Replacing a three-way switch

MATERIALS: Three-way switch, electrician’s tape, masking tape

TOOLS: Tester, screwdriver, combination strippers, long-nose pliers, lineman’s pliers

THREE-WAY DIMMER - If you want two dimmers on a three-way circuit, make sure you get special multi-location dimmers. Standard three-way dimmers can replace one but not both of the switches in a three-way circuit. (Fluorescent fixtures also require special dimmers.)

Replacing switches is easy as long as you get the correct replacement and label the wires for reassembly.

Three-way switches work in pairs to control a light from two locations—handy for controlling a light from the top and the bottom of a stairway or from either end of a hallway. The toggle isn’t marked OFF and ON. Either up or down can be ON depending on the position of the toggle of the other three-way. Before you begin, shut off power to the circuit. Use pieces of tape to label each wire as you detach it from the old switch. At each switch, one of the wires goes to the common terminal, which is darker than the other terminals. The other two wires, called travelers, go to the lighter terminals. Restrip any damaged wires. Most of the steps for replacing a three-way switch are the same as for a single-pole switch, but with three-ways, you must know which wire is which. You can also control a circuit with three switches, but the wiring can be confusing. Consider hiring a professional electrician

1 TAG THE COMMON WIRE. Shut off power, remove the cover plate, and test to make sure there is no power in the box. Label the common wire with a piece of masking tape. The common terminal is colored differently from the others (it’s not the green ground screw) and may be marked "common" on the switch body.

2 OPTION A: WIRING ONE CABLE. When only one cable enters the box, it will have three wires plus a ground. Identify the hot wire using a voltage detector, or by touching one prong of a voltage tester to a ground and the other to each wire in turn. Attach the hotwire to the common terminal, which is a different color. Attach the other two wires to the traveler terminals. Connect the grounds.

3 OPTION B: WIRING TWO CABLES. If two cables enter the box, one cable will have two wires plus ground and the other will have three wires plus ground. Despite all the extra wires, you’ll find only three wire ends. Proceed just as you would for a one-cable installation.

Replacing a dimmer switch

MATERIALS: Dimmer switch, wire nuts, electrician's tape

TOOLS: Screwdriver, side-cutting pliers, strippers

Make sure the new dimmer switch is rated for the total wattage of the fixture. A chandelier with eight 100-watt bulbs is too much for a 600-watt dimmer to handle. Don’t use a standard dimmer for a fan or you will burn out the motor. Install no more than one three-way dimmer per receptacle; the other switch must be a three-way toggle. You can buy rotary dimmers (the least expensive), dimmers that look like standard switches (the toggle can be placed anywhere between on and off position), or rotary models with their own included ON-OFF switch so the dimmer will turn on at the level of your choice. The wide selection of dimmer switches at home centers and hardware stores gives you many control options.

1 REMOVE THE DIMMER KNOB. Shut off power at the service panel. Pull off the rotary knob with firm outward pressure. Underneath is a standard switch plate. Remove it. Remove the mounting screws and carefully pull out the switch body.

2 TEST FOR POWER. A dimmer has wire leads instead of terminals. Test for power by slipping the probes of the tester against the base of the wire nuts. If power is detected, shut off the correct circuit in the service panel.

3 OPTION A: INSTALLING A STANDARD DIMMER. Attach the ground wire if there is one. Strip 3/4 inch of insulation from each solid house wire and 1 inch from each stranded dimmer lead. Wrap a lead around a wire with your fingers so that the lead protrudes past the wire about 1/8 inch. Slip on a wire nut and twist until tight. Test the strength of the connection by gently tugging on both wires.

3 OPTION B: INSTALLING A THREE-WAY DIMMER. If you replace a three-way dimmer, tag the existing lead wires to connect the new dimmer in the same way as the old one. If only one cable enters the box, attach the black wire to the common terminal and the other two wires to the traveler terminals. If you replace a three-way toggle switch with a dimmer, tag the wire that leads to the common terminal. The other two wires are interchangeable.

Choosing electrical boxes

All electrical connections must be contained inside a metal or plastic box that complies with local codes. And all boxes, including junction boxes, must be accessible. Never cover a box with drywall or paneling. Electrical codes specify the number and size of wires that each box can accommodate because crowding wires in a box is a potential hazard. Check codes carefully and, to be safe, install larger boxes than you need now so you have the option to upgrade. Compared to metal boxes, plastic boxes are easily damaged and should only be installed in a wall. Use metal boxes in exposed locations. Inspectors generally expect to see 1/4-inch or more of wire sheathing inside the box.


Cable clamp. Buy clamps made for NM cable. Remove the knockout. Screw the clamp to the cable, then slip it through the hole and screw on the locknut. Pinch the locknut by using a hammer to tap a screwdriver on it. Or attach it to the box first, slide the cable through the clamp, then tighten the screws.

Poke and staple. For many plastic boxes, to run cable you may need to push the cable past a plastic flap or knock out a plastic tab. Once you've inserted the cable into the box, staple the cable on a framing member within 8 inches of the box.

Built-in clamp. Plastic boxes large enough to hold more than one device have internal clamps, as do most remodel boxes. Tighten the clamp screw to firmly secure the cable.

Pop-in plastic connector. Remove the knockout and push this connector in place. Then push the cable through; if accessible, staple the cable within 8 inches of the box.

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