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Conductor Sizing for Solar Sub Panel Output

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    Conductor Sizing for Solar Sub Panel Output

    Hi everyone and thanks for looking at this post. I am on the 2011 NEC code and have a system with 3x Fronius Symo 12.0-3-Delta inverters. Each inverter has an output of 33.3 A at 208V. Each inverter is going to a solar subpanel and connected to 3x 50A 3 pole breakers. The bus on the subpanel is 400A and the MCB is 150A. My question is, do I have to size my wire running from the MCB to the MDP breaker according to the size of the MCB or the total output of the inverters? Essentially, does the wire need to carry 150A or can I get away with a wire that only carries 100A since 33.3 x 3 = 99.9.

    #2
    Originally posted by roblsmith3 View Post
    Hi everyone and thanks for looking at this post. I am on the 2011 NEC code and have a system with 3x Fronius Symo 12.0-3-Delta inverters. Each inverter has an output of 33.3 A at 208V. Each inverter is going to a solar subpanel and connected to 3x 50A 3 pole breakers. The bus on the subpanel is 400A and the MCB is 150A. My question is, do I have to size my wire running from the MCB to the MDP breaker according to the size of the MCB or the total output of the inverters? Essentially, does the wire need to carry 150A or can I get away with a wire that only carries 100A since 33.3 x 3 = 99.9.
    You have the continuous load factor on this, which means you'd need 125A worth of wiring by default, prior to considering the wire protection.

    If you are using a 150A breaker, you need at least 126A worth of wiring. Essentially, it must exceed the previous breaker size, so that the wire's ampacity rounds up to the next standard breaker that you are using. See 240.4(B).

    Comment


      #3
      In my opinion you can size your breaker and wiring for 125A instead of 150. It is still an output circuit required to be sized to 125% of the output, but you can base it on the combined output of the inverters and not the breakers that protect them.

      With respect to the busbar for the combiner panel, an AHJ might require the busbar to be (150+125)/120%.

      Incidentally 45A is a standard size, although whether you can find 45A 3-pole breakers is another question.

      Comment


        #4
        Originally posted by jaggedben View Post
        In my opinion you can size your breaker and wiring for 125A instead of 150. It is still an output circuit required to be sized to 125% of the output, but you can base it on the combined output of the inverters and not the breakers that protect them.
        That is the case, if you are only using as large of a breaker sized as large as you need. If you are using a breaker in excess of what you need, you need at least as much ampacity as the previous standard breaker (with a fraction of an ampere greater). See 240.4(B).

        So if you choose to connect this to a 200A breaker, instead of 125A as you need per the 125% rule, you'd need at least 175.000001 Amps of wire. This is the case if you have an overcurrent device that is required. If an OCPD isn't required, but is there anyway, you can treat it as if it isn't there, and it wouldn't affect wire sizing.

        Comment


          #5
          Originally posted by Carultch View Post
          That is the case, if you are only using as large of a breaker sized as large as you need. If you are using a breaker in excess of what you need, you need at least as much ampacity as the previous standard breaker (with a fraction of an ampere greater). See 240.4(B).

          So if you choose to connect this to a 200A breaker, instead of 125A as you need per the 125% rule, you'd need at least 175.000001 Amps of wire. This is the case if you have an overcurrent device that is required. If an OCPD isn't required, but is there anyway, you can treat it as if it isn't there, and it wouldn't affect wire sizing.
          Point taken, although in this case it seems he would need to use #1 copper at 130A and the difference between a 125A and 150A breaker is moot. I overlooked that the OP said he was using a 150A breaker; I thought he was asking after the smallest breaker he could use as well as the wire. I'd use a 125A breaker myself.

          I'm curious why you say the wire ampacity must be a millionth of an amp over the breaker rating, instead of a thousandth or a billionth.

          Comment


            #6
            Originally posted by Carultch View Post
            ... 175.000001 Amps of wire. ...
            The NEC does not require significant digit rounding for calculations... but it should.

            I always go by 220.5(B) rounding >0.5 up and lesser fractions down... so a 126A ampacity minimum (because ampacity is only given in whole number values) would be required to use a 150A breaker.
            [COLOR=RoyalBlue]I will have achieved my life's goal if I die with a smile on my face.[/COLOR]

            Comment


              #7
              Originally posted by jaggedben View Post
              I'm curious why you say the wire ampacity must be a millionth of an amp over the breaker rating, instead of a thousandth or a billionth.
              It was arbitrary. I just held the zero key down a random amount of time, and it happened to coincide with a millionth. I'm exaggerating to make my point about the next size up rule, in the sense that the ampacity of the conductor cannot be an exact match to the previous standard OCPD rating, and that it has to be able to exceed the previous and round up to the next. The rule doesn't specify just how much it needs to exceed the previous standard size.

              I've been in situations where after adjustment and correction factors, the ampacity of the conductor is small fractions (less than 0.5A) of amperes above a standard breaker size. It can also depend on how you calculate the temperature correction, whether you use the square root formula, or the rounded values based on the 5C temperature ranges.

              Comment


                #8
                Thank you all for replying! I originally had 1 AWG and wanted to be sure it was still good to use. JaggedBen, i hear you on the 125A MCB thing.....That's what I would like to do but a lot of the AHJs around here don't see it that way and want us to add up the breakers.....I've been fighting it for years! Anyways, thank you all again for replying. Till next time!

                Comment


                  #9
                  Originally posted by roblsmith3 View Post
                  Thank you all for replying! I originally had 1 AWG and wanted to be sure it was still good to use. JaggedBen, i hear you on the 125A MCB thing.....That's what I would like to do but a lot of the AHJs around here don't see it that way and want us to add up the breakers.....I've been fighting it for years! Anyways, thank you all again for replying. Till next time!
                  Do they ever provide a code reference on why they want you to add up the breakers, and accumulate all the rounding errors? Instead of adding up the current, and applying a master 1.25 factor. The AHJ is not supposed to make up their own rules. Any violation claim needs to be based on a specific code requirement.

                  I can understand accumulating rounding errors, if you are sizing conductors for use with an MLO panel with no master OCPD anywhere within the main output circuit. But once you have a master OCPD, the master OCPD should only need to be as large as 125% of the total maximum continuous current, and the wire should only need to be as large as needed for the load and to be protected by the master OCPD.

                  Comment


                    #10
                    Pre 2014 (e.g. 2011) the code specifically says to add up the values of the first breaker on each inverter output.

                    Comment


                      #11
                      Originally posted by GoldDigger View Post
                      Pre 2014 (e.g. 2011) the code specifically says to add up the values of the first breaker on each inverter output.
                      That's only true for calculating the 120% rule. The code never said that the breaker for a feeder supplying an AC combiner has to be done that way.

                      Comment


                        #12
                        Originally posted by jaggedben View Post
                        That's only true for calculating the 120% rule. The code never said that the breaker for a feeder supplying an AC combiner has to be done that way.
                        Some interpret the 120% rule of earlier editions as applying to all buses and feeders between PV inverter and service.
                        [COLOR=RoyalBlue]I will have achieved my life's goal if I die with a smile on my face.[/COLOR]

                        Comment


                          #13
                          Originally posted by jaggedben View Post
                          That's only true for calculating the 120% rule. The code never said that the breaker for a feeder supplying an AC combiner has to be done that way.
                          True.
                          In exactly the same way that you do not add up load breakers to get the load on a feeder.

                          Comment


                            #14
                            Originally posted by Smart $ View Post
                            Some interpret the 120% rule of earlier editions as applying to all buses and feeders between PV inverter and service.
                            Yes, but still not the breaker for the feeder. Some AHJs will apply it to all busses and conductors, and some will apply it to none if the bus is in a combiner panel dedicated to inverters. Most the AHJs I encounter will not apply it to the feeder even under the 2011 code.

                            Comment


                              #15
                              Originally posted by Carultch View Post
                              . The AHJ is not supposed to make up their own rules. Any violation claim needs to be based on a specific code requirement.
                              The AHJ can adopt the NEC or not, and it can selectively enforce it as well as amend it as they see fit. My local AHJ has adopted the 2014 NEC but is not yet enforcing rapid shutdown. The AHJ is the last word.

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