Answers to properly sizing landscape lighting transformers for your outdoor lighting projects
I have been getting a lot of questions lately about landscape lighting transformers.
How much load is too much? Why is it hot? Why isn’t it working? And more. So let’s try to help you decide what kind of transformer you need.
The very first thing you must know when choosing a transformer for your low voltage outdoor lighting system is the load. I’m not going to go into all of the little things that we professionals also consider, but let’s look at the primary stuff. What is the total wattage of the lights that are being installed? In the case of halogen or incandescent lights this is pretty straight forward. If you have 5 lights and each one has a 20 watt halogen lamp in it, your total load is 100 watts. That was easy, right? Now let me say that you can NOT use a 100 watt transformer for this job because it will be overloaded! The reason is this: there are load losses in the wire being used, and in the connections as well as the load of the lights themselves. Also, the National Electrical Code says that you cannot load any device above 80% of its maximum load. This means that you cannot put more than 80 watts of total load on a 100 watt transformer.
Now let’s look at LED fixtures and lamps for a moment. LEDs are electronics, not light bulbs. They run on DC power, not the AC that comes out of most low voltage landscape lighting transformers. This is also true for your laptop computer and other electronic gadgets that have those little black boxes on the cord between your device and the wall outlet. When a DC product is using AC current, the power drawn is measured by Volt Amps (VA) which is a higher number than the wattage labeled on the LED lamp or fixture. For example: If an LED fixture uses 4 watts, but has a 5.5 VA, the 5.5 is the measurement used when choosing the size of the transformer. Failure to account for VA in these calculation can cause some critical failures when you have 10 LED fixtures using 4 watts each and attempt to install a 50 watt transformer. Those 10 fixtures may be using as much as 60 VA and your transformer will be overloaded.
I almost always install twice as much transformer as what is actually NEEDED at the time of original installation, and here is why. I like to give far more than just the 20% minimum extra room required on a transformer. It’s great to have some cushion in case a miscalculation has been made when adding up all of the loads, wire and connections. And it has been our experience, in most cases, the project will expand over time as the landscape matures. Adding more lighting is much easier when there is plenty of space on a transformer. Finally, the cost difference between a 100 and 200 watt transformer is very small compared to the huge headaches that erupt if the transformer is undersized.
Of course if you would like us to evaluate your current or needed transformer situation, please contact us here.
