The Outdoor Lighting Expert on 'Choosing the right size Transformer'

By Maria Bargellini

Transformers are used when installingB low voltage lighting. Its function is to step down your line voltage, which is approx. 120 volts, and reduce it to 12 volts to operate your landscape lights. Transformers are typically the most expensive part of yourB landscape lighting project and often the least thought about. Properly sizing your transformer is the first step in your installation. Not sure what that means? No worries. This post will help you choose the right transformer for your outdoor lighting needs.

First Step (Math)

Adding up the total wattage: If you are using (5) – 50 watt up lights, (6) 20 watt path lights and (2) 35 watt well lights your wattage total will be 440 watts. Note: Just because a fixture comes with and is rated for 50 watts doesn’t mean that you have to use a 50 watt bulb in there. 20 and 30 watt bulbs are often used in place of 50 watt bulbs. You can adjust your total wattage according to what you plan to actually use. If you plan to add more fixtures in the future add that wattage as well. It is less expensive to oversize your transformer in the beginning than to buy a larger one or an additional one later.

Second Step (More Math)

Transformers are sold by wattages. The most commonly found sizes are 300w-600w-900w-1200watts. Here’s where it gets tricky, you can only use 80% of the wattage that a transformer is rated for. So that 300 watt transformer can only hold 240 watts from your fixtures. For the project listed previously you will need a 600 watt transformer to run the 440 watts with only 40 watts available for additional fixtures. Confused yet?

  • A 300 watt transformer can be used for 240 watts of fixtures.
  • A 600 watt transformer can be used for 480 watts of fixtures.
  • A 900 watt transformer can be used for 720 watts of fixtures.
  • A 1200 watt transformer can be used for 960 watts of fixtures.

If you are planning to install landscape lighting in the front and back, B unless you plan to have them turn on and off at the same time you will need 2 separate transformers, as you cannot run 2 different timers off of 1 transformer. If you do want them to be controlled by 1 timer then go ahead and add all the wattageB together and make sure you are within 80% of the transformers capacity.

For pictures and more information clink on these links:B Alliance Fixtures Kichler Fixtures

Related articles: B B Outdoor Lighting Fixtures Whats up with up lights Landscape lighting design DIY landscape lighting Area Lights / Path Lights


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