Low Voltage Garden Lighting Facts
Most modern garden lighting these days is low voltage. That makes the transformer a basic fact of life. A transformer takes the 120 volts that you get from your wall socket and changes it to a friendly 12 volts, making it much safer to use. If the cabling from the transformer becomes damaged, the danger is slight. Damaged 120 volt cabling, by contrast, could seriously injure or kill the person who touches it.
So it's important to remember that any cabling that runs from the house to the transformer is still at 120 volts. It's only the cabling that comes out the other end that is safe. To run 120 volt cabling to the transformer, you may require a building permit or the services of a licensed electrician. Make sure it is buried at least 18 inches. Once the cabling to the transformer is safely in place, the rest is easy. The cabling that comes out of the transformer doesn't need to be buried at all. It can be covered in mulch, run along the underside of a fence, or otherwise hidden.
Because it only uses 12 volts, this makes low voltage lighting an economical choice as well. Your power bill won't take as much of a hit.
To decide what capacity transformer you need, you have to add up the wattage of the bulbs you intend to use. The wattage should not exceed 80% of the transformer's capacity. This gives you some leeway if you want to add another bulb somewhere down the road. However, don't load the transformer at less than 60% capacity. The bulbs will run too hot, shortening their life.
Low voltage cable can drop in voltage if the cable is too long and the number of lights placed on it is too great. You may want to give higher wattage lamps their own dedicated cable in order to avoid this problem. In general, the thicker the cable, the farther you can run it before voltage drop enters in. Voltage drop isn't a danger, but it will result in a dimming of the lamps.
Even though low voltage lighting is safe, it would be unwise to become overconfident. The fittings to these fixtures can still become quite hot (even more so if the transformer is underloaded). The material used for the fitting will make a difference in this as well. Stainless steel retains its heat far longer than aluminum. So even if the lamp has been turned off for a while it may still be quite hot to the touch.
The easiest way to deal with this problem is by wearing thick gloves whenever you need to change a fixture or bulb. Since low voltage lights are often small, this isn't always feasible. If you can't wear gloves, wait until you're sure the fixture has cooled down. And then wait a little longer before you cautiously touch it just to see.