low voltage lighting transformers

i recently picked found a couple of close out cable-light kits that i picked up cheap. my thinking was that i could combine the fixtures from several kits onto a single cable.
of course, when i unpacked the kits, i discovered that the transformers in each kit are only rated to 60 W - each kit comes with 3 fixtures, each of which uses a 20 W MR-16 bulb.
what i want to do is put six fixtures on each cable (well, pair of cables, to be more precise). so, can i:
a) just put more fixtures on each cable? these things must have some extra capacity built in, right? mostly just kidding with this option, but does anyone know how much of a safety factor consumer-grade transformers might be designed to? each transformer has a 15 A fuse built in, so i assume that if i tried this, i probably wouldn't burn the house down
b) purchase a couple of cheap 120 V to 12 V transformers rated to 120 W or higher? this sounds like a reasonable solution if the transformers are cheap enough. i don't even know where to buy something like that. radio shack? contractor's electric supply house? internet? how much would this be likely to cost, given that size and appearance are not important? [i have located the j-boxes for this in the attic space above, with the low-voltage cable dropping down to the cable via a piece of conduit - i hate the look of a big, bulky transformer perched on the ceiling like a bug - so the transformer could be as big and ugly as necessary]would it be necessary to have some kind of dust-protector around the transformer?
c) since i bought 4 kits, is there some way to wire up 2 transformers to each pair of cables? in line? in parallel? i was thinking that it might be possible to bring power to both transformers, and then merge the power leads from the 12 V sides so that the hot cable is being supplied with 12 V from BOTH transformers, (doing the same in reverse on the neutral side), but this *feels* wrong. even if this works electrically, would it solve the problem?
thanks for any help
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forrest_m
forrest underscore m at hotmail
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forrest wrote:

I love bargain tables at stores. I love the idea of saving money. But if I stop and think about it, I realize the items on the table are of no use to me, and I'd end up wasting my money.

Was this mentioned on the packaging?

There is a reason the manufacturer rated the transformers at 60 watts. If they could get away with more wattage, I'm sure they would. (It would be more cost effective, if the transformer could really handle 100 watts, they wouldn't rate them at 60 watts without good reason.)

Are you talking about those cheapy transformers that are designed for small radio projects? These are way underpowered for what you want to do. How much does it cost for a good 300 watt transformer?

It's amazing how our society ignores their "gut feeling". Looks like you were about to ignore yours. Good thing you decided to "double check" with us. If you bought 4 kits, each capable of 60 watts, and each kits comes with 3 20 watt lights, then why not use all 4 transformers, each powering 3 lights? Keep in mind, the transformers will consume some power also, and instead of having 3 60 watt transformers, perhaps you can sell them on ebay and get a good 300 watt transformer? --Mike

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Why not just buy smaller bulbs?
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"Mike Fritz" wrote (single carets) in response to my OP (double carets)

mike, thanks for the response. this is supplementary, decorative lighting, so if i have to live with only 3 fixtures per cable, it was still a good deal, i just want to make it "perfect"

no, there only a picture on the packaging. they had one set up, so i knew what it all looked like, but the transformer was on the ceiling and i couldn't read the label...

that's why i said i was mostly kidding :-)

no, i'd love to find a decent heavy duty transformer, but i'm not sure where to go shop for such a thing. in a bit of research on the net, i found plenty of electronic transformers... for $125-$150 each for 120 W capacity. since i need at least 2, it's starting to get expensive. yes, electronic are much smaller and lighter, but that's not really an issue for me here. i'd be grateful for a source for old fashioned, heavy duty magnetic transformers that don't cost too much.

the problem with using all 4 transformers is that in the most straightforward setup, i would then end up with 4 sets of suspended cables (each consisting of a pair of cables) visible within the room, which would be too much. thus my question, is there any way to "pair up" the transformers i already own and thus get 6 lights onto each pair of cables:
_____ _____| T1 |___h1___ / |____| @12v \ 120v / \ --------< >----------hot cable---O--O--O--O--O--0-- \ _____ / | | | | | | \_____| T2 |___h2___/ /---neutral cable---O--O--O--O--O--0-- |____| @12v / / to T1 <--n1--------/ / to T2 <-----n2-----/
where O are the light fixtures themselves and the 120v neutral is not shown | O
this goes beyond my apprentice level electrical knowledge, what i wonder about is the point where the two 12v hot wires h1 and h2 come together and where n1 & n2 split apart again. part of me says "why, it's just a parallel circuit" and the other part of me worries that even if the joining hot wires is ok, that the electrons in the neutral wires won't know to split themselves evenly and will all go back to 1 transformer and overload it... i.e. one transformer will end up trying to ramp up all 120 W worth of power back up to line voltage. Or would it be OK because if it got uneven, it would increase potential in one or the other, thus correcting the problem?
Can someone explain why this setup would or wouldn't work? if it's dodgy, don't worry, i won't do it, but i'd like to understand how it works.
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forrest_m
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No.
Not much.

No, but the fuse could protect the input or the output. I'm going to assume the output because a 15a fused input would cover up to a 1800 watt load.

Look in the kinds of stores that sell landscape lighting. Home improvement stores come to mind. Electrical supply houses often have them as well, along with lighting fixture stores which also sell outdoor low-voltage fixtures.
A good 300 watt 12 volt transformer runs about 100 bucks.

No.
And keep this in mind when you're using the cable supplied with your discount kits...
Low voltage ( 12 volts - 10 times lower the ordinary household current) loads actually run at higher amperages than high volt (120 v) loads.
At 12 volts, a 12 watt load is 1 amp, and a 60 watt load (3 -20 watt bulbs) will require a cable capacity of 10 amperes. 6 20 watt bulbs would require 20 amp cable.
Most kits use #18 or #16 guage wire which is rated 10 amperes or less. Feeding 12 - 12 volt lights from one 300 watt transformer would require you to replace your cable as well, unless you run 2 separate cables back to the transformer.

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<snip>

bulbs)
20
Almost right. 60 watts will draw 5 amps (not 10), therefore 120 watts will draw 10 amps.

Feeding
replace
transformer.
Remember, the OP said he bought 4 sets in the hope of getting some working. I didn't see where he ever said he wanted 12 - 20 watt lamps.
BTW, you can wire the secondaries (12 volt side) of 2 transformers together to gain current capacity. As has been discussed here, they must be "in phase". This gets a little tricky in practice because if the secondary wires are not marked and you wire then "out of phase", you effectively have a 24 volt power source "shorted". The resulting high current could cause overheating or a blown fuse. Also, as has been stated here, if you choose to wire the secondaries together, it is mandatory you also wire the primaries together, for safety.
I installed these "hockey puck" lights in our kitchen a couple of years ago and chose to use 2 separate transfomers. It's a little more work but sometimes simple is better.

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Whoops - brain fart. My bad.

I believe he did.
"i recently picked found a couple of close out cable-light kits that i picked up cheap. my thinking was that i could combine the fixtures from several kits onto a single cable.
of course, when i unpacked the kits, i discovered that the transformers in each kit are only rated to 60 W - each kit comes with 3 fixtures, each of which uses a 20 W MR-16 bulb.
what i want to do is put six fixtures on each cable "

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I believe you can run the transformers in parallel but you must insure that they are in phase. Is there any color coding of the wires? If so, white 120 and white 120 (e.g.,) must be tied together; ditto black 120v. Similarly, colors should be matched on the 12volt side. If things are out of phase, your lamps won't light. If that happens, just swap one set of wires, (either input or output but no both.)
You may need to increase the cable size since you will be running lights than the manufacturer assumed.
forrest wrote:

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On Fri, 12 Nov 2004 01:47:47 GMT, Bennett Price

If you run the transformers in parallel and someone (could be yourself or a young child) unplugs just one of them, you are going to get deadly voltages on the male plug of the transfomer that was just unplugged. It could start a fire and/or kill you, or a family member, or a member in your neighbor's family. There is no difference in the theory between a step down and a step up transformer. The 120 volts would get stepped down to 12 V. and then back up to 120 volts on the backfeed side. I would not risk doing this under any circumstances.
Parallel transformers do have a purpose, such as your local electric utility substation, where proper protection is provided, or in an industrial plant.
Buy one properly sized transformer for garden lighting to do the job. Also realize that garden lighting systems are limited power for a reason. Even though the voltage is only 12 volts, you don't want to draw 300 watt arcs if the wires are accidently shorted while they are connected to some big-ass arc welding transformer.
Beachcomber
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Landscape transformers might also have an internal safety shield to prevent the primary and secondary from shorting together (particulalry those used near water)
wrote:

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Go to home depot and look at larger LV transformers - they have them all the way up to about 600W, water resistent, complete with timers, not too expensive.
I just bought a 150W unit for about $40CDN.

Very little safety factor. The transformers themselves aren't too bad, but I'd expect them to fry if you went to 120W. I've blown a couple of these "right at the limit" - electronics failures, not transformer.
15A _fuse_? In the 12V or the 120V? 60W Malibu units have 5A breakers. That fuse is to protect against dead shorts. But if you over-lamped it, by more than a little bit, I'm sure you'll fry it.

Don't bother. By the time you buy a raw transformer (ie: from Hammond), box it, add wiring and timers etc., it'll cost 4 times as much as going to HD and buying a larger LV transformer.
Blown LV controllers are in fact the cheapest source of 12V transformers...
[I've recycled the transformers from each of the blown units]

Really really really don't try wiring them in parallel.
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Chris Lewis, Una confibula non set est
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"Chris Lewis" wrote in message >

wow, i wish i could find that in my neighborhood - i looked at a couple of different places here in town over the weekend, and the cheapest i found was around $125 US for a 150w unit.

yeah, i may still try this at a later date, but i decided to try an interim solution of buying 10W MR-11 lamps, which fit into the receptacle (would you believe that 10W MR-16s are $15 each!!). it's already all wired up, and these guys are like $2 each, so i'll try it for a month or two and see if it's enough light. simpler is better, right? i can always go back to the parallel solution later if necessary.
but i'm curious now, about the wiring in parallel, even if it's just theoretical. on the 120v side, the wires are color coded, so that's easy. on the 12v side, the transformers have 2 red wires. so the question is, how to ensure that they are in phase? trial and error? if it's out of phase, what are the consequenses? lights don't work? transformer blown? bolts of lightning kill everyone within a 5-block radius?
suspension cable is 14 ga, so i think it can handle the 10 amp load. (i think it's oversized because it's under tension supporting the fixtures and they don't want it to sag over time...)
thanks for all the responses,
--
forrest_m
forrest underscore m at hotmail
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No, on the bench check for the proper voltage between one red wire from one transformer, and another red wire from the other transformer.
Use a bulb. No light = in phase and you have a matching pair. Lit bulb means out of phase and you have an incorrect pair.

You'd likely burn out at least one of the secondary transformer windings.

Yup.
Now you're just being silly. It would only be a 4 block radius!

Sounds like a very odd landscape kit.

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cables,
Agree. Don't overload the transformers whether the electromagnetic or electronic type. The electronic ones have very little overcurrent capacity in my experience. Even electromagnetic ones will overheat and fail early if overloaded.

or
cheap
HD
transformers...
If this equipment is for an outdoor system, be aware that transformers made for such service have a "guard" between the primary and secondary. It's a UL/CSA requirement. In electromagnetic units, the guard is a piece of grounded metal; so if the insulation on the 120 volt primary fails, it shorts to ground rather than possibly leaking over to the 12 volt circuit and killing somebody. Lightning strikes and surges are the often the reason such things happen.
TKM

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FYI: these are three-wire outdoor 60W Noma Malibu LV transformers where the control electronics (timer/light sensor) have fried.
Reuse of the transformers:
One was used outdoors in its original casing to feed 12V to a shed, where "ordinary" electrical wiring devices (boxes and switches) were used to install some LV lighting.
Scavenging it consists of pulling out the electronics card, and causing the output of the transformer to be directly connected to the outlet terminals via the 5A breaker. The transformer frame grounding is retained. The unit is situated in a very sheltered area and doesn't get rained on.
The boxes in the shed aren't grounded, but they probably should be. This is probably iffy w.r.t. the NEC (because the NEC seems to cover "anything using standard electrical devices", whether or not you're actually running it off 120V), but the wiring _is_ NEC class II LV wiring compliant. This isn't an issue with CEC. It isn't a UL/CSA issue (at least here, unless, perhaps, I sell the shed).
In Canada, the Noma units aren't approved for "near pool" use per-se (the specialized grounding), but neither the old nor the new use have anything to do with a pool. So, the new use isn't anything "worse" than the old.
The second transformer was used to replace a fried transformer in a QH 50W floor lamp. Not approved in a "repair by a licensed technician using original parts" sense, but how many home repairs are? ;-)
A third one is going to have the transformer ripped out and built in place in my workshop to provide workbench LV power supply for hobby use. And perhaps to power a emergency lighting battery charger.
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