Step 1: Project overview
The best time to install wiring for undercabinet lights is during a kitchen remodel, before the walls are covered with drywall. But if you want undercabinet lighting and aren't planning any major renovations, don't despair. The wiring plan we show in this article is designed to work in almost any kitchen, and can be installed without visible damage to the walls. And since we're using the inside of the base cabinets to run wires, you don't even need access to a basement or attic.
The National Electrical Code requires that plastic-sheathed cable (commonly called Romex) be protected in areas where it's subject to abuse. Since we're running the cable in the back of cabinets where pots and pans could bump it, we've chosen to be safe and run the cable inside a flexible steel conduit (called “flex”). We'll show you how to cut and install flex and then how to pull the cable through it. If you can run the plastic-sheathed cable high in the cabinets or behind drawers, you may not need conduit. Ask your local electrical inspector which method is acceptable.
Even though the wiring is simple, you'll still have to tie in to a source of power to provide 120 volts for the lights. We'll tell you how to locate a suitable circuit. But if you're uncomfortable with this part of the job, consider hiring an electrician to bring power to the junction box (Figure A), and then complete the remainder of the wiring yourself following Figures A and B and the photos. In either case, check with your local building department to see what type of electrical permit and what inspections are required.
Expect to spend a day running the flex, pulling in the plastic-sheathed cables and installing the lights. In addition to standard hand tools, you'll need a voltage tester, a wire stripper, a hacksaw and a drill with both 1/2-in. and 1-1/8 in. spade bits. If you plan to mount the switch in a tile backsplash like ours, buy a glass bit for cutting the switch hole. Otherwise, a sharp keyhole saw (Photo 2) will work for cutting the hole for the new switch as well as the access hole in the back of the cabinet. All these tools are available at a hardware store or home center.
Figure A: Undercabinet Wiring
The horizontal runs of wiring are hidden in the backs of the lower cabinets. Vertical runs to the lights are fished through the stud cavities.
Step 2: Finding power
There are many potential power sources, but unfortunately the electrical code prohibits any connections to “dedicated” circuits. This rules out the 20-amp small-appliance circuits in your kitchen (you can't use the countertop outlets for power) or dining areas, 20-amp laundry room circuits and 20-amp bathroom circuits. If a light switch box has a hot, neutral and ground, you can take power from it. Other possibilities include a junction box in the basement or an outlet on the other side of the wall from the cabinets (we used a hallway outlet). Make sure there's a protected route to get an electrical cable from your chosen power source into the base cabinets.
Even though the new undercabinet lights don't need much power, make sure they won't overload an existing circuit. This process is tedious and may take you several hours, but it's a necessary step for a safe job. Here's how you do it. To determine whether the circuit you want to use can handle the additional lights without overloading, first shut off the circuit in the main panel. Then go through the house turning on lights and other electrical items. Add up the wattage of everything that doesn't go on, that is, everything that's on that circuit.
Then add the wattage of the lights you'll be adding. We recommend a maximum connected load of 1,440 watts for a 15-amp circuit and 1,920 watts for a 20-amp circuit. (The circuit amperage is stamped on the breaker or fuse.) If the total wattage exceeds these amounts, find a different circuit. If you're confused, call in a licensed electrician to help with this part.
Finally, check to make sure the electrical box is large enough to accommodate the wires you'll be adding (see “Calculating Box Sizes”). If the box is too small, replace it with a larger one.
After you've chosen the electrical box to tie in to, turn off the circuit breaker or unscrew the fuse that controls the circuit. Some electrical boxes contain more than one circuit. Before doing any work in the box, test all the wires with a non-contact voltage tester to make sure they’re “dead.”
Calculating Box Sizes
To figure the minimum box size required by the National Electrical Code, add: 1 for each hot and neutral wire entering the box, 1 for all the ground wires combined, 1 for all the clamps combined, and 2 for each device (switch or receptacle, but usually not light fixtures) installed in the box. Multiply this figure by 2 for 14-gauge wire and 2.25 for 12-gauge wire to get the minimum box volume in cubic inches. Plastic boxes have the volume stamped inside.
If you have aluminum wiring, call in a licensed pro who is certified to work with it. This wiring is dull gray, not the dull orange that's characteristic of copper.
Step 3: Plan the wiring route
The first step is to determine which base cabinet you'll run the power into. To locate the position of the outlet in the base cabinet, chuck an 8-in. length of coat hanger into your drill and drill a hole alongside the outlet and through the back of the cabinet. Then mark a 12-in. square and cut an access hole (Photo 2) directly behind the electrical box. A sharp keyhole saw works well. Make sure to keep the cut shallow to avoid hitting electrical or plumbing lines. Next screw the junction box to the back of the cabinet just below the level of the drawer (Photo 3). The 4 x 4 x 2-1/8 in. deep metal box we're using is large enough to accommodate the wires shown. If your installation requires more cables, calculate the box size needed and buy a larger junction box if necessary.
Drill 1-1/8 in. holes through the cabinet sides and run 1/2-in. flex for each light or group of lights, for the switch and for the power (Photo 4 and Figure A). Allow an extra 6 in. of flex where it enters the walls. Ream the cut end of the flex to remove burrs and install flex connectors on both ends.
You may have to pull your stove away from the wall or slide your dishwasher out so you can run the flex behind them. Make sure to route it where it won't interfere when you slide the appliances back into place. If you have an inaccessible corner, you may have to cut an access hole in the side of the cabinet in order to run the flex. Route the cable through the attic or basement if necessary to get across areas that aren't connected by base cabinets.
Push or use a fish tape to pull the plastic-sheathed cable through the flex (Photos 7 and 8). Allow an extra 12 in. of cable at the junction box and several feet beyond where the cables should end. It's better to waste a few feet of cable than to end up short.
After the cable is pulled into the flex, drill holes in the back of the base cabinet (Photo 8) and the bottom of the wall cabinet (Photos 8 and 10) and cut the hole for the switch (Photo 11). Fish the cable up to these locations (Photo 8). The fishing method we show in Photo 8 also works for insulated walls. You'll just have to work a little harder to hook the fish tape with the hanger. Using your saw, ream the hole through the back of the cabinet at an angle to better fit the flex. Push the end of the flex into the wall and secure the flex with straps placed every 4-1/2 ft. and within 12 in. of every hole in the cabinet and within 12 in. of the junction box (Photo 6).
Figure B: Connections at Junction Box
Connect the power source to the switch and from the switch to the hot wires (black) to the lights.
Step 4: Mount the Fixtures and make final connections
The final step is to mount and connect the light fixtures and connect the wires in the switch box, outlet box and junction box. Photos 13 and 14 and Figure B show how. Double-check the wires in the outlet box with a non-contact voltage tester to make sure the power is off before making the final connections in this box.
If possible, install a dimmer in place of a regular switch. Check with the manufacturer to find out whether a regular dimmer, a magnetic dimmer or an electronic dimmer is required. Then follow the package instructions for connecting the dimmer.
When you're through connecting the wires in the junction box according to Figure B, screw a metal cover plate to the junction box. Then close up the hole in the back of the cabinet by screwing an oversized square of 1/4-in. plywood over the opening.
Buying Undercabinet Lights
The top-of-the-line fixtures we're installing use low-voltage xenon bulbs. The fixtures cost about 20 percent more than similar halogen fixtures. But the xenon bulbs last many times longer, burn cooler and don't require special handling like halogen bulbs. Each of our fixtures has a built-in electronic transformer to power the low-voltage xenon bulbs but is powered by standard 120-volt current. Xenon bulbs are dimmable but require a special electronic dimmer. Basic fluorescent fixtures are much cheaper, but usually can't be dimmed. The wiring method we show works for any 120-volt undercabinet light.
Plan to install a continuous row of lights for the most evenly distributed light. Some types of undercabinet lights are provided with plug-in connectors to join fixtures end to end. Others, like the ones we're using, can be joined by running wires from one fixture to the next. Wire them together according to the manufacturer’s instructions.