To choose the right size transformer for landscape lighting, add up the wattage of every fixture you plan to connect (use the VA rating for LED fixtures if the spec sheet lists one), then pick a transformer large enough that this load is no more than 80 percent of its rating. The quick way to do the math is to divide your total by 0.8 and round up to the next standard size. A 600 watt load needs at least 750 watts of capacity, so you would buy a 900 watt transformer.
What a landscape lighting transformer does
A low-voltage transformer takes the 120 volts from your house and steps it down to about 12 volts for the fixtures. Low voltage is safer to work with, and in most places the cable can be buried directly in the ground without conduit (check your local code, and use cable rated for direct burial). The transformer plugs into or is wired to a GFCI-protected outdoor circuit, and most have a timer or photocell built in to turn the lights on and off.
Step 1: Add up the load
Make a list of every fixture that will connect to the transformer and write down its wattage. You will find it on the fixture label, the box or the spec sheet. For LED fixtures, use the VA (volt-amp) rating if one is listed. The driver inside an LED fixture draws a bit more from the transformer than its wattage suggests, and VA accounts for that.
Here is an example for a mid-size yard. The fixture ratings are typical for LED landscape fixtures, but use the numbers from your own fixtures.
| Fixture | Quantity | VA each | Total VA |
|---|---|---|---|
| Path lights | 12 | 4 | 48 |
| Tree and wall uplights | 10 | 7 | 70 |
| In-ground well lights | 6 | 9 | 54 |
| Floodlights | 4 | 15 | 60 |
| Total | 32 | 232 |
Step 2: Apply the 80 percent rule
A transformer should not run at its full rating. A fully loaded transformer runs hot, wears out sooner and can trip its breaker. Keep the connected load at or below 80 percent of the rating. That is the same as choosing a transformer at least 25 percent larger than your load.
In the example above, 232 divided by 0.8 is 290 watts. A 300 watt transformer would technically work, but it would be nearly full from day one.
Step 3: Leave room for what you will add later
Most people add lights after the first season, for a new garden bed, a patio, the pool area or a tree that has grown in. It is much easier to buy a larger transformer now than to add a second one later. Add the fixtures you might want in a few years to your total before you apply the 80 percent rule. In the example, a 600 watt transformer gives plenty of room to grow, and transformers commonly come in 150, 300, 600, 900 and 1,200 watt sizes.
A transformer that is larger than you need does no harm. It only supplies what the connected fixtures draw.
Step 4: Plan for voltage drop
Voltage drops as power travels along the cable. On a long run or a heavily loaded run, the fixtures at the far end get less voltage and look dimmer than the ones near the transformer. A few things keep that in check:
- Use heavier cable on longer runs. 14-gauge can work for a short run with a few low-wattage fixtures. Most runs are 12-gauge, and long or heavily loaded runs often need 10-gauge.
- Split the load into several runs. Rather than one long daisy chain, run several cables from the transformer to different areas. Some installers use a hub layout, where one cable feeds a junction point that branches out to a group of fixtures, so each fixture sees about the same voltage.
- Group similar fixtures. Put fixtures with similar loads on the same run so the voltage along it stays even.
- Use a multi-tap transformer. Professional transformers have several output taps, often 12, 13, 14 and 15 volts. A long run can start on a higher tap so the fixtures at the end still get the voltage they need.
- Measure at the fixture. Check the voltage at the last fixture on each run with a meter and compare it with the input range on the fixture’s spec sheet.
Magnetic or electronic transformer?
Magnetic transformers are heavier and use a wound core. They are very durable, hold up well outdoors in stainless steel enclosures, and usually offer multiple voltage taps. Most professionally installed systems use them.
Electronic transformers are smaller and lighter, and many put out a steady regulated 12 volts. They are fine for small systems with short cable runs. They usually have a single output voltage, so they give you fewer ways to correct voltage drop on a larger layout. Check that your fixtures and any dimmer you plan to use are rated to work with the transformer type you choose.
What happens if the transformer is too small?
An overloaded transformer runs hot, and its circuit protection may trip, shutting the lights off or making them cycle on and off. Over time the heat shortens its life. If your lights have started going out after you added fixtures, add up the load again. You may need a larger transformer or a second one for part of the yard.
Can I add more lights later?
Yes, as long as the new total stays within 80 percent of the transformer rating and the cable run can carry the extra load without too much voltage drop. If the run is already long, it is often better to start a new run from the transformer than to extend the old one.
When a professional system makes sense
Sizing one transformer for a small yard is a reasonable weekend project. On a larger property, the design gets more involved. There are several transformers to place, long cable runs to plan, conduit to get in before the hardscape goes down, and controls that tie the outdoor zones into the rest of the house. That is where professional design is worth the cost. Our guide to planning landscape lighting for large estates covers how zones and transformers are laid out, and our landscape light timer guide covers scheduling.
Planning a landscape lighting project?
We design and install low-voltage landscape lighting, from transformer sizing to Lutron and Savant control, for homes across Greater Boston, the South Shore and Rhode Island. Book a free consultation or read more about our landscape lighting.
