Which low-voltage landscape lighting transformer size should you buy

What the transformer does

A landscape lighting transformer plugs into a standard 120 V outdoor outlet and steps the power down to a low voltage, commonly 12 V, for the fixtures along a cable. UL Solutions says low-voltage power units are limited to 15 Vac or 30 Vdc output, and that low-voltage luminaires are evaluated for fire risk because their power is limited. UL 1838 is the standard for low-voltage landscape lighting systems, and UL 2108 covers low-voltage lighting systems; both were revised so luminaires and power units can be matched in the field using marked ratings. In plain terms, read the ratings on the transformer and the fixtures and match them.

Step 1: Add up the fixture wattage

Find the watts on each fixture’s label or listing. Multiply by the number of fixtures, then add the groups together. LED fixtures use far less power than the older halogen kind; the Department of Energy says LEDs use at least 75% less energy than incandescent lighting, and they last much longer. That is why a modern LED yard often needs a much smaller transformer than an older halogen setup.

The 20% headroom is a common rule of thumb. It covers future additions and keeps the unit from running at its limit all night. If the manufacturer states a different maximum load for your product, follow that.

Step 2: Match the cable to the run length

Voltage drops along a long cable, so lights at the far end look dimmer than the first ones. Thicker cable has less loss. Typical guidance from fixture and cable makers is: 16-gauge for short runs and low loads, 14-gauge for most yards, and 12-gauge for long runs or heavy loads. Cable manufacturers publish charts relating gauge, watts and length; use the one that matches your product. Splitting the yard into two runs from the transformer, instead of one very long loop, is a common way to keep brightness even.

A multi-tap transformer, with terminals such as 12 V, 13 V, 14 V and 15 V, lets you raise the output slightly to offset voltage drop at the far end. Do not exceed the fixture’s rated voltage.

Step 3: Pick the controls

• Built-in photocell and timer. Most landscape transformers include a dusk-to-dawn sensor and an on-time timer. That is enough for many yards.
• Smart plug in front of the transformer. This gives you app control, sunset schedules and voice control. Use an outdoor-rated plug whose rating exceeds the transformer’s input load, and read the notes in outdoor smart plugs. Leave the transformer’s own photocell on “always on” if you let the plug do the scheduling, otherwise two timers fight each other.
• Smart-ready landscape lights. Some fixtures are individually addressable or connect to an app. They are an alternative when you want color or per-zone control; see outdoor smart lighting.

Installation and safety

Plug the transformer into a GFCI-protected outdoor outlet under a weatherproof cover, mounted at least a few inches above the ground per the manufacturer’s instructions. If there is no outdoor outlet, hire a licensed electrician to add one, and check local code. The low-voltage cable on the lawn side can be buried a few inches or laid under mulch. Call 811 before digging deeper than a shallow trench for cable. Do not connect the transformer to 120 V wiring yourself unless the work is within your skills and local rules.

If you are planning a whole yard, the backyard smart kit shows how lighting, watering and a camera fit together.

Common mistakes

• Buying a transformer that exactly equals the load. Leave headroom.
• Mixing fixtures of very different wattage on one long run, so the nearest lights glow brightly and the far ones dim.
• Using lamp wattage instead of fixture wattage. Read the fixture rating.
• Putting a smart plug with a low-amp rating on a 300 W transformer without checking the input draw.