A detailed explanation of one of the hardest electrical circuits to figure out. This is one of the easiest ways to do 3-way light switch wiring.
Step 1
The 'feed' cable should be 12-2 Romex (12 gauge for 20A Circuit, 14 gauge for 15A Circuit) right up to the first switch, then run 12-3 Romex 'traveler cable' to the light and then on to the second switch.
Step 2
The 12-3 Romex has an extra wire called a traveler (usually red), which is necessary for 3-way switches.
Step 3
At the first 3-way switch, the 12-2 Romex 'feed' from the circuit enters the bottom of the switch box and the hot (black) wire connects to the bottom right terminal on the 3-way switch. This terminal is referred to as the common terminal and is usually identified with a different color screw from the other two and the ground screw.
Step 4
The 12-3 Romex 'traveler cable' enters through the top of the switch box. The red traveler (hot) wire connects to top left terminal on the 3-way switch. The black (hot) wire connects to the top right terminal on the 3-way switch. The red traveler and the black wire are electrically indistinguishable and are actually the two poles that are switched when you flip one of the 3-way switches up or down.
Step 5
The 12-3 neutral (white) wire connects directly to the 12-2 'feed' neutral wire with wire nuts and the copper ground wires are also connected together with wire nuts.
Step 6
At the light box, you will eventually have two ends of 12-3 Romex hanging out. One end comes from the first switch (neutral); the other end comes from the second switch (hot switch leg).
Step 7
At the light box, connect the copper ground wires together with a wire nut to get them out of the way, and then connect the two red traveler wires together with a wire nut as well.
Step 8
At the light box, find the black wire coming from the first 3-way switch and the white wire coming from the second 3-way switch and connect them together with wire nuts. Now, and this is important, you need to mark each end of this white wire by wrapping it with black electrical tape. This means you have made the white wire a 'hot' wire. This is a code requirement and is a common practice for 3 and 4-way switches.
Step 9
Moving on to the second 3-way switch, you will connect the red traveler wire to the top left terminal. The white wire, now labeled as black with tape, connects to the upper right terminal. The black wire, as usual, will connect to the lower right common terminal screw. As in the first switch box and light box, the copper ground wire will connect to the box if it's a metal box or the green ground screw on the switch/fixture if the box is plastic.
Step 10
The light box should now have a black wire, a white wire and perhaps a ground wire (for a plastic box) sticking out for installing your fixture.
Step 11
(Alternative instructions for Australian installations)
Step 12
1 Disconnect circuit power (and check no power on circuit)
Step 13
2 Connect circuit Earth (Green) and circuit Neutral (Black)to light (Green & Blue respectively).
Step 14
3 Connect circuit Active (Red) to switch ONE common terminal (middle); Connect Switching Wire (White) to Terminal 1; Connect second switching wire (white or red) to terminal 2.
Step 15
4 On switch both connect switching wires (terminal 1 and terminal 2 respectively on switch 2) and the common terminal to Red Wire (this then gets connected to light).
Step 16
5 At light fitting connect switch one-switching wire 1 to switch two switching wire 1; and connect switch one-switching wire 2 to switch two-switching wire 2.
Step 17
6 Connect switch 2 red wire (connected to switch 2 common terminal) to active terminal on light fitting (Red or Brown).
Tips
- Use the smaller, easier to work and less expensive #14 (14 gauge) copper Romex cable if the circuit is protected by a 15 amp fuse or circuit breaker. Very few three way circuits switch loads on a 20 amp circuit. It is not necessary to use #12 wire branching from an existing #14 wire circuit. 12 gauge is generally only used for kitchen and bath outlets and for other applications that require 20A service.
- A 15A circuit can support 1500 watts of lighting, so you need a "lot" of lights to max-out a 15A/#14 circuit.
- Remember, however, when remodeling you must always check the fuse or circuit breaker on which you're installing new lights or outlets. If you install #14 wire connected to a circuit that is protected by a fuse or circuit breaker greater than 15 amps it's an National Electrical Code violation and, more importantly, a serious safety/fire hazard. This actually applies to all wire. Never install a gauge wire on a circuit that is protected by a breaker or fuse that has a higher capacity than the wire you're using: 6 gauge-50A, 8 gauge-40A, 10 gauge-30A, 12 gauge-20A, etc.
Warnings
- Check you local wiring rules, your local wiring system may use a different color combination.
- Remember to turn off the power before you do any electrical work.
- Never mix wire sizes or materials (copper and aluminum).
Things You'll Need
- Romex cable both 12.2 and 12.3 (for 20 amp circuits) OR 14.2 and 14.3 (for 15 amp circuits).
- Two 3-way light switches
- One light box
- Two switch boxes.
- Wire cutters
- Wire nuts
- Flathead screwdriver.
