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AC Disconnect Sizing and Type for Line Tap (no backfeed)

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  • AC Disconnect Sizing and Type for Line Tap (no backfeed)

    Should the AC disconnect be fused when there is NO other breaker in the line? (does the fuse count as protection like a breaker would?)

    Must we disconnect just the hots, or also neutral (2 or 3 pole)? And if fused, would I need 2 poles or all 3 (including neutral) to be fused?

    My setup:

    I have Microinverters into an Enphase Combiner 3 box. A single AC feed from the combiner to the AC Disconnect, and then to the meter directly via line tap (no backfeed breaker).
    Screenshot 2020-10-16 163339.png
    (pic above shows a 2 pole AC Disconnect, Neutral is pass-through)

    Right now I have a 46 Amp setup. The Enphase Combiner has breakers for each array (4 max, 1 shown in pic), but NO breaker out the final output, as you can see. So it directly feeds the AC Disconnect, which directly feeds to the main bus (again, no breaker, no back feed). Is a breaker needed at all? Or is the purpose of the AC disconnect strictly about disconnecting, and not about overcurrent protection? If we need overcurrent protection, would too large a fuse be a problem (say, a 100A fuse rather than 60A in my case with a 46A production)

    Do we need overcurrent protection between our PV AC production and the utility meter in order to meet code?
    Attached Files

  • #2
    You spear to have a 200A conductor in the main box. Before you transition to 8ga wire, you need overcurrent protection.
    Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister

    Comment


    • #3
      Originally posted by Mike90250 View Post
      You spear to have a 200A conductor in the main box. Before you transition to 8ga wire, you need overcurrent protection.
      The 200A is a disconnect for the meter combo panel. The bus of that panel is rated at 225A. My solar input is 46A directly onto that bus (via 4ga). I have a 100A fused 2-pole disconnect but am now wondering if that is the correct device for the job, and considering swapping it out.

      EDIT: oops, I missed your point. At 200 (or 225A) the conductor coming off that bus would have to be huge, right?
      Last edited by dapug; 10-16-2020, 07:49 PM.

      Comment


      • #4
        when you leave the 225a bus, you need a breaker rated for the wire used. #4 still will need a 60A breaker
        Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
        || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
        || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

        solar: http://tinyurl.com/LMR-Solar
        gen: http://tinyurl.com/LMR-Lister

        Comment


        • #5
          Here used 4 ga wire with an 80A breaker. Not common, have to order it.
          Bruce Roe

          Comment


          • #6
            You are getting good advice from Mike and Bruce.

            The key is that you should not have wires coming off the line without immediately having a breaker right there. Breakers in the combiner are not enough.

            Imagine if someone drives a nail through something and it shorts the wire at some point along the run between the meter and the disconnect. There's nothing to save you except the huge fuse on the power pole. That's potentially a big fire. If you live through it and call your insurance company, they may disallow the claim due to non-code wiring.

            Mike and Bruce are also guiding you to the right size breaker. You can't have a high current breaker and thin wire. The wire size must be right for the breaker. Again, that's code and required for fire safety.

            Are you in a location that has town or city inspectors? In my town, the inspectors must approve the electrical drawing before I can start work. They also give good advice. If not, you may want to get an electrician to give you advice. This is a job with really high risk.

            In a previous post, PVAndy talked about issues he's seen with line side taps. You may want to read that as well. Those things scare the &#%@ out of me. They will work if everything is right, but there are too many ways in which they can go wrong.
            7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV

            Comment


            • #7
              Thanks all. @bob-n, yes, all good advice, I'm all straightened out now, thank you! Yes, city inspectors are involved but I'm trying to get ahead of things so as not to have so many mistakes when they check.

              I'm abandoning the line tap (would require a huge conductor and still need a breaker between combiner and the tap), and instead, going with a 60-70A backfeed breaker at the bottom of that main panel and do what most solar installs do. This way I can keep using my #4 for the entire thing, which is all that is really necessary.

              Should I keep my (overkill) 100A fused 2-pole (1 phase) disconnect, or swap out for a 60A 2-pole non-fused? In particular, is there any code issue in keeping with said disconnect in my new proposed backfeed arrangement?

              Comment


              • #8
                When you do a line side tap, you have to add a service disconnect - a 100a fused switch in your case. Then the circuit has to have another 100a non-fused disconnect to serve as the Photovoltaic AC Disconnect before going to the AC Combiner. If 46A is your max inverter output, then your circuit ampacity is 58a and you could use 60a disconnects.
                BSEE, R11, NABCEP, Chevy BoltEV, >2500kW installed

                Comment


                • #9
                  Bruce and I use different sizes for the breaker for #4 wire. I think he has direct burial cable, and I was going with stuffed conduit which does not dissipate heat as well. Use the sizing chart that matches the wire insulation ( some is 60C, some rated 90C ) and install ( burial or stuffed conduit ) conditions.
                  Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
                  || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
                  || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

                  solar: http://tinyurl.com/LMR-Solar
                  gen: http://tinyurl.com/LMR-Lister

                  Comment


                  • #10
                    Originally posted by dapug View Post
                    I'm abandoning the line tap (would require a huge conductor and still need a breaker between combiner and the tap),
                    I think doing a line side tap on a combo meter/main panel is basically impossible. All of the ones I have seen utilize bus bars. I don't believe there are any listed products that would let you do a tap on the bus bar. And since the bus bar is not meant to be modified, it's not like you can drill a hole in it and bolt on a connector or something.

                    If you had a separate meter and main panel, you could do a line side tap and I do not believe you would need the conductor going to the solar to be sized the same as the main panel's. You would need an OCPD (over current protection device. ie. fuse/breaker) sized appropriately for the wire it's protecting between the line side tap and the OCPD. And the wire I think needs to be under a certain length.

                    But with a combo meter/breaker panel like you have, IMO you're best off with a regular backfeed breaker like you're already planning.

                    Does your AHJ require a plan review? (Mine did.) If so, whatever you have in your plans should match what you'll be installing.

                    Should I keep my (overkill) 100A fused 2-pole (1 phase) disconnect, or swap out for a 60A 2-pole non-fused? In particular, is there any code issue in keeping with said disconnect in my new proposed backfeed arrangement?
                    Can you return the 100A fused disconnect?
                    If so, I would.
                    I'd expect the 60A non-fused to be cheaper.

                    Comment


                    • #11
                      Originally posted by foo1bar View Post

                      I think doing a line side tap on a combo meter/main panel is basically impossible. All of the ones I have seen utilize bus bars. I don't believe there are any listed products that would let you do a tap on the bus bar. And since the bus bar is not meant to be modified, it's not like you can drill a hole in it and bolt on a connector or something.

                      If you had a separate meter and main panel, you could do a line side tap and I do not believe you would need the conductor going to the solar to be sized the same as the main panel's. You would need an OCPD (over current protection device. ie. fuse/breaker) sized appropriately for the wire it's protecting between the line side tap and the OCPD. And the wire I think needs to be under a certain length.

                      But with a combo meter/breaker panel like you have, IMO you're best off with a regular backfeed breaker like you're already planning.

                      Does your AHJ require a plan review? (Mine did.) If so, whatever you have in your plans should match what you'll be installing.



                      Can you return the 100A fused disconnect?
                      If so, I would.
                      I'd expect the 60A non-fused to be cheaper.
                      Keep in mind that in a supply side connection (line side tap) the fused disconnect must be "Service Rated". Also the wire must be sized fo the max fuse size of that disconnect not inverter output even if you used fuses smalle rthan the disco size

                      Andy

                      Comment


                      • #12
                        "I'm abandoning the line tap..."

                        IMO, that is a good decision.

                        "Should I keep my (overkill) 100A fused 2-pole (1 phase) disconnect, or swap out for a 60A 2-pole non-fused? In particular, is there any code issue in keeping with said disconnect in my new proposed backfeed arrangement?"

                        I am not an electrician and don't know every nuance of the code. That said, I don't know of any reason for a fuse in the disconnect when you have the right & same-size wire to and from the disconnect. Your 60A backfed breaker should protect everything from there out until you get to the combiner, which has additional breakers..

                        The only technical reason to swap for a smaller disconnect is to save money. That assumes you can get your money back on the trade-in.

                        If you're really into saving money, a small 60A air-conditioner disconnect for <$15 may be sufficient. It isn't big, is slightly less convenient to operate, and doesn't look as impressive, but it does the job. In the end, it's just a switch for the firemen to use in an emergency. Here is a candidate:
                        https://www.lowes.com/pd/Square-D-60...onnect/3364886
                        7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV

                        Comment


                        • #13
                          I don't care about cost savings, only about what will pass inspection right now. If being fused is improper for any reason, I'll swap it out. As for 100A (only need 60A), I don't mind the overkill because I might expand my system next year to need a 70A back-feed, at which point a 60A disconnect wont work.

                          Comment


                          • #14
                            Fuses are OK. There is no need to change the disconnect.
                            7kW Roof PV, APsystems QS1 micros, Nissan Leaf EV

                            Comment


                            • #15
                              Take a look at pages 96-97 of UGLYS ELECTRICAL-REFERENCES to get a handle on
                              the range of current a conductor can handle. Depending on the situation, 4 gauge copper
                              can handle something between 70A, and well over 100A. With PV inverter's outputting
                              60A, I needed an 80A breaker to handle it, the only way current could ever trip that breaker
                              would be a short that would do so very quickly. The operating loses are easily dissipated
                              for this direct burial wire.

                              Inverter output here must travel considerable distance, 6 breakers in the path protect every
                              segment of conductor. Still not very efficient, finally got all 4/0 aluminum or 1/0 copper wire
                              in place to do it. Bruce Roe

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