What type of junction box on the roof?

Is there a special type of electrical junction box to use on the rooftop, where the solar panel connector wires enter the conduit going to the inverter? Is it possible to house the MC4 connectors that mate with the lead down wires in the conduit in the junction box? If so, since the connectors are rather large compared to the wires (#10) how do we seal the entry into the box water tight?

I am doing a self install. I am using THHN wires for the lead in, in the conduit. They are not outdoor wires. Any suggestions?


  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: What type of junction box on the roof?

    Many types of raintight junction boxes available. Need to be rated at least Nema 3. Make sure it is mounted correct orientation as many can't be laid down. We now use custom gasket sealed plastic boxes hidden under the array. Use lengths of PV wire with MC4 connectors to mate with modules, then run loose end into j-box to transition to THHN. Wire nut splices are OK, but junction blocks or water-resistant splices (Polaris is one brand) are better. Newer NEC codes are requiring the wires be labeled (string 1,2,3,etc) and have labels saying "Photovoltaic Power Source" on all exposed J-boxes and conduit. If your inverter is a grounded type, grounded wire needs to be white, ungrounded wire is black. If not grounded it can't be white - then most use black as the neg. / red as the positive. Red wire (even USE-2) will fade to white if exposed to sunlight though. Lots of guys just tape the ends of a black wire with red tape. Also, its best not to use the electrical convention of black and white wires for the lead down wires. Substitute gray (code allowed for grounded conductors) and something weird like orange for the ungrounded conductors. The next electrician will think twice before hooking up anything to those.
  • solar_davesolar_dave Solar Expert Posts: 2,394 ✭✭✭✭
    Re: What type of junction box on the roof?

    Mine has the junction box in the attic with a weather-head entrance to it.
  • sunflashxsunflashx Registered Users Posts: 1
    Re: What type of junction box on the roof?

    I'm planning on using a SoloDeck box for my install.


    Flashed out roof top junction box made with PV in mind.
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