Designing with Light

A 120-volt outdoor lighting system offers several advantages over a 12-volt system – especially when security, not aesthetics, is the issue. For starters, 120-volt fixtures usually illuminate larger areas than 12-volt fixtures can – useful both for security and for lighting trees from the ground. The bigger fixtures are also sturdier, and their buried cables and connections provide a look of permanence lacking in some low-voltage systems.

On the other hand, 12-volt systems are simpler to install – especially for homeowners. And the cable and smaller fixtures can snake just about anywhere you need them.
Confused? It helps to choose the bulbs you want first and then the appropriate fixtures. For instance, low-voltage halogen MR-16 bulbs are popular for accenting; PAR spots and floods, available in both standard and low voltage, are best for lighting trees or wide areas.

120-volt systems

A 120-volt outdoor system consists of a set of light fixtures and either type UF (underground feeder) cable, if allowed by local code, or individual wires run inside rigid metal or PVC conduit.

Keep in mind that 120-volt wire splices and fixture connections must always occur inside a housing box. Boxes for exterior use come in two types: so-called driptight boxes that deflect vertically falling water and watertight boxes that keep out water coming from any direction. For anyplace likely to get wet, a watertight box is best. All covers for watertight boxes are sealed with gaskets. The outdoor fixture box is typical.
Fixtures for 120-volt outdoor systems range from well lights and other portable uplights to post lights that mark front walks, spread lights that illuminate paths or bridges, and downlights designed to be anchored to the house wall, eaves, or trees.

Outdoor fixtures come in various sizes, mostly made of bronze, cast or extruded aluminum, copper, or plastic. But you can also find decorative fixtures in stone, concrete, porcelain, and wood (redwood, cedar, and teak weather best). When evaluating fixtures, look for gaskets, high-quality components at joints and pivot points, and locking devices for aiming the fixtures.

Although low-voltage fixtures lack the punch of standard-current fixtures, their output is sufficient for most outdoor applications. Since it carries only 12 volts, low-voltage wiring doesn’t present the dangers of 120 volts, nor does it require the special conduit and boxes of other outdoor wiring. All you need is a plug-in transformer, 12-volt cable, and low-voltage fixtures. To make things even easier, you’ll find kits containing all these components at home and garden centers.

NUTS AND BOLTS. The transformer, usually housed in an integral driptight box, steps down 120-volt household current to 12 volts. Plug it into a nearby receptacle, then run the 12-volt cable from the low-voltage side of the transformer to where you want your lights. The cable can be buried a few inches deep or simply covered with mulch in a planting area; but to avoid accidentally spading through it, consider running the cable alongside structures, walks, and fences where you won’t be likely to cultivate. Some low-voltage light fixtures clip right onto the wire, while others require a clamp connector and still others must be spliced into the system and connected with wire nuts. Be sure to use the wire and connections specified in the instructions. If you don’t already have a receptacle to plug the transformer into, install a GFCI- protected outlet and weatherproof cover.

SIZING YOUR SYSTEM. Most 12-volt transformers are rated for loads of 100 to 300 watts. In most cases, you simply add up the wattages of all the fixtures you wish to install, then choose a transformer and cable size that can handle the load.

For long cable runs, however, you must “de-rate” the circuit to account for “voltage drop” – the accumulated resistance in all that wire. The solution? Drop a fixture or two or beef up the cable size. Your kit or cable will probably come with guidelines.