they help save electricity by using a transformer that converts higher voltage electricity into a 12 v energy efficient electrical current. Sorry to say it's not quite that simple. First of all, there's nothing magical about a transformer. Watts are Watts. With an ideal transformer, watts in would be exactly equal to watts out. But in real world transformers, watts in > watts out - always - because of the transformer's losses (inefficiency). So all other things being equal, a transformer wastes electricity. The evidence for this is that it gets warm. Heat == waste. But things aren't always equal. When you're talking about low intensity incandescent lights, a low voltage lamp can be (though it isn't always) more efficient than a line voltage lamp. This is because its filament can be operated at higher temperatures for a given light output. This is often the case with xenon undercabinet lights for kitchens, for example. I may have missed some, but regrettably I've never seen any landscape lights that appear to operate this way. Every incandescent landscape light I've seen has been a low output (and thus low filament temperature) lamp. So I doubt that you gain much, if anything, by using a transformer. There's also the greater losses in the wiring caused by the higher currents. By Ohm's law, current = voltage / resistance. So, for a given wattage, at 12 volts the current is 10 times as high as at 120 volts. The only real advantage that low voltage landscape lighting has over line voltage is relative ease of installation. That said, low voltage landscape lighting is a good application for LEDs. Even though they're not as efficient as their reputation would suggest, they're usually more efficient than dim incandescents. They're also right at home with low voltage. As a side benefit, they usually last much longer than incandescent lamps.

What I have now is low-voltage xenon lights with internal transformers inside each fixture. I'm looking at replacing them because of continual problems, like multiple burned-out bulbs and a burned-out transformer after 16 months, and innards so full of wires and wire nuts that the fixtures pop apart if I removed the glass diffusers (or spontaneously in the case of the smallest strip.) Judging from online reviews, these are common problems with the model that I have. We have line voltage coming out of the walls for the three separate runs of lights, all on the same circuit. If I understand correctly, the electrician can use the existing wiring to install either low-voltage lights with internal transformers (NOT the same brand!) or line-voltage lights. I bought the lights we have based on looks, function, and the showroom salesman's recommendation. Never would have thought to ask to see the inside of the fixture, though.

I believe that the autotransformer would work. Did not find one of your KVA, but three of these 4KVA in parallel would work. Would need ventilated enclosure with fan cooling preferred. Not low total cost. http://electricalnotes.wordpress.com/2012/07/17/parallel-operation-of-transformers/ I do not jerry rig-- I fashion solutions from alternative materials. Here is a link that might be useful: 4KW

If it is not staying tight, pretwist is your friend. And you might be using the wrong size nut. I disagree with ron on the pretwisting (again everyone has their own opinion on this). I have taken off various nuts not pretwisted and the wires are completely loose, or they arced so much that they were welded together and I couldn't get the nut off. When I take off a nut on my pretwisted connections they are still together tight. So the claim that pretwisting does little in the long run is wrong. I have read an entire thread on a different forum of people going back and forth on this so whatever works for you. I personally have had to redo a lot of junction boxes with loose connections done by someone else and have never had a problem with my connections. The connections of mine that I have undone were for later modifications and they were just as tight as ever after taking the nut off. One of the valid arguments against pretwisting is that when that is all you do all day it is extra wear on your wrist, which I agree with. If the nut alone could twist the wire and get it tight and keep it tight then I wouldn't pretwist, but I have not experienced that in the limited electrical work I have done.