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    tandem breakers and panel design

    New-ish Square D QO 30-space panel, need 2 more spaces.
    Tandem breakers come in two types, as pictured below, courtesy 480Sparky.
    Mechanically, I could install the lower type in any QO panel, and it would fit. It wouldn't be "correct" though. WHY? (And I don't just mean, "because they said so.") What's wrong, electrically, with putting one of these in a 30-space (in contrast to a 30/40-space) panel? I.e., what's different about the two panel designs, and what's different about the two tandem types, besides the hook/slot?

    I'm reading that it's somehow related to current limiting, to CTL vs non-CTL breakers. (I don't quite get that either, though I understand the basic concept of CTL.) IF the difference between tandems is CTL/non-CTL, which is which, and what's the logic behind CTL tandems or non-CTL tandems being OK in some cases vs. other cases, or some panels vs other panels?



    I read every thread I could find, and they don't help me explain the issue to homeowners, short of just saying "because it's not allowed."

    For reference purposes (and complicating matters only slightly) newer breakers (side-by-side) look like this:

    Click this bar to view the full image.

    #2
    I think the keyword is "listed".

    It cost manufacturers lots of money to get a product tested and UL approved.


    It's not allowed may not be good enough for a homeowner but it's good enough for me. If a product is approved then it's not my responsibility, it's already tested for a certain use. If I choose to do something like modify a product or use it in a way that's not approved then it is my responsibility.
    The 95% of people that you can't trust give the other 5% a bad name.

    Comment


      #3
      I think when it comes to residential plug on type panels it is all about marketing.

      Comment


        #4
        I know it comes back to listing, but what is the difference
        between a tandem 20 amp (possibly drawing up to forty
        amps on one clip) and a two pole 40 amp or up in the same
        clip space. It seems to be policy overriding basic electrical
        principles, since the bus that these clip to can handle well
        over the tandem C.B. The only thing I could think of at this
        moment is heat transfer issues.

        JR

        Comment


          #5
          Originally posted by growler View Post
          I think the keyword is "listed".

          It cost manufacturers lots of money to get a product tested and UL approved.


          It's not allowed may not be good enough for a homeowner but it's good enough for me. If a product is approved then it's not my responsibility, it's already tested for a certain use. If I choose to do something like modify a product or use it in a way that's not approved then it is my responsibility.
          I agree with this 100%.

          If we were to look in my own homes electrical panel we would find many different brand breakers in my Murry panel.

          On the other hand I would never do that with a customers panel.

          Comment


            #6
            Originally posted by JRW 70 View Post
            I know it comes back to listing, but what is the difference
            between a tandem 20 amp (possibly drawing up to forty
            amps on one clip) and a two pole 40 amp or up in the same
            clip space. It seems to be policy overriding basic electrical
            principles, since the bus that these clip to can handle well
            over the tandem C.B. The only thing I could think of at this
            moment is heat transfer issues.

            JR
            There is nothing different other than the listing. Nothing.

            Acept it is just a matter of what the manufacturer chose to have it listed for.

            It is not overriding any electrical principles because it is not about electrical principles. It is about making money.

            Comment


              #7
              Originally posted by JRW 70 View Post
              I know it comes back to listing, but what is the difference
              between a tandem 20 amp (possibly drawing up to forty
              amps on one clip) and a two pole 40 amp or up in the same
              clip space. It seems to be policy overriding basic electrical
              principles, since the bus that these clip to can handle well
              over the tandem C.B. The only thing I could think of at this
              moment is heat transfer issues.

              JR
              Forty amps on one clip is not the issue - that bus is rated for up to 125 amps in most panels. The difference is the breaker without the rejection feature is not a CTL rated breaker and the one with the rejection feature is CTL rated. Not saying they may not be the same thing internally, but the listing still is non CTL if it doesn't have the rejection feature.

              ETA

              You don't find this issue with the Homeline series because it was not around yet before they required CTL rated breakers, so all the Homeline tandems have the rejection feature and are CTL rated as there is no existing panels old enough that non CTL is an issue with them.
              Last edited by kwired; 05-25-15, 11:37 AM.
              I live for today, I'm just a day behind.

              Comment


                #8
                Loadcenter style panelboards have always had limitations on the use of tandem breakers. UL required panels be supplied with wiring diagrams showing how many tandems were allowed in the panel and which breaker positions could accommodate them. But people, including electricians, kept installing tandems wherever they wanted.

                So, roughly 50 years ago UL and the NEC came up with the concept of breakers and panels with circuit limiting (CTL) construction. But, because breaker panels had been in existence for several decades, an exception for 'existing installation' was included. CTL usually involved some type of rejection clip, the rest of the breaker remained the same, as it was not cost effective to have two 'lower volume' breaker mechanisms.

                There are inspectors that do not approve the non-CTL breaker in panels boards installed after 1965.
                When I entered the trade in the early 70's, non-CTLs breaker were called 'cheaters'.
                But, to this day it seems many electrical wholesalers probably still sell more non-CTL than they do CTL breakers (many manufacturers sell only CTL breakers to home centers).
                Just because you can, doesn't mean you should.

                Comment


                  #9
                  When I wired panels, the specs. disallowed the use
                  of tandem breakers, ( on many of our projects )
                  so we ended up with more panelboards/breakers and a
                  happy design Engineer. He felt that there were too many
                  small components in the tandems, so they were never really
                  an option. They wanted full size breakers for basically
                  everything ( with the exception of fuses, where appropriate )

                  I definately agree this is a money maker for manufacturers
                  and "overly" cautious Engineers that I have dealt with.

                  JR
                  Last edited by JRW 70; 05-25-15, 05:00 PM. Reason: Grammatical Errors

                  Comment


                    #10
                    Originally posted by jim dungar View Post
                    Loadcenter style panelboards have always had limitations on the use of tandem breakers. UL required panels be supplied with wiring diagrams showing how many tandems were allowed in the panel and which breaker positions could accommodate them. But people, including electricians, kept installing tandems wherever they wanted.

                    So, roughly 50 years ago UL and the NEC came up with the concept of breakers and panels with circuit limiting (CTL) construction. But, because breaker panels had been in existence for several decades, an exception for 'existing installation' was included. CTL usually involved some type of rejection clip, the rest of the breaker remained the same, as it was not cost effective to have two 'lower volume' breaker mechanisms.

                    There are inspectors that do not approve the non-CTL breaker in panels boards installed after 1965.
                    When I entered the trade in the early 70's, non-CTLs breaker were called 'cheaters'.
                    But, to this day it seems many electrical wholesalers probably still sell more non-CTL than they do CTL breakers (many manufacturers sell only CTL breakers to home centers).
                    Jim, do you know why exactly they felt the need to limit the number of tandems or circuits? Was it some sort of formula for number of circuits per physical size That you couldnt exceed?
                    Ethan Brush - East West Electric. NY, WA. MA

                    "You can't generalize"

                    Comment


                      #11
                      Originally posted by electrofelon View Post
                      Jim, do you know why exactly they felt the need to limit the number of tandems or circuits? Was it some sort of formula for number of circuits per physical size That you couldnt exceed?
                      I seem to recall discussion about this coming from the New York area, kind of like the original 42 circuit limitation. But as I said they have had limits on tandems for at least 60 years.
                      Just because you can, doesn't mean you should.

                      Comment


                        #12
                        I wonder if there ever was a real technical reason for limiting the use of tandems? If you have a panel with a 125 amp bus and limited to 30 breakers, what physical change would be made when they had 30/40 panel with a 125 amp bus? I bet none.
                        Don, Illinois
                        (All code citations are 2017 unless otherwise noted)

                        Comment


                          #13
                          Originally posted by don_resqcapt19 View Post
                          I wonder if there ever was a real technical reason for limiting the use of tandems? If you have a panel with a 125 amp bus and limited to 30 breakers, what physical change would be made when they had 30/40 panel with a 125 amp bus? I bet none.
                          Well, it is a little simplistic, but the biggest physical change is probably the inclusion of rejection tabs in some of the positions.
                          I doubt that the bus is any different.

                          Comment


                            #14
                            Originally posted by don_resqcapt19 View Post
                            I wonder if there ever was a real technical reason for limiting the use of tandems? If you have a panel with a 125 amp bus and limited to 30 breakers, what physical change would be made when they had 30/40 panel with a 125 amp bus? I bet none.
                            My guess is that this was a 'trade based' decision to encourage the replacement of equipment and its associated labor skills.
                            Just because you can, doesn't mean you should.

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