The right transformer for the job

Learn how to choose the right transformer size for your landscape lighting project and calculate volt amps accurately.
It's important to choose the correct size of transformer for an LED lamp, and to do that, it's important to account for VA, or volt amps.

I often get asked about how to size a transformer for a project. Since this topic cannot be discussed too much and people are adding landscape lighting services to their green industry businesses, I thought it would be a good subject to revisit.

In the LED industry, and specifically in landscape lighting, the term volt amps is frequently mentioned. Volt amps refer to the amount of energy in an AC circuit that is consumed but does not contribute to light output. This is an important consideration for designing and planning an outdoor lighting system. When VA is calculated, an LED lamp typically draws more energy than listed on the lamp. It is important to choose the correct size of transformer, and to do that correctly, it is important to account for VA.

Most retrofit LED lamps used for landscape lighting have a small circuit board inside called a driver. This driver is what accepts 12 volts AC from a transformer and turns this voltage into DC voltage, which is required to light the LED. Now, this rectified voltage goes through a series of small electronic components which make the LED light up. It gets much more complicated than that, but we won’t get into electrical circuit design.

When the voltage passes through the driver, all the small electronic components consume watts. In the case of small retrofit lamps, it’s a small amount of wattage, but it is certainly measurable and necessary to include in your plan. All LED lamps have a power factor value usually between 0 and 1. In most landscape retrofit lamps, this number can range from 0.5 to 0.9. The power factor figure is important because it is used to calculate volt amps. For example, if you took the apparent power (the wattage listed on a lamp) divided by the power factor, you will get the VA or the real power of the LED lamp.

Below is a quick example of how the formula works using a 4W LED lamp:

4 watts / 0.85 power factor = 4.7 volt amps

In this example, the transformer will see a load of 4.7 watts from a 4W lamp. Most manufacturers have a VA chart available or published on their website to help designers measure the voltage drop and size a transformer correctly.

It is hard to know the actual power factor of every product on the market today. Some products do not show actual specifications for VA. If you encounter these products, you can use these formulas in the field. For standard white lights 2200K to 6000K, multiply the face value of the lamp by 1.67. For instance, 4W × 1.67 = 6.68W. This would be the number you would use to calculate the number of lamps that could be placed on a transformer.

If you are using an RGBW product, multiply the face value by 1.79. The reason for this is that some colors will draw more power than white Kelvin temperatures.

To make sure you can safely add light fixtures to your design over time, it is important to plan in advance. For example, when lighting new trees, find out how much they will grow over the next three to five years. Size the cable and transformer to accommodate more wattage and perhaps more fixtures than the original installation. In most cases, the next size up in transformer is a minor cost in the long run.

Remember, it is always best to confirm a manufacturer’s specifications on VA and actual power draw before you select a transformer.

Kevin Smith is the national technical support and trainer at Brilliance LED LLC, Carefree, Arizona, and can be reached via email.

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