MC4 connection questions...

TucsonAZTucsonAZ Solar Expert Posts: 136 ✭✭
As I get ready to install these this week I was just about to order my MC4 cables and I had a quick question. I have six 230w panels set up as three 48v strings, so on panel a/b I connect the positive of panel a to the negative of panel b giving me my 48v +/- and I do the same with panels c/d as well as e/f leaving me with six wires to run into the motorhome and the combiner box which will be inside.

Would I be able to connect the three negatives on the roof and only run four wires (three + and one -) instead of the six wires? I will effectively be doing that in the combiner box inside anyway and that would make things much easier for me up top as well as allowing for a smaller hole in my roof.

I was planning on buying branch connectors and longer cables so I could save money by cutting the longer ones in half instead of the ends off of the shorter ones.

Thanks a lot all of you for the help, as you can see I'm trying to give input on what I'm learning to others as well.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: MC4 connection questions...

    Yes you could connect the negatives of the three panel strings on the roof and run a single negative wire inside. But that wire would need to be capable of carrying the combined current of all three strings, as would any connectors used. In other words using a couple of those MC4 branch connectors may not be suitable for the current.

    A 230 Watt panel with a Vmp around 30 will have an Imp around 7.6 and Isc just over 8. Three together would be 24 Amps current. This is pretty close to the max 30 Amps for the 10 AWG wire and MC4 connectors normally used, so check your panel specifications.
  • TucsonAZTucsonAZ Solar Expert Posts: 136 ✭✭
    Re: MC4 connection questions...
    Yes you could connect the negatives of the three panel strings on the roof and run a single negative wire inside. But that wire would need to be capable of carrying the combined current of all three strings, as would any connectors used. In other words using a couple of those MC4 branch connectors may not be suitable for the current.

    A 230 Watt panel with a Vmp around 30 will have an Imp around 7.6 and Isc just over 8. Three together would be 24 Amps current. This is pretty close to the max 30 Amps for the 10 AWG wire and MC4 connectors normally used, so check your panel specifications.

    Damn, you know your (*&$, based on the Midnite calculator the PV array current is 23.16 with my system like it is, so I should be good as that's still under 80% of what they're rated for. I wish I had the info you have floating around in your head right now, want to trade me for something?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: MC4 connection questions...
    TucsonAZ wrote: »
    Damn, you know your (*&$, based on the Midnite calculator the PV array current is 23.16 with my system like it is, so I should be good as that's still under 80% of what they're rated for. I wish I had the info you have floating around in your head right now, want to trade me for something?

    Some of the info floating around in my head you would not want. Some of it I'm not allowed to divulge. So I disseminate on a need-to-know basis. ;)
  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: MC4 connection questions...

    Why not do two strings of 3 panels? Your 150V MPPT controller should handle that and you don't even need a combiner with two strings, plus less wire too.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: MC4 connection questions...
    solarix wrote: »
    Why not do two strings of 3 panels? Your 150V MPPT controller should handle that and you don't even need a combiner with two strings, plus less wire too.

    Well because:
    1). It's an RV so shading of individual panels may be an issue.
    2). At Vmp 60 for the array it could already be quite high compared to nominal system Voltage (which may be 12) so a further increase would be a further reduction in controller efficiency.
    3). Depending on the panels' actual Voc plus cold temperature factor three in series could be pretty close to the V-max in on the controller (depending on exactly which controller).
  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    Re: MC4 connection questions...

    Coot's one of our resident old timers. He's set me right a time or two.

    Cable is all about numbers, play around with the vdrop calculator a bit till you are happy:

    http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=60&phase=dc&noofconductor=1&distance=8&distanceunit=meters&amperes=20&x=36&y=12

    for that combined neg feed, 8 gauge it looks like. The panels themselves will usually be 4mm2 (about 11 guage) about 900mm long.
    for the three positives 4mm2 each looks ok at 8m.

    http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=4.132&voltage=60&phase=dc&noofconductor=1&distance=8&distanceunit=meters&amperes=8&x=27&y=13

    Btw better to not think in terms of 24/48V panels. They are current generators, and the voltage is all over the place. If you have to think in voltage terms use Vmp, whch for you is 60V.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


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