3 Way Switch Circuits
Power at Fixture
You will need…
- A 3 wire cable that is routed between switch box 1 and switch box 2 – these wire cables comes with a black / red / white wire plus the bare equipment grounding wire.
- A 2 wire cable that is routed between switch box 1 and the fixture box – this wire cable comes with a black / white wire plus the bare equipment grounding wire.
- A circuit power supply source wire cable that is feed to the fixture box.
- Insulated wire nuts to connect / join wire together, variety of sizes out there so make sure you obtain the correct size.
- Wire staples, to affix the wire cables to wall studs as needed. Wire cable must be secured within 6 inches of entering the electrical box.
- Wire cable / wire strippers.
- Screwdrivers as required.
Holes drilled to feed wires through studs must be 1 1/4″ from edge of stud, in instances where the wire cable is closer to the edge of stud then you must have a 1/16-in. thick protective metal plate over the area where the cable is being feed through (may want to notch the stud so that that the protective plate does not create a bump on the drywall). Also read the article called ‘Wire Routing Basics‘
Wire cables entering the electrical box – secure the cable clamp at the box snug enough that the cable cannot come out but not too tight as to pinch the cable and break through the insulated cover of the cable.
You must have at least 6 inch length of wire inside the electrical box, it can be a bit more but not less, it can fail an inspection if the length of each wire is less than 6 inches. Also read the article called ‘Box Fill Calculations’ as there is a limit on how many wires are allowed for a certain size electrical box.
STEP 1 – Make certain that the power supply cable is dead – turn off the electrical breaker at the service panel. Make certain that everyone in the house is aware of what you are doing so that they do not get the notion to reset the breaker when another light in the home is not working.
STEP 2 – Make the box openings (if a existing home)
STEP 3 – Feed / Route the wire cables.
STEP 4 -Mount / Install the Electrical Boxes (make sure the electrical boxes are secured as some fixtures are heavy and may require additional support) [The electrical box should not extend beyond the edge of the finished wall or ceiling so that the fixture can mount flush to the ceiling and the wall switch cover will mount flush – but it also should not be recessed too far into the wall or ceiling]; then feed the wire cables into the electrical boxes. Sometimes because of tight openings in existing homes, the wire cable may to be feed into the electrical box then the box put in position and secured.
STEP 5 – Connect the wires…
Connection Instructions:
At the Fixture box…
Two cables entering this box, one is the circuit power supply cable, the other cable (wire cable # 1) coming from switch 1.
- a) using an insulated wire nut connect / join the black wire from the ‘circuit power supply cable’ + white wire of the cable coming from switch 1.
- b) connect the white wire from ‘circuit power supply’ cable to the grounded conductor (neutral) of fixture – if screw connections at fixture connect to the silver color screw.
- c) connect the black wire coming from switch 1 to the switched ungrounded conductor (hot) of the fixture – if screw connections at fixture connect to the brass color screw.
- d) please see under important notes in regards to the equipment grounding conductor (bare wire).
At switch 1 box…
Two cables entering this box, one is coming from the fixture (wire cable # 1), the other cable coming from switch 2.
- a) using an insulated wire nut connect / join the white wire coming from fixture + the black wire of cable coming from switch 2.
- b) connect the black wire coming from fixture to the common screw of switch 1.
- c) connect the white wire coming from switch 2 to one of the remaining main screws of switch 1.
- d) connect the red wire coming from switch 2 to the last remaining main screw of switch 1.
- e) please see under important notes in regards to the equipment grounding conductor (bare wire).
At switch 2 box…
One cables entering this box coming from switch 1.
- a) connect the black wire coming from switch 1 to the common screw of switch 2.
- b) connect the white wire coming from switch 1 to one of the remaining main screws of switch 2.
- c) connect the red wire coming from switch 1 to the last remaining main screws of switch 2.
- d) please see under important notes in regards to the equipment grounding conductor (bare wire).
Important Notes:
By electrical codes you MUST have at least 6 inches of wire in the electrical box itself, and also the wire must be able to reach at least 3 inches outside the box, it can fail an electrical inspection if the required min. wire length is not met. You may have the wires a bit longer (within reason) but they cannot be shorter. Also see the note on box fill further down in this article.
You are allowed to re-designate a white wire to be used as a hot (ungrounded conductor) in switch circuits but in those cases where a white wire is used in this manner, you must wrap a piece of black electrical tape around that white wire inside the box to signify that is being used as an ungrounded (hot) conductor.
You cannot re-designate a white wire that is actually connected to the light fixture itself. At the fixture itself, the white wire must be the grounded conductor (neutral) coming from the ‘circuit power supply cable’.
In the 3 way configuration depicted on this page, the white wire going from the fixture to switch 1 and the white wire going from switch 1 to switch 2 have been used to carry a switched ungrounded conductor (hot) part of the circuit and therefore as stated should have a piece of black electrical tape wrapped around that wire in the box. The white wire from the power supply cable that goes to the fixture itself at the fixture box location is a grounded conductor (neutral) and therefore is left as white with NO black tape on it.
If the fixture has screw type connectors, the switched ungrounded conductor (hot) is connected to the brass color screw, and the grounded conductor (neutral) is connected to the silver color screw. If it has black and white wires coming from fixture, then just connect black to black and white to white. Use wire nuts of the correct size to join wires together.
The wire cable running between switch 1 and switch 2 has 3 wires in it – black / red / white (plus the bare equipment grounding wire).
What is not shown in the drawings to avoid confusion, is that each wire cable also has a bare equipment grounding wire included. This wire is connected to a equipment grounding screw in each electrical box (if the box is metal), joined either through the grounding screws in the box itself or via a wire nut to the bare wire of the next cable entering / exiting the box, it is also connected to any equipment grounding screw (if there is one) on the switch itself, as well as any grounding screw at the fixture (green wires that attach to the fixture are grounds). Now if using a plastic box, it is made of a material that is non conductive, however some plastic boxes have a metal strip inside that can still be used to connect equipment grounding wires, in the event that it does not then use wire nuts to join the bare equipment grounding wires together. The equipment grounding wire (bare in most cables) must be electrical conductively joined throughout the circuit. Green wires are also equipment grounding conductors.
At the switch itself there are 3 main electrical screws, one of these 3 screws is distinctly different in color (perhaps darker) than the other 2. This screw connection is called the common screw. It is very important that in order for the 2 switches to work as they are intended to that the correct wire is attached to the common screw. Basically as depicted in the drawings one switch has the ungrounded conductor (hot) from the ‘circuit power supply cable’ attached to the common screw, where the other switch has the switched ungrounded conductor (hot) feed to the light fixture itself.
Please also note that there are a variety of manufactures out there that make 3 way switches so it is possible that the switch you buy may have the common screw located in a different spot or side of the switch then depicted in the drawings in this article, just make sure that the wire depicted to go to the common screw of each of the switches is actually connected to the common screw of the 3 way switch you bought. The other 2 wires going between the remaining screws of one switch to the other switch does not matter as long as one wire of the 2 remaining wires go on each of the remaining 2 screws of the screw.
If there is an equipment grounding screw on the 3 way switch it may be green in color and be separated away from the main connections of the switch and likely part of the metal frame that is also part of the mounting structure of the switch.
Extensions to this Switch circuit…
Be Aware that there is a limit by code on how many wires can be in a given size electrical box, when adding more wires to the same box you may have to install a larger and / or deeper box to allow space for more wires coming and out of the box.
Here is an article / explanation in regards to box fill calculations ‘Box Fill Calculation’
The instructions here are based on the fact that the circuit has already been wired to the configuration shown in this article and we are now making the wiring changes for onward power.
From the switch locations….
No onward connections are possible from either of the switch locations because a grounded conductor (neutral) does not exist with this configuration at these locations.
From the fixture location…
Onward ‘always on circuit power’ and ‘switched power’ is possible from the fixture location.
Onward Always On Circuit Power…
(showing bellow the fixture portion of the circuit)
Not shown in the diagram but the onward cable also has a bare wire (equipment grounding conductor) that also must be connected to all the other bare wires at the fixture electrical box.
Connection Instructions for onward cable (always on state)
- a) Using an insulated wire nut, attach / join – the white wire of onward cable + the white wire from the ‘circuit power supply cable’ + white wire connected to the grounded conductor (neutral) connection at fixture.
- b) Using an insulated wire nut, attach / join – the black wire of onward cable + black wire of ‘circuit power supply cable’ + white wire coming from switch 1 (wire cable # 1).
Identification of wires in onward cable….
Black – Ungrounded conductor (hot)
White – Grounded conductor (neutral)
Option 2: Onward switched power…
(example more than one light fixture controlled by the same 3 way switches)
(showing bellow the fixture portion of the circuit)
Not shown in the diagram but the onward cable also has a bare wire (equipment grounding conductor) that also must be connected to all the other bare wires at the fixture electrical box.
Connection Instructions for onward cable (switched)
- a) Using an insulated wire nut, attach / join – the white wire of onward cable + the white wire from the ‘circuit power supply cable’ + white wire connected to the grounded conductor (neutral) connection at fixture.
- b) Using an insulated wire nut, attach / join the black wire of onward cable + the black wire coming from switch 1 (wire cable # 1) + black wire connected to the switched ungrounded conductor (hot) of fixture.
Identification of wires in onward cable….
Black – Switched Ungrounded conductor (hot – switched)
White – Grounded conductor (neutral)
Option 3: Onward ‘Always On’ & Switched Power…
(showing bellow the fixture portion of the circuit)
Not shown in the diagram but the onward cable also has a bare wire (equipment grounding conductor) that also must be connected to all the other bare wires at the fixture electrical box.
Connection Instructions for onward cable (always on & switched)
- a) Using an insulated wire nut, attach / join the white wire of onward cable + the white wire from the ‘circuit power supply cable’ + the white wire connected to the grounded conductor (neutral) of fixture.
- b) Using an insulated wire nut, attach / join the red wire of onward cable + the black wire coming from switch 1 (wire cable # 1) + the black wire connected to the switched ungrounded conductor (hot) of fixture.
- c) Using an insulated wire nut, attach / join – the black wire of onward cable + the black wire of the ‘circuit power supply cable’ + white wire coming from switch 1 (wire cable # 1).
Identification of wires in onward cable….
Black – Ungrounded conductor (hot – always on)
Red – Switched ungrounded conductor (hot – switched)
White – Grounded conductor (neutral)
Foreign Users:
The colors of the wires depicted are based on the standards and code requirements / configuration in the United States and Canada. If you are viewing this site from a foreign country, your wiring standards and color of wires may be different but the principal and sequence of wiring will still remain the same. Therefore the information in this article may still be of use to you regardless of what country you may be in. Just translate the colors used here to the color of wires used in your country in regards to ungrounded (hot) conductors / grounded conductors (neutral) and equipment grounding conductors as well know applicable code requirements in your country.
WHILE EXTREME CARE HAS BEEN IMPLEMENTED IN THE PREPARATION OF THIS SELF-HELP DOCUMENT, THE AUTHOR AND/OR PROVIDERS OF THIS DOCUMENT ASSUMES NO RESPONSIBILITY FOR ERRORS OR OMISSIONS, NOR IS ANY LIABILITY ASSUMED FROM THE USE OF THE INFORMATION, CONTAINED IN THIS DOCUMENT, BY THE AUTHOR and / OR PROVIDER.
By: Donald Kerr