To choose a landscape lighting transformer, add up the wattage of every fixture it will run, including any you plan to add, and buy a unit big enough that the total sits near 80 percent of its rating. Then match the taps to your longest runs, decide how it switches on, and mount it somewhere dry beside a GFCI outlet.
How big a transformer do you need?
Size it to the load. Add up the rated wattage from the fixture spec sheets for everything that will run on it, plus anything you already know you’ll want later. Every fixture on the system hangs off it, so its size is one of the first decisions in a design and not something to sort out at the end.
A transformer takes 120-volt house current and steps it down to the roughly 12 volts landscape fixtures are built for, which is what makes it safe to have cable running through a wet bed.
Then leave headroom. The rule of thumb you’ll see cited most often in the trade is to load a transformer to no more than about 80 percent of its rated capacity. The margin covers swings in supply voltage and leaves room for the fixture somebody adds a year later. Say a plan has twelve path lights at 4 watts and six uplights at 7 watts. That’s 90 watts. Divided by 0.8, that’s a minimum of about 113 watts, so a 150-watt unit is the sensible buy, and it still leaves room for several more fixtures before it reaches the 80 percent line.
LED fixture sheets often list two numbers, watts and VA, and the VA figure is usually higher because of the way the driver inside draws power. A transformer’s capacity is really a VA limit, whatever the box says, so when a sheet gives both numbers, size on the VA.
When is a multi-tap transformer worth it?
When a long run leaves the far fixtures short of voltage, since closing that gap is the whole point of a multi-tap transformer. Low-voltage cable loses a little voltage over every foot, so the last fixture sees less than the one nearest the transformer.
Our explainer on how low-voltage systems are wired works one case through: 60 watts at the end of 100 feet of 12-gauge cable loses about 1.6 volts on the way out.
Instead of one fixed 12-volt output, a multi-tap unit has several terminals, commonly stepping up from 12 to 15 volts, and some models go higher. A long or heavily loaded run gets wired to a higher tap so the last fixture lands near its intended voltage rather than glowing dimmer and yellower than its neighbors. The target is the operating range printed on the fixture’s spec sheet. The way to confirm you’ve hit it is a voltmeter reading at the far fixture with the system on. Guessing tends to overshoot, and a lamp or LED driver fed above its rated range can fail early.
A run that’s already on a high tap is also a warning sign when someone wants to add fixtures to it. Check whether the transformer has capacity left first; our other landscape lighting questions cover adding lights to an existing system.
Magnetic or electronic: which should you buy?
Magnetic is the traditional kind, a copper winding around an iron core in a heavy box, and it tolerates a wide range of loads and small swings in supply voltage. Electronic does the job with solid-state circuitry, smaller and lighter, often with a timer or photocell built in.
The magnetic coil usually gives off a faint hum you can hear standing next to it.
The catch is compatibility. Some electronic units are fussy about the LED drivers in the fixtures they feed, and a poor pairing shows up as flicker or a buzz the same fixtures wouldn’t make on a magnetic unit. For a larger system, or one that mixes fixture brands or grows over the years, magnetic is the conservative pick. An electronic unit makes more sense on a small, single-brand setup where the fixture maker lists it as compatible.
Timer, photocell or astronomic clock?
A plain timer runs on a fixed clock, which works until the season turns. In Nashville, sunset moves by more than three hours between early December and late June, so a timer set in summer switches on well after dark in winter unless somebody keeps resetting it.
Whichever kind you choose, the control decides when the lights come on, and it usually lives at the transformer.
A photocell reads the sky and switches at dusk and dawn with no schedule to keep. It can be fooled, though, by a porch light or streetlight hitting the sensor, or by a shrub growing over it. An astronomic timer calculates sunrise and sunset from the date and your location, so it tracks the seasons without the sensor problems, and it can still be told to shut off at a set hour instead of running until dawn. Many current transformers add app scheduling and remote control on top of one of these. Getting the programming right is part of the install, and Gemmell’s workmanship warranty covers transformer programming along with the wiring, connections and fixture mounting.
Where should the transformer be mounted?
It plugs into an outdoor outlet, and that outlet should be GFCI-protected with a weatherproof in-use cover that closes around the plug. Mount the transformer on a wall or post, off the ground, where it won’t sit in water after a storm or disappear under mulch and new growth.
Keep it where someone can reach it without cutting back a shrub, because a transformer that’s easy to get to is one that actually gets checked.
Where it goes also shapes the layout. Its position sets how long each run is, and a spot near the middle of the lit area keeps runs shorter and voltage drop smaller, which is why what happens in the ground during installation starts with this decision. If there’s no usable outlet where the transformer needs to be, adding one is line-voltage work for an electrician, separate from the low-voltage side of the job.
Is my transformer overloaded or failing?
From the porch they look much the same. Fixtures dim as more get added, or the GFCI keeps tripping. Sometimes there’s a hum that wasn’t there last year, or the housing is warm when you put a hand on it.
When a whole zone goes dark at once, the cause is usually upstream of the fixtures, in the transformer, the GFCI feeding it or its timer, not a dozen lamps quitting on the same night.
The homeowner checklist for when some landscape lights stop working walks through the GFCI, breaker and timer checks safely. Beyond that, failed transformers are among the faults our landscape lighting repair visits turn up most, along with corroded connections and cut cable. The repair is quoted after the system is tested, since the fault has to be found before anyone can price fixing it. Regular maintenance and repair programs are where loose connections and early overload signs get caught before they become a dark zone.
Common questions
What size transformer do I need for my landscape lights?
Total the wattage (or VA, if the fixture sheets list it) of every fixture the transformer will run, including ones you plan to add, then divide by 0.8. That follows the common rule of thumb of loading a transformer to about 80 percent of its rating. Buy the next standard size above the result.
What is a multi-tap transformer?
It’s a transformer with several output terminals, commonly 12 to 15 volts, instead of one fixed voltage. Long or heavily loaded runs go on a higher tap to make up for voltage lost along the cable, so the fixture at the far end gets close to the voltage it was designed for.
How can I tell if my transformer is overloaded?
Watch for fixtures that dim as more are added, a breaker or GFCI that trips more than it used to, and a housing that feels warm. Compare the total fixture load against the rating on the nameplate. If the load is above about 80 percent, the system needs a larger unit or a second transformer.
Why does my landscape lighting transformer hum?
A faint, steady hum from a magnetic transformer’s coil is normal. A hum that’s new or getting louder can mean something has worked loose inside, the unit is running near or past its rated load, or an electronic transformer is poorly matched to the LED drivers on its fixtures. That kind of hum is worth having tested.
