User Tag List

Page 1 of 2 1 2 LastLast
Results 1 to 10 of 11

Thread: Melted Breaker

  1. #1
    Join Date
    Dec 2009
    Posts
    11
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    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. #2
    Join Date
    Dec 2006
    Location
    Eastern Oregon
    Posts
    3,787
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    It sounds to me like you didn't size your breaker at 125% for continuous loads. 59x1.25%=73.75 amps

  3. #3
    Join Date
    Feb 2003
    Location
    Illinois
    Posts
    24,140
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    You have a bad connection at the bus, within the breaker or at the conductor termination. There should not be that much difference between the two temperatures you measured. Did you measure the votlage drop between the bus and the load side of the breaker?
    Note, I am not saying that the 120F at the termination point is too hot, but with the load on each pole being as close as they are, the temperatures should not have that much difference.
    Don, Illinois
    (All code citations are 2017 unless otherwise noted)

  4. #4
    Join Date
    Sep 2009
    Posts
    1,279
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)

    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.

  5. #5
    Join Date
    May 2004
    Location
    MA
    Posts
    1,472
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote 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

  6. #6
    Join Date
    Feb 2007
    Location
    USA
    Posts
    5,876
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote 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 at 02:26 PM.

  7. #7
    Join Date
    Feb 2003
    Location
    Wisconsin
    Posts
    10,324
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote 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.
    Yes, the 40C on the side of the breaker is its ambient temperature rating.

    The lugs of the breaker should always be significantly cooler (10-15C) than the corresponding conductor insulation. In practice, the conductor acts a heat sink.
    Just because you can, doesn't mean you should.

  8. #8
    Join Date
    Dec 2006
    Location
    Eastern Oregon
    Posts
    3,787
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote 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.

  9. #9
    Join Date
    Dec 2009
    Posts
    11
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    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.

  10. #10
    Join Date
    Dec 2007
    Location
    NE Nebraska
    Posts
    41,346
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote 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.

Tags for this Thread

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •