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  • PV to THNN/ THWN connection

    I have a connection to make where my strings come off the roof and into a 100' pvc conduit to the house. I have #10 PV from the panels, and #10 THWN through the pvc to the inverters. What is the preferred connector? I have seen some people use wire nuts which looks awful. Several clean looking installations that I have looked at used nice insulated set screw butt connectors with dielectric grease, but I've been unable to find a manufacturer or product name. The electrical supply counter guys are less than helpful.
    And advice is much appreciated.

  • #2
    Here is a link to a Burndy product. The first page is the expensive ones. Scroll down for the smaller ones. Home Depot has similar ones for less.
    https://www.amazon.com/Burndy-BIT2-I...ustrial&sr=1-3
    9 kW solar. Driving EVs since 2012

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    • #3
      I'll second burndy unitap.

      split bolt may also be possible to do in a code-compliant way. But doing that right I think requires a lot of taping, and I think with different types of tape.

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      • #4
        Why not a transition/combiner box?
        2.2kw Suntech mono, Classic 200, NEW Trace SW4024

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        • #5
          If you have more than 2 parallel "strings" you will need a combiner box which usually has the proper terminal blocks that allows you to connect those wires.

          If only one or two large strings then you have to make sure the terminations are rated for the higher DC voltage each string can produce.

          Tell us more about what your system is and we can help direct you to the proper terminal blocks.

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          • #6
            The system is three strings from a barn roof, feeding two inverters in the house basement. 2 strings to one and 1 string to the other. The strings are 10 wire, and I have 65' of buried 1" pvc conduit. At some point in the future I may upgrade to a single inverter with more MPPT (sunnyboy 7.7) so I want to have each of the strings run all of the way to the house. The connection is where the conduit comes down from the roof and transitions from PV to THWN to run underground. So, as I am not combining strings, I was hoping to not use a combiner. Typically I think a DC disconnect here might do the job, but my DC disconnect is in the basement with the inverters. The plans examiner said as I have the Tigo RSS it would be redundant to also have a manual disconnect there.
            Let me know what your thoughts are, this is my first time doing this solo, and it's a lot to learn.
            Thanks, SD

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            • #7
              Originally posted by badwithusernames View Post
              The system is three strings from a barn roof, feeding two inverters in the house basement. 2 strings to one and 1 string to the other. The strings are 10 wire, and I have 65' of buried 1" pvc conduit. At some point in the future I may upgrade to a single inverter with more MPPT (sunnyboy 7.7) so I want to have each of the strings run all of the way to the house. The connection is where the conduit comes down from the roof and transitions from PV to THWN to run underground. So, as I am not combining strings, I was hoping to not use a combiner. Typically I think a DC disconnect here might do the job, but my DC disconnect is in the basement with the inverters. The plans examiner said as I have the Tigo RSS it would be redundant to also have a manual disconnect there.
              Let me know what your thoughts are, this is my first time doing this solo, and it's a lot to learn.
              Thanks, SD
              I would think that a basic weather proof junction box with grommet entries for the wires and either a terminal block or split bolt connectors would work.

              Again that kind of detail is usually on the permitted electrical drawings.

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              • #8
                Originally posted by SunEagle View Post

                I would think that a basic weather proof junction box with grommet entries for the wires and either a terminal block or split bolt connectors would work.

                Again that kind of detail is usually on the permitted electrical drawings.
                My drawings show the 8x8 weatherproof box location, just not the connector type. The Bundy connectors look solid, so I think I will look for them locally before mail order.
                Thank you all, this site has proved invaluable to search and get solid answers when I'm stumped.
                SD

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                • #9
                  Originally posted by littleharbor View Post
                  Why not a transition/combiner box?
                  I think that is a given, as well as appropriate breakers which might serve as the connection devices. In my case it was one string so a waterproof junction box, some grommets a drain hole, and Burndy connectors is all I needed to transition from PV wire to THNN..
                  9 kW solar. Driving EVs since 2012

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                  • #10
                    Searching around for different types of connectors and hardware, I found this:
                    https://www.factorymation.com/SI25-PM64R-6
                    One knob to shut down all three strings, it solves my splice problem, and I think it will make the inspector happy. He was here yesterday to insect all of the conduit/ device locations prior to pulling wire, and he was not happy that the plans didn't show a dc disconnect on the barn. It's IP66 rated, so I would just have to mount it in the right weathertight box
                    SD

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                    • #11
                      Originally posted by badwithusernames View Post
                      he was not happy that the plans didn't show a dc disconnect on the barn.
                      *sigh* .... I don't know why people insist on adding more things that can fail into flames...

                      The Greatest Debacle in Solar PV: Australia’s Rooftop DC Isolator Fires

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                      • #12
                        It's your choice what you install. nwdiver makes a good point that simpler is better.

                        DC disconnect can be dangerous because of stored energy in cable inductance and the arc possible when you disconnect. That switch is rated for DC disconnection and high voltage, so should be fine. It looks like it was designed for solar use. You're right that you need to find the right enclosure. If you want the switch lever accessible from outside the box, you also need the right front mounting. That may mean a large round hole or something more complex. It's tough to tell from the web page. If it is not accessible from outside the box, it may not meet the inspector's requirement.

                        I just thought of one alternative that you might not have considered for joining cables. There are low-cost crimps that will make a solid connection between cables. I recently bought a low-cost imported crimp tool with ratchet mechanism and machined hardened jaws and it works remarkably well, especially compared to the old-fashioned combination tools. Crimps are probably equal in performance and dependability to the Burndy device mentioned above, but cheaper.

                        That said, the Burndy connector is reusable, but you won't want to reuse the same wire stub. With the Burndy connector, the DC disconnect switch you found, or any screw terminal, you need to torque the bolts per specification. A Harbor Freight 1/4" torque wrench with 1/4" hex socket and screwdriver bit is fine for that.
                        7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV

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                        • #13
                          Originally posted by bob-n View Post

                          DC disconnect can be dangerous
                          And pointless. A DC disconnect at the inverter or charge controller is probably a good idea so you can de-energize your equipment but that's usually built into the inverter. I've seen ground mounts with DC disconnects mounted at the array. Why? It's not going to de-energize the array. If you need to stop current then open the disconnect at the inverter or CC. Adding to the pointlessness is if you have optimizers or rapid shutdown. 'LOL' there's solar arrays out there with like 4 different ways to shut it down.

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                          • #14
                            With my 2 inverters inside the shed, the disconnects are on the outside. My installer broke
                            only the plus (hot) side of the DC run from the array. Unfortunately while disconnected and
                            repairing an array wire, I must have brushed something against the frame and blew the
                            ground fault fuse in the inverter. This caused some distress and a service call.

                            The moral of this story is to break BOTH DC leads from an array with the disconnect switch.
                            Here can be seen a second DC disconnect box added, for enough poles to break all DC
                            wires from the array. The smaller boxes are AC disconnects. Bruce Roe

                            PVdiscon.jpg

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                            • #15
                              Thank you for the input. I agree that having multiple points of shutoff is pointless. With the inverter based shutoff, and Tigo RSS, you can shut it down at the panel end, or the inverter end; having a disconnect mid way serves what purpose?
                              I have forwarded a bunch of info to my inspector and am waiting for a response. PITA, as he's the kind of guy that doesn't like to be told what to do, so I have to carefully choose my words. I know if I tell him the disconnect doesn't need to be there he'll want me to install three of them, just to be a dick.
                              SD

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