X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • NEC Code on Main Panel Busbar 120% Rule

    Have been talking with my local utility and just initiated a discussion with my regional building department on what I'd love to install ... and what I'm allowed to install. I want to make sure I have the NEC code and local RBD rules correct before I ever spend a dime.

    From what I've read, I cannot go over 120% of my panel busbar rating, which is at 100 amps. (I can not go to 200a without a major move of my panel, the street meter, installing 4/0 wire, and massive conduit in my basement)

    The quick math being:

    (100a * 1.2) - 100a = 20 amp breaker

    If we are pushing 240v AC from the inverter to the panel, this means (240v * 20a) = 4,800w.

    So, to stay within code, I can not go over 4.8Kw for the solar system, correct?

  • #2
    A 20 A breaker is only suitable for use with 16A continuous load (or supply). You are limited to a 3800 W inverter. Most major manufacturers make one at that size... Your​ situation is not unusual.

    You can generally add about 25% more panels to that inverter before clipping starts to matter, getting you back to the 4800 W array you had suggested. You can put up even more if splitting them between different facing orientations (east-southeast or southeast-southwest can be nice).
    Last edited by sensij; 05-24-2017, 10:39 PM.
    CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

    Comment


    • #3
      My experience: 100 amp panel, inverter rated at 16 amps and max 250 volts. My line voltage runs at 245 volts or a couple more most of the time. I normally see 3,920 watts or more most of the time when I am clipping. If you are unfortunate and have 235 volts most of the time you results might not be as great.

      Comment


      • #4
        Originally posted by coronad0 View Post
        From what I've read, I cannot go over 120% of my panel busbar rating, which is at 100 amps. (I can not go to 200a without a major move of my panel, the street meter, installing 4/0 wire, and massive conduit in my basement)
        It is possible to have a 100A service and a panel that has a 100A main breaker, 200A bus-bars, and a 60A breaker for solar.

        Backup and start with what you have:
        ex:

        "I currently have
        ___ Amp service from the POCO to my meter.
        The meter has _____ wire connecting it to the main breaker.
        After the main breaker it
        A> has a bus bar going to all the breakers for the house. And that panel is an end-fed (or center-fed) panel.
        or
        B> has ____ wire that goes to the subpanel for the house which has all the breakers for the house."

        Then look at
        * How difficult will it be to swap out the panel that you want to have the back-feed breakers in?
        * Can you do a "supply side tap"? (Is your meter box separate from your breakers or a combination unit)
        * Is a device that sits in between the meter and the meter socket a possibility (sort of like a psuedo-supply-side-tap but doable even on a combination breakers+meter socket)

        If you don't need more than 3800W inverter, then going that route would be easy choice.

        If you DO need more, then there are possible approaches to solving the problem that would be OK under NEC and would allow you to do that (but not have to upgrade your entire service and replace everything)


        Psuedo drawing of the 100A service I stated above:

        100A POCO wires ==== [100A rated meter/meter box] ======(100A capable wire)==== [100A main breaker] ====== (200A bus bars) ===== [various loads + 60A backfeed]

        The 100A main breaker keeps the wire to the meter under 100A all the time (whichever direction it's going)
        The 200A bus bars support up to 240A total (120% rule - so assuming end-feed bus bars)

        You could do the same thing but with 125A bus bars (fairly commonly used in panels for 100A service), 100A breaker, and 50A of backfeed

        You would need to be sure that the 100A breaker and 100A service was sufficient for your loads. (seems self-evident if that's what you've had for years and haven't tripped the main - but you should do the load analysis to "prove" it. If you have a lot of major items like electric car chargers and an electric oven and electric drier and electric water heater and electric heat and AC etc. Then maybe you'll be close to needing more than 100A service. Most likely the load analysis will show that 100A is fine)

        The main things IMO to keep in mind is
        1> always make sure every wire is fully protected by a breaker (or fuse) so it can't be overloaded.
        2> the 120% rule does allow a bus-bar to be up to 120% for breakers feeding into it - but the buss will always have less than 100% of the current at any spot along it. (because the sources are at each end and the consumption is in the middle)

        Comment


        • #5
          You have 2 options if you want to go larger than 3.8kW
          You can replace the panel with a new 200A panel, and swap out the main breaker to match your service (100A). This would allow 100A of solar.
          Or you can do a line side tap. This requires tapping the inverter output directly into the service conductor between the meter and service panel (with a proper fused disconnect). Again, this would allow up to 100A of solar.

          Comment


          • #6
            Before designing a maximum solar system, I'd check the line voltage level and impedance. So many systems
            starting with high limit or over limit line voltage find the solar driving the voltage beyond the allowed max and
            tripping out. This would likely be TWICE as bad on a 100A box system, as compared to 200A PoCo wiring. A
            quick test would be throw a 3KW load on (pair of cheap 1.5KW electric heaters, at each side of neutral) and
            see how much the line shifts DOWN. It could be expected to shift UP a comparable amount for a 3KW PV
            solar system driving out, roughly proportional for larger systems. The bigger PoCo wiring might turn out
            to be a necessity to keep voltage within limits. good luck, Bruce Roe
            Last edited by bcroe; 05-26-2017, 02:18 PM.

            Comment


            • #7
              Thanks a lot for the info, guys! I can't do a side load, and it will be a substantial cost to move my meter and move my conduit and panel for new 4/0, 4/0 to come from the street to the side of my house, not to mention the huge install of new conduit through my finished basement to the bottom of my panel.

              I have a very, very unique house that was built in '64 on a cul-de-sac that has the main breaker and main meter about 144' from my house, on the street in a transformer box that the neighborhood shares, e.g., I do not have a main breaker in my house ... its on the street. To upgrade to 200a service, I'd need new panel, new conduit from the street to the side of my house for 4/0 wires (the city pays for that) and to move the meter from the street to the side of my house where I have 0 electric entry. That means tearing up the basement to run new conduit to the bottom of my panel. My city electric entry is on the front of the house, my main panel is in my hallway, next to my bedroom. Sooo, this might be expensive if I want to do it right.

              I want to do 6.5kw solar and was willing to accept 4.8kw, but yeah, if 16a is my limit and if I had to do 3.8kw to stay within NEC and code, I'd be WAY below my target of $0 electric bills. Heck, I was willing to go to 50% cost coverage if it meant not replacing panels and meters. I may just have to bite the bullet and do the entire panel upgrade.

              Comment


              • #8
                Originally posted by coronad0 View Post

                I want to do 6.5kw solar and was willing to accept 4.8kw, but yeah, if 16a is my limit and if I had to do 3.8kw to stay within NEC and code, I'd be WAY below my target of $0 electric bills. Heck, I was willing to go to 50% cost coverage if it meant not replacing panels and meters. I may just have to bite the bullet and do the entire panel upgrade.
                Solar that offsets less than 100% of your bill is probably more cost effective than targeting 100%. Especially if you have to incur additional costs to get to 100%. If you are looking for the best financial outcome, it is probably maxing out a 3.8 kW inverter with an array approaching 5 kW rating at STC. It sounds like it won't give a $0 electric bill, but if you look at the total money spent and saved you'll come out ahead.
                CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

                Comment


                • #9
                  Sounds like you are talking yourself into spending a lot of money to save a little money. Don't lose sight of a reasonable payback point. The better choice may be to stick with the smaller system and optimize it for all it's worth.
                  Dave W. Gilbert AZ
                  6.63kW grid-tie owner

                  Comment


                  • #10
                    So your main panel doesn't have a main breaker? That seems really odd.
                    If the main breaker is separate, you can still replace your main panel with a new one with a higher rating. There is no need to replace all of the wire, just replace the main panel in your hallway with a 150A or 200A panel and you are good to go. The 100A breaker feeding your house still protects the service wires, but now you have all the bus bar capacity you need.

                    Comment


                    • #11
                      If one goal is to reduce your electric bill with that bill reduction having maybe a higher priority than just solar for its own sake, the most cost effectiveness effective things you can do involve use reduction before solar.

                      PV is but one of the tools to a lower electric bill that's almost never at the top, and usually pretty far down on the list from most to least cost effective measures to take for bill reduction.

                      If PV is considered, as a bit of a guide, one way of cost effectively sizing a solar PV system for optimum cost effectiveness (after the more cost effective measures) is to stop increasing the array size when the incremental LCOE of the last kWh of solar potential added equals the LCOE of the last kWh of POCO energy it replaces. LCOE being the levelized cost of the power, either from the PV system or the POCO.

                      Take what you want of the above. Scrap the rest.
                      Last edited by J.P.M.; 05-26-2017, 12:57 PM.

                      Comment


                      • #12
                        Originally posted by emartin00 View Post
                        So your main panel doesn't have a main breaker? That seems really odd.
                        If the main breaker is separate, you can still replace your main panel with a new one with a higher rating. There is no need to replace all of the wire, just replace the main panel in your hallway with a 150A or 200A panel and you are good to go. The 100A breaker feeding your house still protects the service wires, but now you have all the bus bar capacity you need.
                        There is nothing inherently wrong with that in terms of the NEC. If there is a breaker accessible to the homeowner anywhere upstream of the main panel the conductors from there to the main panel are feeders rather than service conductors.
                        Even if the POCO service came directly to a panel at the house that panel could have six or fewer branch/feeder breakers and no main and still be NEC compliant.

                        If the OP's loads are small enough to allow replacing that 100A breaker at the street with something smaller, he could still use that method to get more headroom for PV at his main panel.
                        SunnyBoy 3000 US, 18 BP Solar 175B panels.

                        Comment


                        • #13
                          Originally posted by coronad0 View Post
                          To upgrade to 200a service,
                          STOP
                          You probably do not need to upgrade the service.
                          Just upgrade the panel inside your house to be a 200A panel.

                          I'd need new panel, new conduit from the street to the side of my house for 4/0 wires (the city pays for that)
                          That's what I thought - then I found out that from the city's vault (at corner of my property) to my meter was my responsibility.

                          That means tearing up the basement to run new conduit to the bottom of my panel.
                          Maybe conduit - maybe cable. Either way it's going through a finished basement. So I understand your reluctance.

                          My city electric entry is on the front of the house, my main panel is in my hallway, next to my bedroom. Sooo, this might be expensive if I want to do it right.
                          A 200A (or 125A) panel is perfectly fine to have with a 100A main breaker and a 50A solar backfeed breaker.

                          It's still a pain to replace a main panel.

                          But less than what you're talking about.
                          And most places (some locations in southern CA being possible exceptions) it is no problem to have a 100A breaker and 100A service with a panel with 200A bars. It's OK under NEC and if you talk to the AHJ and show them your plan they're likely to be fine with it.


                          bcroe is right in that you should look at what your voltage rise will be from the backfeed. And that you can get a reasonable estimate by looking at voltage drop when you put a large load in place. V=IR works for currents in either direction. So if you have a 15A load current and see a 5V drop you can be fairly certain that if you have a 30A backfeed you'll have a 10V rise.
                          Last edited by foo1bar; 05-26-2017, 06:56 PM.

                          Comment


                          • #14
                            Awesome info and thanks everyone!

                            I found out I do have a 125A bus bar in the panel, so redoing my calcs and then I'm just going to submit my plan to the AHJ and specify NEC code showing it'll be ok and see what they say. My only concern is a possible limit with the 2/0 wires going from the panel to the transformer. Everything in our house is gas and have very low loads on the panel. Not sure if I'll have to "prove" that to the building board and the city utility to ensure I'm not going to be clipping/overloading their end.

                            While it looks like it is a 100A breaker that means I'd be theoretically limited to a 50A backfeed, I don't think I'll be coming in anywhere near that much juice.

                            Will figure out V drop and go from there!

                            Comment

                            Working...
                            X