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Transformer blowing 15a breaker

d4jsmit's picture

I have a circuit that  trips periodically, but I can't trace the cause.  Here's the set-up:

15a pancake breaker, through 14/2 wire for about 25' to a duplex outlet.  The duplex drives two devices.  One device is the timer for the irrigation, minimal load, the other is a 900w transformer for my landscape lighting.  From there, the power goes to a motion sensitive cut-off switch.  (The kind that turns the lights OFF after inactivity, but won't turn them ON without someone pressing the button.)  The switch controls a single bank of 4 can lights (75w each)

The problem I have is that the breaker trips occasionally.  There is no rhyme or reason that I can find.  The lights may be on or off, although they are rarely on.

I'm running out of ideas.  I replaced the breaker, thinking perhaps I just got a bad one.  No dice.  Any thoughts are appreciaited.  I'm trying like heck not to have to rip out the wiring and upgrade to 12/2 so I can run a 20A breaker.  Any thoughts?

Have you split these loads to (post #207128, reply #1 of 11)

Have you split these loads to be absolutely sure which one is tripping the breaker? Take one off and feed it from another circuit with a heavy duty extension cord. Then do the other.

You could also wire a fuse in on the secondary side to see if it is a low voltage short feeding back through the transformer.

Greg

What Greg said. But is (post #207128, reply #2 of 11)

What Greg said.

But is either device perhaps on a GFI?  (I'm assuming you'd have noticed if the breaker itself were a GFI.)  It's vaguely possible a GFI outlet could falt in a way to cause this.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Already split the load (post #207128, reply #3 of 11)

Thanks for the suggestions.  I've already effectively split the load, because I have, at various times, run one without the other.

 

I should add that the breaker trips when the transformer powers on.  I know this, because the timer that controls it is a mechanical timer, and I can see what time it stopped running.  The transformer comes on twice per day, and I've seen it tripped in the morning start-up and in the evening start-up.

Did we read this right that (post #207128, reply #4 of 11)

Did we read this right that this is a 900 watt transformer? ~75 amps at 12v on the secondary?

Does it fail with the irrigation timer unplugged? Is there anything else on this circuit?

That is a big transformer. You might just be seeing an inrush problem. What does the manufacturer say about recommended circuit size. I would get a 12ga extension cord and try it on a 20a circuit for a while and see what happens.

Greg

It sounds like you need to (post #207128, reply #5 of 11)

It sounds like you need to replace the transformer.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Yep, it is the transformers (post #207128, reply #6 of 11)

Yep, it is the transformers fault.  It may not be a 'bad' transformer but combination of load inrush and magnetizing inrush depending on the exact point in the AC waveform where the lights were turned off.

Sound like a marginally designed chinese transfomrer?

Have also seen cheap transformers develop an internal momentary short circuit on turn on - after a few months of operation.  On cheap transformers that were not properly impregnated (or not at all!!), the vibration of the windings from magnetic forces eventually rubs thru a spot of insulation.  Turned on at the right time in the AC waveform that gives maximum magnetic forces on the windings, the spot makes solid contact and you have a shorted turn which causes very high inrush and trips the breaker.  

It's not just "cheap" (post #207128, reply #7 of 11)

It's not just "cheap" transformers -- just about any transformer can develop a turn-to-turn short over the years.  Often the short will be intermittent, as you suggest.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

"cheap" transformer (post #207128, reply #8 of 11)

the transformer was about $300.  I guess compared to teh $700 Kichler, that's cheap.  I didn't think I was going too low end.  It was certainly more expensive than anything offered at my local big box hardware store.

 

Dang...

 

How much of a risk is it to just upgrade the breaker to 20A?

transformer was about (post #207128, reply #9 of 11)

transformer was about $300

OUCH!   For those prices, you could buy a regulated SMPS. 

I have a big 'ol pressure cooker.  When I need a big 2kW transformer for a fast chrge battery charger of something,  I get an old 120 Vac welder off CL or garage sales, and wind my own secondary with 4 parallel strings of 8 AWG THHN, then put the whole thing in a box with epoxy resin* in the old pressure cooker and pull a vacuum.  Made my 200A battery charger that way using some surplus hockey puck diodes & SCRs - it has occasionally tripped 20A breaker when using it at 300A <G>

None have failed yet, some 40 years old. 

* about any type of boat resin works, leftover epoxy floor paint is even better.  I mix fine silica sand in with the epoxy to give better thermal conductivity.

I would do a little more (post #207128, reply #10 of 11)

I would do a little more detective work before I bought a new one and simply changing the breaker is not a good idea until you know what you really have going on.

Obviously it is a NEC violation.

Greg

What you might try is (post #207128, reply #11 of 11)

What you might try is removing the load from the transformer, then switching it on and off multiple times, to see if the breaker trips.  But since problems like this tend to be 90% half thermal, you'd need to try multiple sessions separated by hours.

To eliminate the timer switch as suspect you'd probably be advised to disconnect it and use a regular light switch for the test.

You may also want to disconnect the trasformer from the timer and just switch the timer on/off multiple times -- there could be a bit of metal or something in there causing the problem.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville