business and I do engineerig and design I have my own Installers , and I have a bachelor in electrical also NABCEP certified Installer etc.
    The reason my post is to share with my experience I did it with larger wire size as you posted here guys.
    Thank you for your posts :)
  • BB.BB. Super Moderators, Administrators Posts: 30,642 admin
    Re: Combined Negative!!!!

    It is great to have others here who know what they are talking about (:roll::blush:).

    What SG is warning about, I believe, is that some GT inverters have separate inputs (two or more MPPT inputs). So--it is not simple to combine any leads from a solar array unless you know the exact inverter/system you will be wiring too.

    For those GT inverter with separate MPPT inputs (or if you have two or more GT Inverters)--You cannot combine positive or negative leads between the inverters--They need to measure the current flow through their attached arrays and if there are alternate current paths--it will mess up their measurements and possible violate the Ground Fault Detection circuitry too.

    Also--for some panels (SunPower from Germany)--Require a positive grounded inverter (otherwise, the panels build up an electrical charge the reduces their output power). So, in that case, the fusing would be on the negative array leads and the positive would be grounded through the Ground Fault system.

    And to add to the mix--there are now Non-Isolated GT inverters now on the market (no transformer isolation between the array and the 240 VAC utility power)... So combining arrays can cause issues there across different MPPT inputs/inverter inputs.

    I think I got that correct--Solar Guppy is a man of few words and I -- obviously -- have the other problem.;)

    In any case, we strive for clarity and safety on the forum.

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