Install Deck Post Cap Lights

Deck post cap lights turn a good outdoor space into one you actually use after dark. They provide just enough illumination to define edges and steps without overwhelming conversation or creating harsh shadows, and they mark the perimeter of your deck in a way that feels intentional rather than merely functional. The difference between a deck with cap lights and one without is the difference between walking carefully with your phone flashlight and moving freely through a space that feels finished. Installation takes an afternoon, requires basic electrical work with forgiving low-voltage wire, and results in lighting that works every evening without you thinking about it. The project breaks into two phases: running wire through your deck structure and mounting the fixtures themselves. Most failures come from poor wire connections that corrode over a season or from choosing lights that don't match your post dimensions. Get those right, and you're installing lights that will work reliably for years with nothing more than an occasional bulb swap.

1. Map your circuit and measure posts. Count your deck posts and measure the actual dimensions of the post tops—most are 4×4 nominal, which means 3.5×3.5 actual. Sketch your deck layout and plan a wire run that connects all posts back to a single transformer location near an outdoor outlet. Calculate total wattage by multiplying number of lights by bulb wattage to size your transformer correctly.
2. Install the transformer and test voltage. Mount the low-voltage transformer on the wall near your outdoor GFCI outlet, following manufacturer height requirements. Plug it in, attach short test wires to the output terminals, and use a multimeter to verify you're getting 12-14 volts AC. Set the timer or photocell switch according to your preference before running permanent wire.
3. Run wire through deck structure. Starting at the transformer, run 12/2 landscape wire along joists using cable staples every 18 inches. Route wire up through the deck boards to each post location by drilling a 1/2-inch hole at the base of each post. Leave 24 inches of extra wire at each post location, coiled and waiting inside the deck structure. Use a wire fish tape if you need to cross perpendicular joists.
4. Prep each post top. Remove any existing post caps and clean the post tops thoroughly. Check for rot or splitting—replace any compromised posts before proceeding. Pull the wire up through each drilled hole so 12 inches extends above the post top. If your post tops aren't level, shim them now with treated wood shims and exterior construction adhesive.
5. Mount light bases and connect wires. Feed the wire through the base of each post cap light fixture. Strip 1/2 inch of insulation from both the fixture wires and your main run wire. Connect using waterproof wire connectors filled with dielectric grease, matching polarities if marked. Secure the cap base to the post top with stainless screws through the pre-drilled holes, ensuring the base sits flush all around.
6. Install bulbs and attach caps. Insert the correct wattage bulb into each fixture socket, being careful not to touch halogen bulbs with bare fingers. Snap or screw the decorative cap portion onto each base according to the manufacturer's design. Most caps have a gasket that compresses to seal against weather—ensure it seats evenly.
7. Power up and check voltage at endpoints. Turn on the transformer and verify all lights illuminate. Use your multimeter to check voltage at the first and last light in the circuit—if the last light shows less than 10.5 volts, you have voltage drop and need heavier gauge wire or a second circuit. Walk your deck after dark to confirm even lighting and pleasant spacing.
8. Secure all connections and set schedule. Tuck excess wire into the deck structure and secure with additional staples. Apply silicone sealant around each wire entry point at post bases to prevent water infiltration. Program your transformer timer to match your actual sunset time, or adjust the photocell sensitivity if your model uses one. Note the installation date and bulb type somewhere you'll remember for future maintenance.