Can a shorted secondary on an xfmr travel back to primary breaker? - Page 2
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  1. #14
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    My guess it was doing it long before you got there.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  2. #15
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    From your description, someone needs to go there and have a look at their electrical distribution....
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    2 Tim 3:16-17

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  3. #16
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    Feb 2013
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    Central Pennsylvania
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    My advice:

    Don't let the smoke out

  4. #17
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    Nov 2006
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    I can't think of any bad transformer that I have found where the secondary side failed. It has always been the primary even though the short was on the secondary side. I have never encountered one where the short caused enough load on the primary side to cause a line voltage breaker to trip. The failure has always been in the windings of the primary. Has anyone else had a different type of failure?

  5. #18
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    Feb 2013
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    Central Pennsylvania
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    In residential I would think this less likely, but it can be a real issue in commercial (Think Superbowl) a Ground Fault Circuit Interrupter (GFCI) is designed to be very touchy - even moisture can cause enough leakage current (translates into load imbalance) to get it to trip.

    Codes might be requiring GFCI even on an equipment circuit, if it goes outside, I dunno.

    A GFCI will save your life though - I checked again this summer!

    Jim A.

  6. #19
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    I never tested which side burnt up, but I've never had a transformer trip the primary that I am aware of. Granted most transformers I deal with are 24V and 100VA or less.
    100VA at 24V = 100VA/24V = 4.167 amps continuous amps available on the secondary, thus many times these are 5A circuit breakers on the transformers with circuit breakers.

    The conductors in the windings are so small that they will eventually be your fuse if you don't have one, assuming your smallest CB in the sub-panel was 15A.

    I think it was coincidence, but something else likely happened at the same time, the homeowner plugged something they shouldn't have in while you were messing with their humidifier or a bird/string/branch/gopher/mouse crossed the wires in the main.
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  7. #20
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    Nov 2011
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    Yeah...ran into the boss days later and he said I just got caught at the wrong place, wrong time. Still bothersome to me that 120+ worth of breakers are being fed by a 100. Oven and resistive heater being the chief consumers...but that's the way it's been for years.

  8. #21
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    Most panels have more circuit breaker capacity than the main breaker can handle. This is mainly due to the use of demand factors, which is a perfectly legal design criteria, based on the idea that most devices and circuits will never be asked to perform at their full capacity.

  9. #22
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    Quote Originally Posted by timebuilder View Post
    Most panels have more circuit breaker capacity than the main breaker can handle. This is mainly due to the use of demand factors, which is a perfectly legal design criteria, based on the idea that most devices and circuits will never be asked to perform at their full capacity.
    Makes sense.

  10. #23
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    Apr 2003
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    Maryville, Tennessee
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    Quote Originally Posted by timebuilder View Post
    Most panels have more circuit breaker capacity than the main breaker can handle. This is mainly due to the use of demand factors, which is a perfectly legal design criteria, based on the idea that most devices and circuits will never be asked to perform at their full capacity.
    This is true and and reference specifically in the NEC. I ran across that recently while studying to take the Tennessee limited electrical license test.
    -Marty

  11. #24
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    Jun 2001
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    Moore, Oklahoma, United States
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    The primary winding of a standard 24v transformer is a MUCH finer wire than the secondary. The primary wire is the one that almost always goes bad in the event of a transformer overload. The wire is a small enough that a 20A breaker ain't gonna protect it, it will smoke before the 20A breaker trips. Some transformers (especially newer ones) have internal fuses on the primary that are designed to blow before a total transformer meltdown.

  12. #25
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    wow is that some incredible bad luck you had nothing to do with the breaker tripping.

  13. #26
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    Winter Haven, FL
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    The other thing to look at too is that on most of our equipment, the wires supplying the transformer are too small to carry enough current to trip the breaker.

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