Dear Mr. Electrician: What schematic wiring diagram do I use to wire a light switch?
Answer: There is no set method of wiring lights and switches. The choice of materials and wiring diagrams is usually determined by the electrician who does the electrical work, and by the electrical and building codes in force at the time of construction.
In commercial and industrial construction the wiring methods and materials are sometimes determined by the architects and engineers who designed the project. Below are some of the possible light switch wiring diagrams that could be used. For WiFi smart switches you must adhere to the manufacturer’s wiring instructions. NOTE: Text links below go to applicable products on Amazon.com
A recent update to the National Electrical Code (NFPA 70, 2020) in article 314.27(C) now requires that all ceiling light electrical boxes in habitable rooms at likely fan locations, be rated for ceiling fan support.
CHOICES OF LIGHT SWITCH WIRING DIAGRAMS
The above two conductor switch wiring diagram is from older homes and is likely not used much anymore depending on which code book is in force in each jurisdiction. The two conductor cable from the ceiling box to the light switch is supposed to have the white conductor re-identified with another color because it is not being used as a neutral, but as a hot wire.
Although re-identifying a wire is the code requirement, it has not been put into practice as much as it should. Consequently it is quite common to find a white wire in a switch box that is the hot LINE and the black wire is the switch leg connected to the LOAD. You cannot connect a receptacle outlet to this type of switch wiring because there is no neutral wire in the switch box, but you can at the ceiling electrical box, assuming there is enough room for the extra wires.
It is required by the National Electrical Code (NFPA 70) that a neutral conductor be available in each light switch electrical box. See Article 404.2(C). The neutral is needed for the use of electronic dimmers, timers, and WiFi smart home devices that can be installed instead of an ordinary light switch. This is why the above wiring diagram is not used much anymore. All of article 404 pertains to the installation of switches.
It is very important to follow the electrical code and other building codes for the safety and protection of your home and family.
The photo above depicts the wiring diagram of a ceiling light and light switch with the power from the circuit breaker panel entering the ceiling electrical box. From the ceiling a three conductor cable with a grounding conductor is used to send power to a light switch. The grounding conductor is not shown in order to simplify the wiring diagram.
It is easy to supply power to more than one light fixture as the wiring diagram above shows. You just install a cable or conduit from a ceiling box to the next ceiling box. The main thing to watch out for is having too many wires in one electrical box. There are code mandated limitations as to the number of wires that can be in an electrical box as well as physical constraints.
Generally speaking each individual #14 wire requires two cubic inches of space inside of an electrical box. A #12 wire requires 2.25 cubic inches. A switch counts as two wires. In addition deductions must be made for connectors, clamps, and studs inside of the electrical box. Read Article 314 in the National Electrical Code (NFPA 70).
If you wanted to control the two lights separately from two different switches in the same box, a three conductor cable with ground would need to be installed to the first light instead of a two conductor cable. At the first light you connect the black and the white wires. The red continues to the next light along with a white.
Of course you could also install two separate two conductor cables to each light from the switches.
Multiple receptacle outlets can be connected with lighting outlets as depicted in the above light switch wiring diagram. Duplex receptacle outlets are made for feed through of the power from one receptacle to the next. The above wiring circuit was made using only a two conductor cable with ground. The grounding conductor is not shown in order to simplify the diagram.
If a ceiling fan is going to be mounted instead of a light fixture, a ceiling fan rated electrical box would need to be installed. In addition I would install a three conductor cable with ground from the switch to the ceiling box. This would allow for the option of having two switches, one for the fan, and one for the light kit on the fan. You can read my post about installing an old work ceiling fan box.
Typically the grounding conductors would be joined together in each switch and receptacle box and a pigtail would be connected as well to supply the ground connection on the green screw of each duplex receptacle and single pole switch.
If the electrical boxes are metal, then they must also be grounded using an additional grounding pigtail that gets connected using a 10/32 machine screw to a tapped hole in the back of the metal electrical box.
Sheet metal, Tek screws, and wood screws are not permitted for ground connections. See Part VII beginning with Article 250.130 in the National Electrical Code (NFPA 70).
The diagram above shows a two conductor cable from the circuit breaker panel going to a wall switch. From the switch a three conductor cable continues to a ceiling light. From the ceiling electrical box a receptacle outlet is fed power using a two conductor cable with ground. The grounding conductor is not shown in order to keep the diagram simple.
When I rough-in the wiring for ceiling lights I usually install the power from the circuit breaker panel to the wall switch and only install one cable to the ceiling light electrical box or ceiling fan box. I tend to avoid feeding other things from the ceiling box so that the box can be moved easier if the homeowner decides to move the light location before construction is finished or many years after the construction work is done.
Having the neutral conductor in the switch box makes it handy to send power to other things such as receptacles as shown in the above diagram. It could also be used to supply power to another switch that controls lights in another part of the house. A three conductor cable must be used from the light to the switch.
Although it is permitted to wire lights and receptacles on the same circuit I usually avoid doing that for the sake of the people living in the home. If by chance someone plugs an electric heater, or something else into a receptacle outlet and it overloads the circuit, the circuit breaker will trip off. With lights and receptacles connected together in this scenario the room will have no lights until the problem is resolved.
If the lights are wired separately from the receptacles, they will remain on while the problem is fixed. If the lights and receptacles are wired with a multi-wire circuit than it doesn’t matter as both circuits, if wired correctly on the two pole circuit breakers, will shut off anyway.
WIFI SMART SWITCHS FOR ALEXA, GOOGLE DOT, AND APPLE HOMEKIT
Electronic smart switches get wired a little differently than standard wall switches. For one thing the LINE and LOAD connection must be done correctly. With a standard single pole wall switch the LINE and LOAD can switch terminals. The smart switches also require a neutral conductor to operate.
If there is no neutral wire in the switch box, such as in the diagram at the top of this page, you likely cannot use a smart switch. Though there may be a few types of smart switches available that are designed to operate without a neutral connection, the type of functions that they offer may be limited. Research before purchasing any smart switch or lighting dimmer.
The requirement for a neutral in a switch box has only been introduced into the National Electrical Code a few years ago. Prior to that the person that wired your home originally would install the wiring in a manner that was suitable for him or her.
You cannot use the grounding conductor to connect the smart switch neutral wire to. This can create an unsafe condition throughout the home and may cause problems with electrical appliances. It is forbidden by article 404.22 in the National Electrical Code.
Before purchasing a smart switch or lighting dimmer, check the manufacturer’s installation instructions and specifications. In the United States standard residential lighting circuits are 120 volts at 60 hertz (HZ). The switch must be compatible to that. Some of the smart switches that I have seen have a voltage range from 120 to as high as 277 volts, which is for commercial lighting.
To replace a wall switch with a smart switch first turn the power off using the circuit breaker for that circuit. Remove the wall plate covering the existing switch by unscrewing the two screws. Remove the switch by unscrewing the screw at the top and the screw at the bottom. Gently pull the switch away from the wall. Sometimes the wires are too short to pull the switch far.
If you see a white wire on the existing switch, that is not a neutral. It is either a LINE or a LOAD wire. It is a common wiring practice when wiring with only a two conductor cable to use the white as LINE and the black as LOAD. As I explained above the white is supposed to be re-identified with another color, but is often neglected.
You must identify the LINE and the LOAD wires inside of the existing switch box before installing the smart switch. You cannot assume by color code which wire is the LINE and the LOAD wire. Check that the power is off using a non-contact voltage detector.
Confirm that the power is off using a category III (CAT III) or category IV (CAT IV) volt meter or you can use a pigtail light socket with a light bulb.
Remove the two wires from the screw terminals on the side or the push-in connections in the back of the existing single pole switch. You can use a tiny screwdriver or a paper clip to remove the pushed in wires. Do not cut the wires. The wires in the switch box are all that you have to work with.
Now pull out the white wires that are inside of the switch box, but do not take them apart. Remove the wire connector on the white wires so that the bare wire is exposed. Keep the joined white wires separated from the wires off of the switch. Turn the power back on and wearing electrically insulated gloves with glove protectors, and a face shield CAREFULLY (You can get shocked or electrocuted) check with a volt meter or pigtail light bulb to see which wire is the LINE.
Put one of your volt meter leads (Or pigtail light bulb) on the white wires and touch the other lead to one of the wires that were removed from the switch. Then check the other wire that was removed from the switch. The switch wire that shows approximately 120 volts with the white wires is the LINE. The other wire with no voltage is the LOAD. Ignore voltage readings that are much lower than 120 volts. Turn the power off again.
Once you have identified the LINE, LOAD, and NEUTRAL wires inside of the existing wall switch electrical box and the power is off you can proceed to connect the smart switch following the manufacturers instructions.
I have noticed that some smart switch wiring diagrams from the manufacturer have the equipment grounding conductor colored yellow. Standard electrical code wiring practices in the United States require the grounding conductor to be green or a non-insulated bare wire. Other countries may have the ground wire as yellow with a green stripe, or green with a yellow stripe.
In your switch box you should have one or more grounding conductors spliced together with a wire connector or wire crimp. The ground wire from the smart switch needs to be connected to that. If the wires are crimped and there is no pigtail to connect to, you will need to remove the crimp. I do this by carefully cutting the crimp parallel to the wires using my Knipex high leverage diagonal pliers. Try not to cut the wires.
With the ground wires now separated from the crimp, twist the ends together tightly using pliers. Tightly twist the ground wire from the smart switch onto this group of ground wires. Twist an appropriately sized wire connector onto the end.
In some cases the grounding path is established through the metal armor of the cable or metal conduit. In those instances the metal box will be grounded and you can connect a grounding pigtail to the back of the metal switch box using a 10/32 machine screw. Some very old black enamel metal switch boxes use 10/24 machine screws. Remove all unused clamps from inside of the box.
The smart switches are generally compatible with wall plates suitable for Decora switches though the colors may not be precisely the same.
For three way switch wiring diagrams go to my post.
Four way switch wiring diagrams are posted here.
For switched and half switched outlet wiring diagrams see my post here.