mc4 junction box???

Newbie here:

I am wondering if anyone can provide some guidance on installing a mc4 junction box to a home made solar panel? I have the solar panel complete with two leads (pos and neg) coming out the back of the panel. I have seen a few youtube video's showing how to install a mc4 junction box but it only shows 4 tabbing or bus wires coming out of the back of the panel, but the video only says if the wires are pos or neg. I'm at a loss right now on how to wire it up. Does anyone have any diagrams showing what wires go into the mc4 junction box?

Thanks

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,517 admin
    Re: mc4 junction box???

    Welcome to the forum... Your first post did come through--You should not need to create a second thread. It will spread the same discussion across several threads.

    I will past some links for you to read/watch... Hopefully they will help you understand. Otherwise, you may need to post pictures of the parts and real of the panel if you need more information:

    http://www.electricaltechnology.org/2013/03/how-to-get-connection-from-solar-panel.html

    This is a video--Note that the narration is wrong--that is a BYPASS DIODE, not a blocking diode (above link shows bypass diodes and how they are installed).

    http://www.instructables.com/id/How-To-Build-A-Solar-Panel/step8/Installing-the-Junction-Box/

    If you have glass panels with teldar or some sort of thin plastic backing, then you will probably use type 2 neutral cure silicon seal. If you are using wood backing--you probably will add a couple screws to hold the J-box down (and some silicon sealant).

    Does this help... If not, can you be a little more specific with your question(s)?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • fcSolarfcSolar Registered Users Posts: 5
    Re: mc4 junction box???

    Sorry about the repost. I thought my initial post probably went to the wrong forum so I reposted. I am attaching two files, one file shows my solar panel wiring without the mc4 connector. The second file shows the mc4 connector itself. I guess the question is how do you wire the mc4 connector into the panel based on the wiring in the attached file? I am assuming that I will probably need to slightly change the way I have wired the cells together to accommodate a mc4 junction box.

    Thanks for the links I will check them out.

    Shawn

    Attachment not found.
    Attachment not found.
  • NorthGuyNorthGuy Solar Expert Posts: 1,925 ✭✭
    Re: mc4 junction box???

    Looks like your box is for 3 strings connected in series, and you have only one string. I would connect it as if it was one of the three.

    Unless you need a blocking diode for whatever reason, I would short where the other two non-existent strings were supposed to be.
  • fcSolarfcSolar Registered Users Posts: 5
    Re: mc4 junction box???

    That makes sense. I will try that.

    Thanks
  • fcSolarfcSolar Registered Users Posts: 5
    Re: mc4 junction box???

    Ok I was thinking about this last night. I am hoping that the attached diagram makes sense. Does it look like the mc5 junction box that I have pictured could be wired in this fashion? The panel I have built is only a 60 watt panel so I am actually thinking the junction box that I have purchased is probably way overkill?

    As you stated earlier the diodes in the junction box are bypass diodes. Within the attached cell configuration this would allow the panel to still produce half the wattage if half the panel is fully shaded (assuming the shading is hitting the correct cells :) ).

    Thanks again for your assistance. I would like to string in parallel 4 or 5 panels together to charge a pack of batteries. I haven't yet determined the number of batteries that I should use. At this point I'm just trying to sort it all out :)

    Attachment not found.
  • NorthGuyNorthGuy Solar Expert Posts: 1,925 ✭✭
    Re: mc4 junction box???

    You do not need a separate blocking diode because the bypass diode intended for the missing string will work as a blocking diode.

    Most likely, you do not need a blocking diode at all. That depends on your controller. Most controllers simply switch the array off at night, so blocking diode is not needed. It won't hurt, but it will detract from performance. If you do not need it, simply short between terminals 2 and 3 on your diagram.

    Make sure you use correct polarity!
  • BB.BB. Super Moderators, Administrators Posts: 29,517 admin
    Re: mc4 junction box???

    If I did not make any mistakes--The "middle diode" bypass diode should be replaced with a jumper wire (or you can put a jumper wire across the two middle terminals if they will hold the extra wire with the panel connections too). You only have two strings and a box designed for three strings.

    I would not even use the blocking diode unless you have no solar charge controller and are directly connecting the panels to a 24 volt or higher battery bank. And if true, I would use a single external diode per string (less power drop as you only have one blocking diode and better cooling for the diode).

    Lastly--Just to be safe--Home made solar panels, especially when you are getting larger panels and strings of panels can be a real fire hazard. If you install these panels unattended, they should be somewhere that if there is a fire, it does not put property or people at risk.

    And for any panels, there should be an inline fuse/breaker (such as a protected combiner box) per string if you have (typically) three or more strings connected in parallel.

    Panel Fire Question

    There are other details about DIY solar panel construction that make them very difficult to justify for building them at home (for example, many times EBay'ed cells are mfg. rejects because, for example, they have current defects which can cause cell failures/overheating).

    With commercial products below $2 (and even sometimes near/below $1 per Watt), we try to discurage people from making there own panels as other than a learning experience. Most DIY panels will only last weeks to a couple months if permanently mounted outside in weather.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • fcSolarfcSolar Registered Users Posts: 5
    Re: mc4 junction box???

    Thanks Bill for your input. It is very much appreciated. Right now I have a small 30 to 40 watt solar panel and a cheap charge controller in my shed. The panel is charging a 12 volt battery and I have a 400watt inverter used for lights in the shed. Very basic setup. I am hoping to start with new hardware (charge controller, battery pack, inverter). I will probably end up just buying the solar panels rather than making them as it doesn't feel like you can make a solar panel when you include time as money for the price the panels are now.

    The panels I am building now are made with:
    - tempered glass
    - 3x6 inch solar cells
    - solar cell encapsulant
    - aluminum solar panel frame

    I will send a picture of a frame when complete. I was also worried about fire hazard, mostly from my cheap charge controller or inverter. I wasn't as worried about the solar panel as being a fire hazard, however I did read an article about grounding the panel (I believe its called earthing in the UK, it was an article originating from the UK). I hope to be done with my second panel tomorrow. I am waiting for instructions on the solar panel junction box from the distributor.

    Thanks again for your comments.

    Shawn
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