Landscape Lighting Transformer... Choosing the Right Low Voltage Transfomer
A landscape lighting transformer is the most imporant part of your landscape lighting system. The low voltage transformer converts the (115V-120V AC) to a low voltage range from 12 volts to 18 volts.
Light Fixtures and Total Wattage
But in order to choose the best transformer for your landscape lighting you first must know how many light fixtures you will have and what is the total wattage of all the fixtures. When figuring total wattage always use the maximum wattage capacity for each light fixture.
Also remember that it is best not to run a transformer over 80% of its maximum rated capacity. So a 300 watt transformer should only handle 260 watts.
For example let's say you have 5 landscape spot lights with a maximum wattage of 35 watts each. Then you have 5 dome path lights with a maximum wattage of 16.25 watts each. So we have 5 spot lights at 35 which equals 175 watts and we have 5 path lights at 16.25 which is 81.25. This gives us a total of 256.25. Which tells you that a 300 watt transformer will do the job.
You also need to take in consideration that in the future you may want to expand your landscape lighting. You have 2 options, buy a larger size transformer now or when the time comes to expand you will need to purchase another transformer. If you look at transformer prices it is much cheaper to buy a 600 watt than 2-300 watt transformers.
If you are doing a fairly large landscape lighting project it might be easier to install 2, 3 or 4 transformers instead of just one.
Low Volatage Multi Tap Transformer
Professional grade transformers will have multi taps inside, giving you the option to select different ranges. The reason for the selection is that you can have power loss through your wire runs due to the length of the run or the wattage load you have on it.
For instance on a long wire run you might have to tap the wire in to a 14 volt tap to maintain the operating range of your landscape light bulbs, which is between 10.8 volts and 12.0 volts.