It could be that the wiring is too long and too small, resulting in excessive voltage drop.

Voltage drop is a big problem with low voltage lighting. IMO low voltage lighting is really practical only for applications where the light is intentionally rather dim.

"IMO low voltage lighting is really practical only for applications where the light is intentionally rather dim."

Practical in what sense, David? I've seen an awful lot of brilliantly lit merchandise displays illuminated by low-voltage track.

Voltage drop is something to watch out for if the transformer is mounted far from the lights. I'm not familiar with the fixtures the OP used but I'm guessing the transformer might be located in the monorail canopy, which would eliminate that explanation.

Jillypie, can you link to the specific fixture model you installed?

I think you are right about the transformer. It was a remote installation. Last nite they jury-rigged aninstallation with the transformer closer and it was brilliant-we would need a dimmer! I don't quite understand the difficulty. Can they not put the transformer in the ceiling? Too much heat? We can get the surface mount transformer but it is much clunkier looking, expensive and a long wait. Sigh. So on Wed I am going to try and find out where they could mount that transformer.
I have included the link to the monorail and will try to post the link to the heads. If the link within the body of my message doesn't work, the heads are the first ones after the monorail in the "Fusion" collection, monorail chandeliers. the twin tube heads. Thanks.

http://www.lbllighting.com/index.php?lang=2&sid=ac7cb5829990e454495347f1b33d3e52&func=a_7&subfunc=&col=38&prod=743&item=743&detail=1

Here is a link that might be useful: Monorail Chandelier

I'm glad you found the problem. The transformer does have to be mounted somewhere accessible (not hidden behind drywall) and some types require air circulation to keep them cool.

Voltage drop occurs when the wire between transformer and light is too small for the amount of current it's being asked to carry. For a given wattage demand (such as your 300W light fixture), voltage and amperage have an inverse relationship. Lowering the voltage has the effect of increasing the amperage, thus increasing the required size of the wire. If your electrician knew that this was the sort of fixture you'd be using (low voltage with a remote transformer) back when he roughed in the ciruit, he should've used larger gauge wire.

Thanks Jon,
Yes the electrician did know, but failed to use the correct gauge wire. Unfortunately, we are all closed in, there is a room over head and I am not sure how they are going to resolve this. Thanks for you help. Nice to not have to wait until next week to understand the issue.

For what its worth, most good electricians can figure out a way to pull a wire just about anywhere. It might require cutting a hole here or there, but it is possible. Hang in there and let them fix their mistake. If you are like me, you are disheartened to think that they didn't get it right the first time. I am learning the hard way that most contractors don't get things right the first time--sad situation of construction.

The issue with LV lighting is as Jon mentioned - since P = IE (watts = current * voltage), the lower the voltage, the higher the current must be for a given wattage. By Ohm's law (E = IR; voltage = current * resistance) higher current means more voltage drop across a given resistance. Finally, lower nominal voltage means that a given voltage drop will be a higher percentage of the nominal. So with low voltage you have everything against you when you need high power transmitted over significant distances. That's why I say LV is really practical only for rather dim light.

Admittedly, it's a bit of a generalization since there are lighting systems that can yield bright light on low power. But most people who are using LV lighting are using incandescent lamps of some kind.

A fixture with the transformer in its base is another matter entirely. Because the LV wiring is very short, for all intents and purposes it's a line voltage fixture.

Hello all and thanks for all the info. It would appear (and I stress appear) that they have solved the problem by cutting a hole in my attic floor and slithering along to some spot near the light where they can mount transformer and thread line to "slitherer." The reason I say "appear" is that as soon as they did the above, I had to leave and of course, as soon as I left they did too, leaving the chandelier still sitting on the floor. At this point it seems I am supposed to be grateful if the electricians appear for an hour or so every other day.

Peeler, I don't even want to get started on contracters not doing it right the first time- how about the 3rd or 4th time! Today we have floor guys ripping up the second install of my tile/wood combo and the paver guys ripping out my front stoop for the 3rd time because twice they used the wrong color paver and next they used the wrong size. I feel like evrything has been done at least twice, and that I was often told to order the wrong size or model etc, so I have had the restocking fee syndrome. AAH, the joys of new construction.
Sorry for the vent. Jon and David have given me a wealth of info which has been very helpful in my understanding of the problem. An additional benefit of the info is that my electricians no longer mutter "silly woman" under their breath!

...my electricians no longer mutter "silly woman" under their breath!

Awesome.

Update: New electricians managed to move transformer closer to my chandelier and it is much brighter. I don't think it is as bright as when they had it jury rigged, but I think it is fine. So on to the next crisis. Thanks all.