Low voltage systems are popular choices for landscape lighting because they are safe and energy efficient. Transformers reduce standard household current from 120 volts to 12 volts so there's no danger of electrocuting yourself or your child if a wire is accidentally cut.
To avoid overloading the transformer, you will need to figure the wattage of all of the lights you plan to use and select a transformer rated for that capacity. The most common transformer sizes are 300W, 600W, 1000W, and 1200W. You should also keep in mind that you don't want to set up a system that operates right at its maximum load capacity. A good rule of thumb is to use less than 90% of a transformer's capacity; this will increase the life of both the transformer and the light fixtures that are attached to it.
To figure your total wattage, you'll need to know how many watts each light fixture uses. For example: if you have 10 fixtures rated at 20 watts each, multiply 10 x 20 = 200 watts. The length, and gauge, of your wire also needs to be factored in. If the wire is too long, or too many lights are connected, you'll notice a drop in voltage. There are some highly complicated tables that will show you how to figure your voltage drop exactly, but if you add 25% to your total, you're sure to be covered. So, using the 200 watts from our earlier example, we'll add 25%: 200 x.25 = 50. Now, we have 200 watts for our light fixtures and 50 watts for our wiring, for a total of 250 watts. Keeping in mind that we don't want to use more than 90% of our total capacity, we'll see that a 300W transformer is perfect for this situation (300 x.90 = 270).
If you want to be more precise, there are numerous places on the internet that publish the tables you are looking for. You'll still need to know how many lights you'll be using and their wattage, but you'll also need to know exactly how many feet of wire you plan on using, plus the gauge of that wire. But for type of landscape lighting that most homeowners do, the simple math version will give you enough information to select the correct size of transformer for your low voltage garden lighting project.
