Landscape Lighting, continued

Transformers and Circuits

The power plant of any low-voltage lighting system is the transformer (Figure 6), which steps the home's 120-volt alternating current down to 12-volt direct current. Transformers are rated in watts, and units are available with capacities ranging from less than 100 watts up to 1,000 watts or more. The 500-watt, 600-watt, and bigger units are really banks of 250-watt or 300-watt transformer cores racked together in one box, with multiple taps you can use to supply multiple circuits. Most units now have optional taps available with slightly boosted voltage, giving you 13-volt, 14-volt, or 15-volt power when you need it to serve a particularly long run of cable.

Figure 6.This 550-watt transformer box holds two 275-watt transformer banks, which the author can set independently to supply 12, 13, 14, or 15 volts. The author's control circuits typically use both a timer and a light-sensitive switch. At top left, he strips one of the wires that will link a timer mounted in the transformer box to an exterior-mounted light-sensitive switch, then mounts the timer into the box (top). After connecting the lighting circuits (above left), he runs a voltage check (above right).

Transformer boxes come with knockouts and brackets designed to let the installer include manual switches, timers, and photocell controls. I prefer to set up systems so that a photocell switch turns them on at dusk and a timer turns them off at a preset time.

We almost always put our transformers on the outside of the house, mounting the photocell switch device right on the transformer box. But the photos here show a job where we put the transformer on the wall inside a garage, and I had to drill through the wall to run my lighting circuits. In such cases, we mount the photocell on the junction box on the outside wall face (Figure 7).

Figure 7.The author drills a hole at the base of the garage wall (above), then pulls 12-volt power supply and control wires through a watertight junction box (above right), and mounts a light-sensitive control switch on the box (right). The individual red, white, and black wires serve the photocell switch; the larger black cables entering the underside of the box carry power for the light circuits.

We bury cables 6 to 12 inches deep and bury canister lights roughly flush with the surface of the ground. Spotlights typically mount on a spike; most suppliers have one or two spike types that will work with their whole range of spotlights (Figure 8).

Figure 8.After pulling wire through a mounting stake and threading on the fixture (below), a worker sets the spotlight in the ground, angling it slightly to create shadows on a stone wall face.

Constant exposure to rain and ground moisture is a given with landscape lighting, so water-resistant details are important (Figure 9). For wire connections, I rely on Blazing Wire Connectors from Blazing Products (877/304-2111, www.blazingproducts.com). These connectors resemble ordinary wire nuts, but they have a tube filled with silicone sealant and a locking cap. I've never known one to fail.

Figure 9.Landscape lighting circuits and fixtures are constantly exposed to weather and have to be able to withstand moisture. To protect wire splices, the author uses a two-piece fitting from Blazing Products (left) that uses an O-ring and a silicone-filled cap to seal out moisture. The author chooses MR ("metallic reflector") lamps equipped with glass shields that protect the lamp from any drips of condensation that may form inside the weather-sealed lighting fixture. At right, he taps on a lamp's glass protector shield.

I also use lamp assemblies with a glass shield, which protect the lamp itself, not from rainwater — the fixtures keep that out pretty well — but from condensation on the inside of the fixture lens. We've had individual lamps fail in the past because small amounts of moisture got vaporized inside the fixture, condensed on the underside of the lens, and dripped back onto the lamp. The secondary glass shield keeps those drips away from the bulb.

Accounting for Voltage Drop

Choosing an appropriate transformer, and determining the length and total wattage of the individual lighting circuits it will have to serve, is complicated by a fact of life called voltage drop. With low-voltage systems, power is lost in the wiring, in proportion to the distance the electricity must travel. The farther down a run of cable a light is located, the lower the voltage supplied to that light; but the thicker the wire, the less voltage drop occurs over a given cable length. Low-voltage cable is specified in gauges — the lower the gauge, the thicker the wire — and there is a published "cable constant" for each gauge of wire that lets the user calculate voltage drop for that gauge. We typically use heavy 12-2 (12-gauge, two-wire) cable, but we sometimes run 18-2 cable for lights mounted in trees, because the thinner wires are less noticeable. It's important to calculate the voltage drop in each circuit, factoring in the cable constant and the distance the power must travel, to make sure all the installed lights will be supplied with adequate voltage.

Lamps manufactured for 12-volt systems will work fine at 10 or 11 volts — they'll shine brightly, and will in fact last much longer than they would at the full 12 volts. But go much below 10 volts, and you'll notice dim lights at the end of the wiring run. I've seen cases (not on my jobs, thankfully) where lights that should have been throwing a brilliant white flood of light were giving off only a feeble orange glow. That's what happens if you lay out circuits that involve too many watts' worth of fixtures, or use too long a run of cable.

When you have a distant load to deal with — a long driveway, for example, or a group of trees far away from the house — you do have options. Heavier cable may help, but it can be stiff and awkward to work with; you might prefer to hook that circuit to a higher-voltage tap on a multiple-tap transformer. In extreme cases, you may need heavier wire and higher voltage. You may even need to have an electrician run a new 120-volt circuit out from the house so you can locate a transformer closer to the spot. A good lighting supplier can help you figure out a solution. Once you run through the calculations for different transformer and cable options a few times, you'll quickly get a feel for what the choices imply; with experience, it gets easier to take unusual cases in stride.

For routine layout of simple circuits, however, rules of thumb are a good starting point. During the design phase, we strive to limit the length of runs to 100 feet or less, and to keep the total wattage of lights served by each transformer at 80% or less of the transformer's power rating.

We also avoid straight runs involving many lights in series — instead, we try to lay out the circuits in a "star" or radial pattern, running one wire to a central point and then branching out to individual fixture loads. That works better, because voltage drop depends on the distance between the load and the source. Adding more lights doesn't create more voltage drop as long as the power doesn't have to travel more feet to get to each one. So if I have a 50-foot run to a branch point, and five lights at the end of five 10-foot branch wires, my voltage drop for each light is based on 60 feet of cable. If I string those five lights in series, with 10 feet of wire running from each light to the next, the last one will be at the end of a 100-foot run and will experience more voltage drop as a result.

These rules of thumb give generally satisfactory results, pretty reliably. Even so, I always calculate the voltage drop for every circuit and note what the voltage should be at each fixture. As we hook up each light, we check the voltage at the fixture. This reveals any problems with the wiring and assures us that each light will perform as intended. I'd rather get it right today than have to come back and fix it next month.

Plan of Attack

It's always possible to put lighting into a property as an afterthought. But for optimum effect, it should be designed in advance. It helps to have an advance plan that accounts for practical requirements such as locating the transformers and running wires below paving. I typically add the lighting specifications to the overall landscape plan that I've drawn. The final installation may be different from the original lighting plan; ideas change, and something that looked perfect on paper might need adjustment when plantings and other features are installed. Also, we may discover other features that we would like to highlight (for example, a boulder that we've unearthed on the site). But the plan is useful for keeping track of things that probably will not change, like the transformer location and the wiring layout. And a plan drawn to scale is helpful in laying out the individual circuits, so as to keep the total wattage and cable length for each circuit within the limits of the transformer's capacity.

The plan also reminds you to think ahead. If you are landscaping from scratch and installing a paved element such as a walkway, driveway, or patio, place a piece of conduit under the paving. If it only has to contain lighting wires, 1-inch-diameter conduit should be large enough. If you are adding lighting to a landscape, existing paving can be a hindrance; you'll have to decide whether you would rather bore under the paving (assuming it's solid), remove and replace it, or run the wires around it.

Learning by Doing

Nightlighting has become an accepted part of landscape design, almost as common as trees and shrubs. It doesn't have to be a large part of the budget, however — the first rule of nightscaping is "Less is more," so you might as well start with less. One of the best things about low-voltage lighting is that you don't have to do it all at once.

If you're new to landscape lighting, start slowly and experiment. Try one angle, then another; put a light here, then move it over there. Try a path light, then hang a light in a tree to see the difference. Learn what works by trying anything that strikes your fancy. In the end, you'll have a landscape that your client doesn't need a day off to appreciate.