Dear Mr. Electrician: What is the correct light switch wiring diagram I should use to connect ceiling lights?
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. NOTE: Text links below go to applicable products on Amazon.com
Choices of Light Switch Wiring Diagrams
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 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). This is for the use of electronic dimmers, timers, and smart home devices that can be installed instead of an ordinary light switch.
The two conductor wiring diagram at the top of this page 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 that 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 hot and the black wire is the switch leg connected to the LOAD. You cannot connect a receptacle outlet to this switch wiring, but you can at the ceiling electrical box, assuming there is enough room for the extra wires.
It is very important to follow the electrical code and other building codes for the safety and protection of your home and family.
It is easy to 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. In addition deductions must be made for connectors, clamps, and studs inside of the electrical box. See 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.
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 circuit breakers, will shut off anyway.
For three way switch wiring diagrams go to my post.
Four way switch wiring diagrams are posted here.