Let's start with an introduction for those not familiar with transformers in the lighting industry (Hint: they are not shape-shifting creatures). There are two types of lighting systems: Line voltage and low voltage. Line voltage simply means that your lighting fixtures may be plugged in directly to an electrical outlet or hardwired directly to a power source. The voltage that the light fixture needs is the same as the voltage supplied from your wall. Low voltage lighting systems require lower voltage to operate than the typical power source supplies. So, if the power source in your wall outputs 120 volts, low voltage lighting fixtures require an input of just 12 or 24 volts. That's why you need a low voltage transformer - to convert the voltage from your power source to the amount your lighting fixture needs!
Occasionally, the transformer is built right in to the lighting fixture - in which case there's no need for this guide. The decision has been made for you! However, if the transformer is separate, you'll need to make sure it is compatible with your lighting fixtures. Follow this guide to make your selection in four steps :
Decision 1: Volts
Low voltage lighting systems operate on either 12 or 24 volts. The easiest way to start filtering out the right transformer is to determine if the low voltage light fixture you need to power requires 12 or 24 volts. You should be able to find this information in the description/specifications of any product. Also, determine whether your power source outputs 120 or 277 volts. Most power sources in the U.S. output 120 volts, but there are transformers available for power sources that output 270.
Decision 2: Magnetic v. Electronic
There are a few pros and cons to consider here. Electronic transformers have an additional electronic device called an inverter that enables them to be much smaller than magnetic transformers. So, if you need to tuck the transformer into a tight space, you might want to veer toward an electronic transformer. Another advantage to an electronic transformer is that it can be reset at the wall switch if there's an overload.
Although magnetic transformers are somewhat larger and heavier, they are more durable than electronic transformers and they tend to last longer.
Decision 3: Wattage
Now that you've determined whether you're looking for a 12-volt magnetic transformer, 24-volt electronic transformer, 12-volt electronic transformer, or 24-volt magnetic transformer (that's a mouthful!), you can narrow it down by wattage. Each transformer has a "maximum wattage load." Typically, this is what the actual product is labeled. The rule here is that the maximum wattage load of your transformer must be equal to or larger than the total wattage of all the light fixtures you are connecting to it. Let's say you have one light fixture with one 60-watt light bulb for the transformer mentioned above. That's fine. However, if you install a light fixture with three 60-watt light bulbs, you'll need a transformer with a maximum wattage load of at least 180 watts.
Decision 4: Enclosure
If you see the word "enclosed" in any of the product descriptions, this simply means the transformer is encased in a metal case that provides storage space for terminal block connections you may need to make. It's fine to choose a transformer that's not enclosed, but you should probably store it in some sort of metal housing (i.e., a junction box).