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wiring details of ground-mount system w/ string inverter, plus generator backup

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  • wiring details of ground-mount system w/ string inverter, plus generator backup

    I'm confused about many of the details of wiring my DIY ground-mount system. I have extensive experience with ordinary home wiring. My jurisdiction is subject to the 2017 NEC; DIY wiring is allowed by a homeowner (subject to inspection of course). Here is my current thinking:

    Screen Shot 2020-02-26 at 4.44.23 PM.png

    I'll have 16 panels connected in two strings to the two MPPT inputs on an SMA SunnyBoy SB 3.8-1SP-US-41 inverter.; the inverter has an integrated DC disconnect. The inverter will be attached to the ground-mount, so I'll be running 240vac to the house. My grid connection is 240v split-phase.

    I'll also be adding an essential-loads sub-panel next to my main panel, for use with a small generator. It'll be a main lug panel, with two interlocked double-pole breakers, one connected to a generator jack and the other to a double-pole breaker in the main panel. Possibly if the generator has an un-defeatable ground/neutral bond, I'll need to use a transfer switch or interlocked 3-pole breakers.

    Being ground-mount, the system does not require rapid-shutdown. In fact, the DC will remain active during grid outages, so that I may use the inverter's "secure power" outlet (thus enabling a little grid-down power when the sun is shining). The UL1741 compliant inverter will shut down the 240vac feed coming into the utility room during grid outages.

    Questions:

    1. Am I missing any components, or are there some I might omit ?
    - I don't need a DC disconnect as that is built into the inverter.
    - Do I need an AC disconnect near the POCO meter, since the AC line will go dead when the grid is down anyhow ?
    - Must I (or might I want to) add a meter for solar generated power ? Or will the inverter monitor that for me ?

    2. How do I go from the male and female MC4 connections at each end of each string to the bare wire ends for connection to the inverter ? Are there simply MC4 cables I can buy ?

    3. Should the AC from the inverter be connected to a double-pole breaker in the main panel (as shown) ? I'm pretty sure I've seen diagrams where the AC is simply spliced into the cables from the meter to the main breaker.

    4. I understand the AC connection between inverter and house should be 3-conductor plus ground (this inverter requires neutral connection). The panels will be mounted a treated-wood structure, so each will require a grounding lug. The panel grounds are connected to the inverter, the ground in the AC cable and a grounding rod. However, I'm unclear if it's necessary (or possible) to connected the '-' MPPT inputs to ground.
    Attached Files
    Last edited by RShackleford; 02-26-2020, 05:48 PM.

  • #2
    Rusty, it is Rusty, isn't it?
    As for the MC-4 connections, you can buy cables with the connectors on both ends. you simply buy the proper length cable to cover the total +ive and -ive length and cut the cable appropriately.
    One thing that might help you to use shorter cable runs is by using the leapfrog wiring method shown below.

    You do know the limitations of the secure power feature don't you? One you are limited to 1500 or 2000 watts output, depends on inverter model. Of course if your solar array isn't producing that much power, ie; heavy cloud cover, early or late in the day or nighttime you get what you get/ This feature needs to be turned on at the inverter and ALSO needs to be turned off, manually. If the power comes back on from the poco you won't know it until you turn off the secure power and the inverter requalifies the grid.

    Leapfrog wiring of series wired panels.jpg
    Last edited by littleharbor; 12-10-2019, 09:07 AM.
    2.2kw Suntech mono, Classic 200, NEW Trace SW4024

    Comment


    • #3
      Originally posted by littleharbor View Post
      Rusty, it is Rusty, isn't it?
      No, not really . I need to add a pic to my profile ...

      As for the MC-4 connections, you can buy cables with the connectors on both ends. you simply buy the proper length cable to cover the total +ive and -ive length and cut the cable appropriately.
      Thanks.

      One thing that might help you to use shorter cable runs is by using the leapfrog wiring method shown below.
      Cool. If the panels' cables will reach two panels over. I gotta figure out how I'm going to arrange my two strings also.


      You do know the limitations of the secure power feature don't you?
      Yep.

      This feature needs to be turned on at the inverter and ALSO needs to be turned off, manually. If the power comes back on from the poco you won't know it until you turn off the secure power and the inverter requalifies the grid.
      Not following. Referring to my drawing, let's say the poco goes out. I flip the interlock on the critical-loads subpanel, and attach the "generator" (more likely, a 1kW inverter connected to our Nissan Leaf with its 40kWh battery). So the critical loads are powered, but the other loads are still connected to the grid. When grid comes back, I'll see power in those loads.

      Not really sure how the "secure power" will fit into this, just seems like a cool thing to have. Probably what happens is I use the Leaf's charging cord, during the day, to put juice from the secure power into its battery.

      Comment


      • #4
        Originally posted by RShackleford View Post
        No, not really . I need to add a pic to my profile ...


        Thanks.


        Cool. If the panels' cables will reach two panels over. I gotta figure out how I'm going to arrange my two strings also.



        Yep.


        Not following. Referring to my drawing, let's say the poco goes out. I flip the interlock on the critical-loads subpanel, and attach the "generator" (more likely, a 1kW inverter connected to our Nissan Leaf with its 40kWh battery). So the critical loads are powered, but the other loads are still connected to the grid. When grid comes back, I'll see power in those loads.

        Not really sure how the "secure power" will fit into this, just seems like a cool thing to have. Probably what happens is I use the Leaf's charging cord, during the day, to put juice from the secure power into its battery.



        "Not following. Referring to my drawing, let's say the poco goes out. I flip the interlock on the critical-loads subpanel, and attach the "generator" (more likely, a 1kW inverter connected to our Nissan Leaf with its 40kWh battery). So the critical loads are powered, but the other loads are still connected to the grid. When grid comes back, I'll see power in those loads."

        Sounds like you could use any GTI then, being you are powering your critical loads with a generator

        .Not really sure how the "secure power" will fit into this, just seems like a cool thing to have. Probably what happens is I use the Leaf's charging cord, during the day, to put juice from the secure power into its battery.
        [/QUOTE]
        I guess you could do that when the power goes out but then again you won't know when the power comes back on if using the secure power feature. What if the outage is a relatively short one. All outages in this area are less than 15 minutes.
        2.2kw Suntech mono, Classic 200, NEW Trace SW4024

        Comment


        • #5
          Originally posted by RShackleford View Post
          No, not really . I need to add a pic to my profile ...


          Thanks.


          Cool. If the panels' cables will reach two panels over. I gotta figure out how I'm going to arrange my two strings also.



          Yep.


          Not following. Referring to my drawing, let's say the poco goes out. I flip the interlock on the critical-loads subpanel, and attach the "generator" (more likely, a 1kW inverter connected to our Nissan Leaf with its 40kWh battery). So the critical loads are powered, but the other loads are still connected to the grid. When grid comes back, I'll see power in those loads.

          Not really sure how the "secure power" will fit into this, just seems like a cool thing to have. Probably what happens is I use the Leaf's charging cord, during the day, to put juice from the secure power into its battery.



          I just figured you were using The name Rusty Shackelford from the King of the hill cartoon series.


          Pretty much all panels can be leapfrogged in Portrait orientation, not all panels work in Landscape though.
          Last edited by littleharbor; 12-11-2019, 04:01 PM.
          2.2kw Suntech mono, Classic 200, NEW Trace SW4024

          Comment


          • #6
            Originally posted by littleharbor
            Pretty much all panels can be leapfrogged in Portrait orientation, not all panels work in Landscape though.
            In my preferred landscape mounting, no way would the leads reach. So I alternated the direction
            of the panels, so I would only need an extension jumper for every other panel. Bruce Roe

            24ReSiSec.JPG

            Comment


            • #7
              Originally posted by littleharbor View Post
              I just figured you were using The name Rusty Shackelford from the Hing of the hill cartoon series.
              That what I've thought this whole time, and a pic of Dale would have been a great avatar.

              Comment


              • #8
                Originally posted by sdold View Post
                That what I've thought this whole time, and a pic of Dale would have been a great avatar.
                Ok, you guys happy now ?

                Comment


                • #9

                  Sounds like you could use any GTI then, being you are powering your critical loads with a generator
                  What is "GTI" ?

                  I guess you could do that when the power goes out but then again you won't know when the power comes back on if using the secure power feature.
                  I'm a bit confused about people bringing up not knowing the grid is back up; isn't this always an issue ?

                  Also, since my non-critical loads will remain connected to the grid (per drawing in my OP), they will clue me in that the grid is back, right ?

                  Comment


                  • #10
                    No one has addressed the issue of how the inverter's AC output connects to my main panel. Some places I see that it is done via a breaker in the panel (as in my drawing), but I've also heard of it simply being spliced into the service-entry cables (from the meter to the main breaker).

                    I wonder if the breaker option has to do with some POCOs requiring a way to disconnect the inverter from the grid. I think mine does, but I think it has to be on the outside of the house; that's the box labeled "disconnect" in my drawing - I can't imagine the disconnect and the breaker would both be required.

                    Comment


                    • #11
                      Sounds like you could use any GTI then, being you are powering your critical loads with a generator What is "GTI" ?
                      Grid Tie Inverter.

                      I guess you could do that when the power goes out but then again you won't know when the power comes back on if using the secure power feature. I'm a bit confused about people bringing up not knowing the grid is back up; isn't this always an issue ? Also, since my non-critical loads will remain connected to the grid (per drawing in my OP), they will clue me in that the grid is back, right ?

                      The SMA Inverter will disconnect from the grid and won't requalify to the grid till the Secure Power feature is switched off. You are probably right about other non critical loads lighting back up when the grid comes back on. You will need to look for them being other loads are being powered by your generator and secure power feature.

                      Sorry about this non standard quote reply. Something is acting funny and I couldn't get the "Quote" feature to work.
                      Last edited by littleharbor; 12-14-2019, 09:25 AM.
                      2.2kw Suntech mono, Classic 200, NEW Trace SW4024

                      Comment


                      • #12
                        @RShackleford , Thanks for the link to this thread. I don't know why I didn't see it earlier. Now that the array issues are hashed out, I guess it is time to revisit this thread. It seems there are two issues that haven't been addressed. One, the question about the line side tap. Two, I am not sure you have purchased an inverter yet and the question might be what type of back up source to use in the event of power outage.
                        Do I have that right?

                        I am also intrigued with the comment that you have considered some way to access your Nissan Leaf battery. Even though I have a hybrid inverter, powered by Nissan Leaf modules I have assembled a 1500W inverter that I can connect to my 12 volt battery in either of my Teslas to provide emergency power to my daughter and/or my sister who live nearby and have been subject to some of the same public safety power outages recently.
                        9 kW solar. Driving EVs since 2012

                        Comment


                        • #13
                          Originally posted by littleharbor View Post
                          .....
                          One thing that might help you to use shorter cable runs is by using the leapfrog wiring method shown below.
                          ........
                          Just a shoutout to @littleharbor to say thanks for this trick. I have a similar situation and this will simplify my wiring.
                          9 kW solar. Driving EVs since 2012

                          Comment


                          • #14
                            Originally posted by Ampster View Post
                            One, the question about the line side tap.
                            Talked to a fellow who consults on local installations and he says the AHJ is fine with line side taps. POCO guy wasn't thrilled, but mostly because it means I'm without power after they pull the meter and before I finish the wiring, get it inspected, and have them re-install the meter. The AHJ actually wanted to know if I was doing it this way, so that they can expedite that final inspection for me; so that's cool. Plus, it'll give me a chance to take my Leaf backup system for another test drive

                            Two, I am not sure you have purchased an inverter yet and the question might be what type of back up source to use in the event of power outage.
                            I have the inverter, and the model is in my OP. It has that "secure power" supply thing; dunno how well it'll really work, but it's a good inverter, so I figured what the heck.


                            I am also intrigued with the comment that you have considered some way to access your Nissan Leaf battery. Even though I have a hybrid inverter, powered by Nissan Leaf modules I have assembled a 1500W inverter that I can connect to my 12 volt battery in either of my Teslas to provide emergency power to my daughter and/or my sister who live nearby and have been subject to some of the same public safety power outages recently.
                            Not quite following. You have a hybrid solar system, and have loose Leaf modules as the storage ? And, separately, you're doing the same thing with your Teslas as I'm doing with my Leaf ?


                            Comment


                            • #15
                              Originally posted by RShackleford View Post
                              Talked to a fellow who consults on local installations and he says the AHJ is fine with line side taps. POCO guy wasn't thrilled, but mostly because it means I'm without power after they pull the meter and before I finish the wiring, get it inspected, and have them re-install the meter. The AHJ actually wanted to know if I was doing it this way, so that they can expedite that final inspection for me; so that's cool. Plus, it'll give me a chance to take my Leaf backup system for another test drive
                              My understanding is most line side taps are done because of buss capacity issue. Either the calculation about the buss Amperage capacity or just no more room for an additional breaker. Clearly adding a breaker is simpler with no downtime or need to coordinate.
                              I have the inverter, and the model is in my OP. It has that "secure power" supply thing; dunno how well it'll really work, but it's a good inverter, so I figured what the heck.
                              I understand now. Those are a cost effective solution to the issue of power outages.

                              Not quite following. You have a hybrid solar system, and have loose Leaf modules as the storage ? And, separately, you're doing the same thing with your Teslas as I'm doing with my Leaf ?
                              I made it more complicated than needed.My hybrid inverter is irrelevant for your purposes.
                              My separate inverter is a way to provide some portable backup to my relatives in case of an emergency. In the process I discovered the capacity of the DC to DC converter that allows me to pull over 100 Amps at 12 volts that running 1500 Watts of power would require. I currently can connect the inverte with jumper cables to my 12v battery. Long term want to set up Anderson connectors to make it easier and safer. It is only intended to be used on an 120 volt extension cord and I have no plans to connect it to to an Generator input for a critical loads panel. In my case my critical load panel would require 240 volts anyway.

                              9 kW solar. Driving EVs since 2012

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