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    Melted Breaker

    I had a service call where a 70 amp Square D QO breaker and loadcenter that serve as a disconnect for an electric strip heat unit had melted down and fused to each other. After installing a new breaker and loadcenter I observed that the new breaker is getting too hot. Read with a Fluke laser thermometer it reaches 120 degrees on the load side lugs of one phase. When reading the current draw with a Fluke meter, A phase reads 59 amps and B phase reads 54 amps, however the A phase is reaching 120 degrees while the B phase is at 84 degrees. The breaker is rated at 40 degrees Celsius ( 104 Fahrenheit). All connections are tight and the breaker is stabbed in correctly and making good contact. Can anyone provide any ideas as to why this is happening? I fear that the new breaker and loadcenter will melt down also. The HVAC man says there is nothing wrong with his unit.

    #2
    It sounds to me like you didn't size your breaker at 125% for continuous loads. 59x1.25%=73.75 amps

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      #3
      Don, Illinois
      (All code citations are 2017 unless otherwise noted)

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        #4
        modulating control?

        Check the control, how does the heat load modulate? Is it on / off by pulling in staging relays or is it an electronic SCR type modulating control?

        If you have on / off staging relays the load is clean and not causing the heating. If you have SCR's that are firing and chopping part of the sinewave, the load is noisy, nasty non sinusoidal with high frequency components, and is likely causing the heating.

        In addition to sizing at 125% for continuous loading there would have to be some derating for the high harmonic loading, if present. I do not know exactly what to prescribe, I would call the equipment manufacturer and get their recommended field wiring requirements. Ask them about the type of modulating control if it's not obvious.
        Lasciate ogne speranza, voi ch'intrate

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          #5
          Originally posted by patron View Post
          The breaker is rated at 40 degrees Celsius ( 104 Fahrenheit). .
          I'm pretty sure that the 40 deg is the operating air temp of the breaker (ambient)
          The breaker lugs are rated for 60 / 75 deg C????

          Thats 140-167F.
          Charlie

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            #6
            Originally posted by Cow View Post
            It sounds to me like you didn't size your breaker at 125% for continuous loads. 59x1.25%=73.75 amps
            Can you please explain the above. I did not think there was anything electrically befier inside a 70, 75 or even a 80 amp breaker. I would be that even a 100 amp would have the same contact area. It's just a matter or trip setting.

            Patron, what size wire are you using?
            Last edited by Sierrasparky; 02-16-10, 02:26 PM.

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              #7
              Originally posted by cpal View Post
              I'm pretty sure that the 40 deg is the operating air temp of the breaker (ambient)
              The breaker lugs are rated for 60 / 75 deg C????

              Thats 140-167F.
              Just because you can, doesn't mean you should.

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                #8
                Originally posted by Sierrasparky View Post
                Can you please explain the above. I did not think there was anything electrically befier inside a 70, 75 or even a 80 amp breaker. I would be that even a 100 amp would have the same contact area. It's just a matter or trip setting.
                I honestly couldn't tell you about the breaker contact size either.

                Troubleshooting based on assumptions gets me in trouble though. I'd rather install the correct size breaker from the start and then see if they still have problems.

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                  #9
                  I agree with the 125% sizing of the breaker, however, the unit nameplate lists the maximum overcurrent device as 70 amp. It has been my experience lately that inspectors are are paying close attention the the nameplate vs installed breaker size.

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                    #10
                    Originally posted by Sierrasparky View Post
                    Can you please explain the above. I did not think there was anything electrically befier inside a 70, 75 or even a 80 amp breaker. I would be that even a 100 amp would have the same contact area. It's just a matter or trip setting.

                    Patron, what size wire are you using?
                    the thermal element in a 70 amp breaker better be different than a 100 amp breaker otherwise they have the same trip setting.
                    I live for today, I'm just a day behind.

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                      #11
                      Originally posted by cpal View Post
                      I'm pretty sure that the 40 deg is the operating air temp of the breaker (ambient)
                      The breaker lugs are rated for 60 / 75 deg C????

                      Thats 140-167F.
                      Exactly.

                      What type of loads/heat are on the adjacent breakers? With higher continuous load 2 pole/3 pole circuits I like to leave an open space on either side of the breaker(if possible) for heat dissipation.
                      "Soon after I embraced the sport of angling I became convinced that I should never be able to enjoy it if I had to rely on the cooperation of the fish."

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