Low-voltage lighting isn't just post caps. A wide variety of fixtures provide general, decorative, accent, and safety lighting.

A great deck deserves great lighting, not just a couple of wall-mounted 100-watt floodlights that glare down on the deck — which is almost worse than no light at all.

There are two different types of deck lighting systems: One operates on household voltage (120 volts, also called "line voltage"), and the other runs on low voltage (12 volts). Each has pros and cons, but except for very large residential projects, low-voltage systems are usually a better choice.

Low-voltage systems are inherently safer. Because there's virtually no electrocution hazard, codes don't require exposed low-voltage cable to be run in conduit, and in most jurisdictions, no electrician's license is required to install low-voltage wiring (if you need to run an additional line-voltage circuit to feed the transformer, call an electrician). Additionally, there are a wide variety of low-voltage outdoor fixtures available.

Low-voltage systems do have a few downsides, though. The fixtures put out less light than line-voltage fixtures, and if the distance from the transformer to the farthest fixture is 100 feet or more, voltage drop can cause a dim, yellow light unless you use special installation techniques. You'll also need to find an unobtrusive place to install the transformer. And even though installation is faster because you don't need conduit, it does leave the cables exposed and vulnerable to damage.

For large-scale projects, line voltage is great. A circuit can accommodate many fixtures and each one can put out a lot of light. No transformer is needed, which saves a few bucks, and adding dimmers to line-voltage systems is easier than adding them to low-voltage ones. And since you'd be running conduit for the cable anyway, it's simple to add outlets around the deck.

However, the higher shock and electrocution hazards of line voltage, combined with fewer fixture choices, make me favor low-voltage systems for smaller projects.

Transformers and Voltage

The device that changes 120-volt electricity to 12-volt is called a transformer. Transformers are made with different capacities (rated in watts), and each one runs a single circuit. I strongly recommend using a transformer that has a UL 1838 listing, which means that its voltage output is limited to 15 volts and the wattage to 300.

Why would you want the option of 15 volts on a nominal 12-volt system? Because the greater the distance the electricity has to travel, the more voltage is lost to the resistance of the wire. The extra 3 volts can compensate for this voltage drop. Feed fixtures from a loop at the end of a long supply line at 14 volts or 15 volts, and even the farthest fixtures will still provide light that's bright white.

Because of the danger of fire, it's safer to limit the power to any circuit (fed from one transformer) to 300 watts. An output of 300 watts at 12 volts means you have 25 amps of current available. That's a lot. If there's a dead short with a UL 1838-listed transformer, the breaker protecting the secondary winding of the transformer will trip. However, if there's a moderate-resistance short circuit, or a short to ground, the breaker might not trip or might take a while to trip.

The current translates into heat at the point of the short, at a weak connection, or where a cable is damaged. The higher the amperage, the greater the chance of a fire if something goes wrong. The risk increases, therefore, with larger wattage transformers; 12-volt 600-watt transformers have 50 amps of available current.

If you need more than 300 watts, install another transformer. Also, there are products available with two separate transformers in a single housing, and the whole shebang is UL 1838 listed. Each transformer in such a combination has one set of output terminals rated at 300 watts.

LED or Incandescent?

Some fixtures have a light-emitting diode (LED) instead of an incandescent lamp as the light source. An LED is a semiconductor device like a transistor; it will last virtually forever (typically 50,000 hours) and use a lot less energy than an incandescent bulb. Most solar-powered deck lighting uses LEDs.

Generally, though, a single LED produces much less light than a standard 5- or 7-watt lamp, and so-called white LEDs give off a cool, bluish light, noticeably different than an incandescent lamp. LED fixtures cost a lot more than similar ones that use incandescent lamps, and the variety of styles of fixtures with LED light sources is pretty limited.

If long lamp life and energy savings are paramount and cost is not an issue, LEDs can be a good choice. First, though, I recommend demonstrating an LED and an incandescent fixture side by side, so the client knows what to expect from the LED.

Have both individuals of a couple there to see the LED; often one is okay with it (and loves the energy-saving aspect), and the other hates the way it looks. It's not good to find that out when the installation is complete. As it stands now, you can't just swap out LEDs and put in incandescent lamps; the fixtures are different.


Additionally, UL 1838-compliant transformers offer overcurrent protection (that circuit breaker I mentioned above, usually with a push-button reset) and a thermal cutoff switch, which cuts the power to an overheated transformer.

I usually plan for a load (the sum of the wattage of all the lights on the circuit) on any transformer of no more than 80 percent of the transformer rating (this is called derating). It's pretty easy to figure out the circuit load. Each lamp has a wattage rating, typically 5 or 7 watts. A 300-watt transformer derated to 240 watts can power up to 34 7-watt bulbs.

Derating does a couple of things. First, it allows for adding a fixture here or there. Second, a derated transformer will last longer, because a maxed-out transformer runs hot and heat breaks down the insulation, eventually causing a short in the windings.

This is also why most transformers are listed for outdoor mounting only and the installation instructions state that the cooling louvers/vents on the housing have to be kept unobstructed.

Lighting for Tasks, Safety, and Decoration

You can use light several ways. Spotlights and hanging fixtures are just the ticket for task lighting that illuminates a specific area — say for grilling or for reading (Figure 1). Safety lighting is used on stairs, transitions, and spas (Figure 2). Mini recessed lights that fit into holes drilled along stairs are another option. Lighting these areas also accentuates them, but true accent lighting focuses on a feature, such as a tree or mural (Figure 3).

Figure 1. Task lighting can be a small work light for checking to see if the steaks are medium rare (top), or a chandelier that lights a friendly game of Texas Hold 'Em (bottom).

Figure 2. Safety lighting helps to keep people on their feet going down stairs or walking around spas.

Figure 3. Accent lighting creates memorable grace notes by illuminating such features as flowers, trees, or murals.

Decks are fun places, and decorative lighting that is a feature in itself underscores this point (Figure 4). Post-top lights can serve as both decorative and general lighting (Figure 5).

Figure 4. Lighting can be whimsical (left) or artistic (right).

Figure 5. Probably the most common deck lighting, post-cap fixtures provide general lighting and decoration at once.

No matter how you combine the lights, don't overdo it. You want enough light, but it's the contrast between darker and lighter areas that makes a space interesting.

Lighting Stairs

People trip and fall on stairs, and in many cases, building codes require lighting to minimize this hazard. Poorly done stair lighting can be even more dangerous than none, as inconsistent shadow and light confuse the eye. Below are three reliable ways to illuminate stairs.

Perhaps the most obvious way to light stairs is with fixtures recessed into the risers. One downside of this approach can be that light shines right into the eyes of those climbing the stairs.

Lighting a stair from the side may be easier on the eyes of climbers. One recessed fixture placed low on the stringer and centered on each tread provides plenty of light.

Another way to light stairs is to mount the lights a little higher and slightly forward of each riser. This lights each tread and uses one less fixture per flight.


I set up systems to allow for different levels of light, one for entertaining and one for private relaxing. To this end, it's a good idea to set up the fixtures on two or more circuits that can be switched independently. Put the general lighting and task lighting on one circuit and the decorative lighting on another.

For relaxing on the deck, it's nice to have the lights low. You could run just the decorative lights, but a more sophisticated approach is to use a dimmer. While there are dimmers designed for 12-volt systems, I find that it's less expensive to dim the 120-volt line going to the transformer. Make sure the dimmer is rated for the size (wattage) of the transformer and for an inductive load. This information should be available in the product literature.

Dimmer switches should be installed inside, though this complicates matters somewhat. While it's possible to install a regular switch outside in a weatherproof box with the right weatherproof cover, it's not a good idea with a dimmer switch because its electronics don't handle outdoor humidity, heat, and cold well.

You'll need to extend a circuit and add the dimmer switch and an outside GFI receptacle outlet for the transformer(s), or possibly run a new circuit with the dimmer switch and GFI receptacle.

Planning for Changes

I always talk to the clients about how they expect to use the deck, and I'm not afraid to offer thoughts on how other clients have used their decks. Fine-tuning a system with the clients' input gets their emotional buy-in, makes it easier to charge custom prices, and minimizes changes after the installation. Still, clients do sometimes request changes. I prepare for this when I design the system and bid the work, so any changes requested after the client sees the lighting aren't hard to accommodate.

For example, when running the supply line, I put a short loop of cable at the midpoint between the fixtures. If you want to add another fixture later, having 4 inches to 6 inches of slack will make all the difference.

If the client wants to implement the design in stages, or has a track record of wanting changes, I load a transformer to 50 or 60 percent of its capacity, rather than 80 percent. Having the unused capacity means you can add fixtures and not overload the transformer. Sure, you could add another transformer later, but then you have to add more supply cable along the same path. It's a lot more efficient to have read the client right and planned for change.

When you're designing the lighting layout, place the light fixtures symmetrically so if you add additional fixtures, it will still look good. For instance, place a fixture on every other post. If the client wants more light (and I usually encourage them to live with the layout for a week before we make changes), you can add fixtures so that every post has one.

When adding fixtures that require cutting an opening in a post or in the deck, I first mount a dummy block on the surface and run temporary wiring to give the client a preview. It gives the client a good idea of the effect of more fixtures and can be removed easily, leaving little damage.

Loops and Lollipops

The cable used for low-voltage deck lighting is the same two-wire cable used for landscape lighting, which is like lamp cord or zip cord, only heavier gauge, more rugged, and able to handle sunlight exposure. As a rule of thumb, use 12-gauge cable for the supply lines unless the transformer is more than 50 feet from the first fixture; then use 10-gauge cable to minimize voltage drop.

If you're loading a circuit to the max, it's a good idea to run the supply cable in a loop that's fed from each end. Otherwise, the fixtures at the far end will be dim due to voltage drop.

Just be sure not to mix up the conductors of the supply line — opposite ends of each conductor have to attach to the same lug on the transformer (maintaining "polarity"). Mix them up and you'll short out the system, and the breaker will trip.

I've been called in to troubleshoot where the installer didn't understand polarity and had reset the breaker a dozen times. I guess the thought was that one more reset would fix it. A circuit breaker generally is good for a few trips, but each trip stresses the mechanism. At some point, it just won't reset anymore.

Usually, one of the sides (conductors) in landscape cable has smooth insulation, and the other side has very fine longitudinal ribs in the plastic. This is one way to check polarity. Do not use the cable labeling for this purpose; the printing isn't always on the same side of the cable from spool to spool.

On decks requiring multiple circuits and transformers, using loops from the transformers to the more distant circuits can eat up a lot of cable. I often put just the closer fixtures on a loop circuit, as described above.

The more distant fixtures are also on a loop circuit, but the loop doesn't start and end at the transformer — it starts at and loops back to the fixture in that circuit closest to the transformer, and a single supply line then runs back to the transformer. Such circuits look like a lollipop. To minimize voltage drop, I get the power to the first fixture on a 10-gauge cable, and from there create a loop of 12-gauge cable in the area where the fixtures are located.

Lollipop circuits work great when the transformer is 75 feet to 100 feet from the first of a group of fixtures. This is also the right time for a transformer output greater than 12 volts.

Try the 13-volt tap first. Use a volt meter to measure the voltage at the farthest fixture. If it's between 11.5 volts and 12.5 volts, it's good. If it's lower than 11.5 volts, check it again using the 14-volt tap, and so on. If 15 volts at the transformer doesn't produce 11.5 volts at the distant fixture, have an electrician run a 120-volt line out closer to the group of fixtures and put a transformer there.

Connecting the Cable

You have to connect the supply cable to each fixture. The fixture wire or tap is usually 14- or 16-gauge wire. There are several ways to make these splices, but never use regular wire nuts. Corrosion will make the connection fail sooner rather than later.

The old-school way to make connections is to use silicone-filled wire connectors (Figure 6) designed for wet (direct burial) conditions (DryConn, King Innovation; 800/633-0232, www.kingsafety.com). These work fine, but you have to cut the supply cable at each tap, and you have to pay attention to polarity. Another approach is to use clip-on connectors (Figure 7) that pierce the insulation of the supply cable (Kichler Lighting; 866/558-5706, www.kichler.com).

Figure 6. Silicon-filled wire nuts twist on like regular wire nuts, but they keep out the water that corrodes connections.

Figure 7. Clip-on connectors are a speedy way of splicing the fixture's wires to the main supply cable.

Figure 8. Finding truly corrosion-resistant cable staples is nearly impossible. A plastic coating will help to protect the exposed portion of the staple, but avoid using staples in places where rust trailing down part of the deck will be objectionable.

Figure 9. Concealing wire runs to the fixtures takes ingenuity. Be careful not to place cable where it can be damaged by fasteners added later, and to avoid a short, be sure the power is off when working with low-voltage wiring.

Below the deck, the cable should be attached with electrical-cable staples (Figure 8). Above, it can be concealed inside holes drilled in the deck posts, run in molding, or hidden in routed grooves behind battens (Figure 9).

Cliff Popejoy is a licensed electrical contractor in Sacramento, Calif. He does both residential and commercial work on lighting and power, data communications, and photovoltaic systems. When he's not wearing a toolbelt, he's usually on a hiking trail somewhere.