Some lo-voltage transformers can use a standard incandescent dimmer. MOST cannot.
You need to find out what kind of transformers you have and does it require a special lo-vo dimmer.

You also can not dim an outlet, well with very few exceptions such as theatrical lighting systems and special receptacles/cord plugs made by lutron.

I have low voltage "hocky puck" undercounter lights which are dimmed by a standard IC dimmer.

Many of the small 60 W 'transformers' are actually AC-DC power supplies.
They can be easily designed to cope with the chopped voltage waveform from a triac type dimmer since they are already chopping the voltage at 20 kHz.
Pegasus lighting has a line of single and dual 120 V in 60 watt 'per' supplies that operate from standard dimmers.

My contractor, who is not an electrician but has done the wiring for his projects, tells me he has done it this way (plugging the transformer into an outlet and using a dimmer on the outlet) before, in fact he did it for another low voltage application in my house. If it is working as expected, should I assume all is well or are there other consequences: fire hazard, lowered lifetime of transformer, etc?

"who is not an electrician"

"he has done it this way (plugging the transformer into an outlet and using a dimmer on the outlet) before, in fact he did it for another low voltage application in my house."

No one knows what else he has messed up.
It is illegal to do electrical work for hire almost everywhere without the correct license, and he does not have one.
You are not allowed to dim a receptacle unless it is not accessible (by location or device type) since someone could easily plug in a device that could start a fire if operated from a dimmer.
Without consulting the manufacturer there is no way to determine if the installation is a safety hazard.
Some transformers and power supplies operate from conventional dimmers, others need special magnetic dimmers, and some cannot be dimmed at all.

Hi,

Dimming a socket will work but it is a bad idea and against code. This is because a dimmer is designed for lighting and has a limited power range that it can handle. If you plugged in an appliance, say a heater into the socket, the dimmer would overheat and sort of burn up. Hence the risk.

Dimmers come in three "flavors" incandescent the ones you see normally in Home Depot and three others, one designed for floursecent lighting and twofor low voltage transformers / electronic. The way they differ is in how the chop the power wave from, and how they are designed to handle reactive (i.e. high inductance) loads.

Note if your lighting system has a switch mode power supply, then dimmers will not work. This is because the switch mode power supply will compensate for the lower power.

My thoughs on proceeding are as follows:

1) Is your lighting system a switch mode of not. If it is a switch mode then do not dim it, or choose another lighting fixture with a "plain vanilla" transformer power supply.

2) Get a dimmer for low voltage. E.g.

http://www.lutron.com/diva/

Note there are two kinds one for magnetic, and one for electronic.

3) Wire the thing as per code on a lighting circuit.

best, Mike.

Perhaps this will provide more information as regards safety:

It is a Juno Trac 12 remote mount magnetic transformer that is being dimmed by a dimmer meant for magnetic transformers (www.junolightinggroup.com p 586 in catalog). Thus, little danger of someone plugging something unrelated into the outlet but it would have been nice to have had this hardwired to eliminate the chance. I shall label the outlet as such for future owners.

It appears to be set up according to manufacturer's specifications but I welcome any suggestions of the experts for me to examine further problems. Thanks.

If you plugged in an appliance, say a heater into the socket, the dimmer would overheat and sort of burn up.

no, not sort of burn up, it WILL burn up. my cousin did this for my grandmother back in the mid '80's so she could have a lamp on a dimmer. fast forward 20 years and she decides to plug in a 1500 watt heater. within 2 hours of pluggin it in, the dimmer caught fire and this ran up the wall and into the attic. MAJOR structural damage and she could not live in her house for 9 months during repairs. if a neighbor had not seen the smoke and called the FD when they did, who knows what could have happened. there were 3 oxygen tanks in the room, if they had gone off the house would probably been leveled.

to the OP, rip out this bogus crap and do it correctly. if you later sell this house and some one plugs in a heater or other high draw appliance, you MAY be held responsible.

OK, everyone got me sufficiently paranoid to contact Juno. The tech rep responded as such to my question about using a dimmer designed for a magnetic transformer on a dedicated outlet for a magnetic transformer:

"Electrically that arrangement would work. Since you state that the outlet is located in a remote location, and presumably will be dedicated to only provide dimmable power for the T550N transformer you should be OK. The only reservation I have about this type of setup, is the possibility of some other electrical device being plugged into a wall outlet wired off a dimmer. It might be a good idea to use a single receptacle instead of a duplex to minimize the chance of this happening. Perhaps even labeling the outlet "for low voltage lighting only" or something of that nature as an added precaution. You will have to install a cord and plug on the transformer. If you have not ordered the T550N already, you can order it with a 6 foot cord and plug option. You would order the transformer as catalog# T550N-BL-CP6. Otherwise you can attach your own cordset."

I apoligize for not getting all the facts beforehand. Would those who recommended trashing the existing systems or felt there was imminent danger of fire consider this sufficient or are there still logical objections?

i would buy the special outlets spencer mentioned earlier and use them in both locations. they will not allow a standard 120v plug to be inserted. you would hav eto change the plug to match these outlets.

you could also ask the inspector what it would take for him to pass them. he may require something totally different.