Can a shorted secondary on an xfmr travel back to primary breaker?

[firecontrol]

My guess it was doing it long before you got there.

[timebuilder]

From your description, someone needs to go there and have a look at their electrical distribution....

[Mr_Greenjeans]

My advice:

Don't let the smoke out

[kdean1]

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?

[Mr_Greenjeans]

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.

[crab master]

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.

[hurtinhvac]

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.

[timebuilder]

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.

[hurtinhvac]

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.

[m singer]

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.