Smoking Transformer

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gtm

gtm

LifeTime Supporter
Aug 11, 2008
209
St. Petersburg, Florida
I bought an Intermatic PX-300 stepdown transformer to replace the one that feeds my (single) pool light. I hooked it up over the weekend and as soon as I powered it up it started putting out wispy, whitish smoke. It never tripped the breaker, but the supply wires got a little hot before I could trip the breaker myself. I was in the process of testing the voltage on the load side when this happened, and I don't think there was any voltage there. Could be wrong about that, though, since I was rudely interrupted by the smoke. In what I generally consider to be a non-smoking zone, BTW.

The transformer takes in a 3-conductor (hot, neutral and ground) 120 line. On the high voltage side there are 3 choices for the the hot connection depending on whether you want 12, 13 or 14 volt output, and neutral wire and there's a block in the middle for the ground wire. I checked the supply line with a multimeter before doing any connections since my house is a older and you never trust the color of the wire 'round here. I'm positive I didn't cross up the connections.

I did some googling and read in a few places that this was consistent with the transformer having a short, probably on the low voltage side, but I don't really know that much about transformers.

To get to the questions, I took it back to the electrical supply store today and, although they replaced it, they were pretty ***** about it. Pretty much said I must've screwed it up. I don't think so, but thought I would check here in case there's a screwup mode I didn't think about. Is there something important I didn't do? Does anyone know whether a faulty transformer right out of the box is one in a million, one in a thousand, or what; i.e., did I win the lottery or just have a rainy day?

Thanks, Gary
 
You really can't hook up the primary of a single phase transformer wrong in a way that will cause smoke. It doesn't care which line is hot and which is neutral, and the ground is just fed through via the case. You could get smoke if the bulb side is shorted, and the breaker probably wouldn't trip. Post an image of the wiring diagram provided and I can try to give you more info.
 
I work with high voltage transformers and hope this might help. First, we test the transformer to make sure it's working properly. With the power off, we always ground the secondary side(load side), then wire the primary side. Then we turn the power on and test the secondary side of the transformer for proper voltage. If it's correct, we turn the power off and wire the secondary side. Then turn the power on.

If you energize with the load on and something goes wrong, you won't know if it's the transformer or the load that caused the problem, and you could ruin a perfectly good transformer.

Are you certain that there's nothing wrong on the load side? Why are you replacing the transformer?
 
gtm,
You mentioned "on the HIGH voltage side there are 3 choices for the the hot connection depending on whether you want 12, 13 or 14 volt output...".
This is suppose to be for the secondary side, which is low voltage.
I hope you didn't connect your hot or neutral line to this terminal or it could have been the reason why it got damaged.
This is used to accomodate long runs of cord lengths so that the longer the run, you can bump the low voltage up 13 or 14 volts to maintain the brightness.
 
If you hook a step-down transformer up backwards, it becomes a step-up transformer.
So if you were putting 120 volts in the low side, 1200 volts would theoretically come out the high side.
The low side probably has wiring that is too small to handle 120 volts, and would heat up and smoke.
It would probably burn up the transformer windings before the breaker tripped.
If you hooked it up backwards, I hope nothing was attached to the other end.
 
Thanks to everyone for the suggestions and comments. I had a 14-hour day at the office today, and just spent 30 minutes figuring out how to upload an image. Hadn't done that before, but I think I've got it now. In addition to reading your comments I also talked to a local electrician and the Intermatic Tech Support folks. I'll send an update tomorrow. Sorry for the delay, but I've still got work to do for my class tomorrow!

Thanks much again ... Gary
 
Sorry to everyone who replied for being so long getting back, but work has just been a bear lately. First, responses to your messages ...

JohnT - Thanks for confirming what I thought about the hot versus neutral. The schematics appear at the end of this message. At least I hope they do -- this is my first try at attaching images.

tsunami - I think that's what I did. There was no load on the transformer when I was testing it. I tested everything beforehand and the old transformer was definitely not putting out on the load side. The pool light checked out on a simple continuity test, but again, when I connected the "smoking transformer" there was no load on it.

Poolsean - That's what I thought, too. The old transformer definitely had the 12,13,14 option on the load side, but the schematics on the Intermatic PX300 clearly show this choice to be on the supply side.

aaronp - I agree, but I found no voltage on the load side at all.

budster - I also think it's a bad transformer, please see the following.

So here's what I've done in the meantime.

First, I called the Intermatic tech support line. Spoke to a woman who described the connections in a manner that agreed completely with the picture below. I told her we were obviously looking at the same picture, described what happened, and asked if there was any way that there might be a misprint. She asked me to hold on and came back about 10 minutes later saying that she'd checked with several other folks, including one of the engineers that designed the thing, and they were sure that the schematic was correct.

Second, I talked to an electrical contractor who was doing some work in my building and he kindly looked at the schematic shown below. He said it seemed odd to him, but said that there was no reason it couldn't be done that way; i.e., with the voltage selection being done on the supply side. He also said that he had an Intermatic 300W transformer on his own pool light and took the trouble to open his up and call me that evening to say that his was connected just as the schematic shows, much to his surprise as well BTW. Finally, he said that a bad transformer out of the box was not nearly as unusual as one might think.

Taken all together, I've decided to try again just as the schematic shows. But .... if this one blows I'm out more than $100 because there's no way the electrical supply company folks are going to give me a third one. So, if anyone has any reason to say STOP, please let me know.

Thanks again for everyone's help. I'm actually not a bad electrician (though not licensed), but I really would appreciate any advice you all can give.

Cheers, Gary

[attachment=1:1l3ydzth]TransformerDiagram.JPG[/attachment:1l3ydzth]
[attachment=0:1l3ydzth]Transformer.JPG[/attachment:1l3ydzth]
 

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After looking at the schematic, I agree with your contractor. Odd, but I think you are correct. I would put tape or wire nuts on the secondary leads and power the transformer first before you hook up the light.
 

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I did a temporary wiring this afternoon. Didn't want to do all the proper stuff until I knew that the transformer and the schematic were okay. No smoke and I get 11.5 V AC on the load side. I'll do all the hard-wiring when I get a chance and then test the light. Thanks again for all your help!
 
Hello again. I got around to working on this a little more today and have another question if you all don't mind.

I had to cut the metal conduit on the transformer end in order to get the old one off the wall, and I also found that a piece of the conduit on the light end had rusted away. I can repair the conduit, but I also need to pull new wires, which is sort of a bear, as it appears the conduit has a partial blockage about 10 feet from the transformer end. I did, though, finally manage to get a steel tape through and am ready to give it a try.

The old wires to the light were 10 gauge twisted and an 11 gauge solid wire used to ground the box where the light is wired in. There is no ground wire going out from the box to the light, but the light itself is grounded with a clamp to the metal conduit that goes from the box to the light. But note that the schematic for the transformer (see my earlier message for an image) does not show a ground wire going from the transformer to the light at all.

1) Should I run a grounding wire from the transformer to the light, as in old setup, or follow the schematic and not bother with it? Given the blockage I'd have a lot easier time snaking 2 rather than 3 wires, but obviously I want it to be safe.

2) If I do indeed need the ground wire, is there a reason in the old setup that this was a solid wire rather than twisted like the others? The twisted ones are easier to snake, I think. Also, any reason why it was smaller gauge?

Thanks for any advice you can give ... Gary
 
Since DC voltage consists of positive and negative conductors (the two wires), the third wire, or ground wire (which is most likely an electrical BOND wire) should be used as it is intended, a secondary safety bond to discharge stray voltage.
Not sure why it's an 11(?) gauge wire. This should be a solid #8 green wire.

If you're going to do it, you might as well do it right.
 
Thanks Sean. I'm sure it's a grounding wire, as I've traced it back and know that it connects to the house ground. But as you suggested, I'll include it when I pull the wire even though the schematic for the transformer omits it.

Interestingly, although the schematic on the cover doesn't show the ground wire going out on the load side, there's a Spanish version of the schematic in the box and that picture DOES show a ground wire going out to the lamp! I think it just got left off the English version picture somehow. As near as I can tell (my Spanish is not that great), the instructions are otherwise identical.

As far as the 11 gauge wire that was in the old one, I measured that with a wire gauge. It might not be perfectly accurate, but it's definitely not 8 gauge. The wires in that subpanel are sort of mishmash. I suspect that the person who installed it (before I bought the house) used whatever they had on the truck, if you know what I mean.

Thanks again, Gary
 
Thanks Ohm_Boy,

It might be 10 gauge rather than 11, but it's definitely not 12. And I'm sure it's the grounding wire for the light's junction box and not part of the bonding wire. I'm going to run 2 10 gauge stranded wires for the light and whatever ground I need for the box. Any idea what that should be? I've been doing some reading, and one reasonable source says 12 gauge is enough, while another say "as large as the primaries", which would be 10 gauge. At this point, I'm figuring that 10 gauge might be the safe bet.

Also, I'm planning to use stranded wire for the ground. Any reason why the ground has to be solid, like in the old setup? Nothing I've read seems to require solid versus stranded, and the stranded is easier to pull, which is an issue since I seem to have a partial blockage in the conduit.

Any additional info will be appreciated. I probably won't get to this until this weekend at best, so no rush.

Thanks again, Gary
 
The way I understand it, grounding is not usually required for low voltage lighting, but bonding usually is (but not always, depending on several factors). If bonding is required then you must use #8 solid copper wire. If bonding is not required then I don't believe that grounding is required either.

Also, the rules for pool lights are complicated, and vary from place to place somewhat. So there isn't one answer to your questions without looking up your local rules and knowing how to interpret them.
 
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