Add a Switch and Connecting the Wires
How Can I Add a Light Switch to Existing Circuits? Adding Light Fixtures and a Light Switch to Existing Circuit Wiring, The Most Common Mistakes When Adding an Additional Light Switch and How to Fix Them.
Adding a Light Switch
Electrical Question: How Can I Add a Light Switch to Existing Circuits?
I have a switch box with two light switches. Each switch controls separate light fixtures, however one switch apparently draws power from the other switch.
I have tried to add an additional switch to one of the existing switches. Here is the wiring that I have to work with:
- There are five lines into the original box.
- I have a new 3-wire cable from the new switch.
- There is the existing 3-wire cable to one switch.
- Three 2-wire lines from a light, which may be the power source.
The new line is attached to a three way switch in the original box where there is a red and white wire to the switch, and black to the white from the 2-wire line from the appropriate light source. The black from that line is attached to the three way switch and power from the other switch. The other 3-wire cable is hooked up similarly where the red and white connect to the switch, and the black to the black of one of the light source (or power) 2-wire cables. The two whites of the two 2-wire lines from the light source are connected, and the final black from the 2-wire lines from the light sources (or power) is connected to the second three way switch.
The downstairs or other light source works but the new three way and its companion in the old box do not light the upstairs light source as intended.
Can you offer any direction? I have tried a number of different connections and either get a flipped breaker or still just the two light sources and not the third which is the one I wish to add a switch to.
Background: Rolly, a Homeowner from Indianapolis, Indiana.
Thanks for your electrical wiring question Rolly.
Connecting the Wires for a Switch
Application: Wiring Additional 120Volt Light Fixtures and Light Switches.
Skill Level: Intermediate to Advanced – Best installed by a Licensed Electrician.
Tools Required: Basic Electricians Pouch Hand Tools and Voltage Tester.
Estimated Time: Depends on personal level experience and ability to work with tools.
Precaution: Identify the existing circuit, turn it OFF and Tag it with a Note before working with the wiring.
Notice: Installing additional light switch wiring should be done according to local and national electrical codes, with a permit and be inspected.
Materials: Make sure light switches have the same amperage and voltage rating as the existing light circuit, and the switch is fully compatible with the light fixture and the type of light bulbs that will be used.
Adding a Light Switch or Light Fixtures to Existing Circuit Wiring
It is important to define your project to fully understand the wiring requirements and connections. For example, this electrical question does not describe if an additional switch will be used for existing lights, or if the new switch will control a new light fixture. However, I will discuss both the basic requirements for adding a light switch and adding light fixtures to existing circuit wiring.
Adding a Light Switch
Electric Wires Required when Adding a Light Switch
- The circuit power source containing a neutral wire, the power wire, and a ground wire. NOTE: Electrical codes now require a neutral wire to be located in light switch boxes.
- A circuit cable to the light fixture.
- NOTE: If the new switch will be used as a second switch to control an existing light fixture then two three way switches will be required along with additional wiring. Typically a 3-wire cable is installed between the two three way switches for this type of control, however the specific project will need to be considered before the three way wiring method can be determined.
Adding Light Fixtures and a Light Switch
Electric Circuit Requirements when Adding a Light Switch and Light Fixtures
- The circuit power source containing a neutral wire, the power wire, and a ground wire is required for 120volt light fixtures, or low voltage light fixtures that require a 120volt power source.
- Determine the proposed electrical amperage or watt load required for all of the additional light fixtures to be installed.
- Identify and test the existing circuit load to make sure it has the capacity for the proposed light fixture(s). If the combined load exceeds 80% of the existing circuit load capacity then an additional circuit will be required.
Basic Circuit Wiring Commonly Found in an Existing Switch Box
- The incoming circuit power source.
- Outgoing circuit power to outlets or switches on the same branch circuit.
- A 2-wire cable used for switching circuit power to a light fixture.
- A cable to each light fixture.
- A 3-wire cable to a three way or four way light switch when two or more switches have been installed.
Troubleshooting Light Switch Wiring Problems
Wiring an additional switch incorrectly can cause a variety of results and possible circuit problems. Many of these problems occur when the existing circuit wiring is not fully understood or identified resulting in incorrect wire connections to the switch or light fixture circuit cable. The following will explain problems and solutions for many common switch wiring problems.
The Most Common Mistakes When Adding a Light Switch and How to Fix Them
Problem: The Circuit Breaker Trips Off when the Power is Turned On or the Light Switch is Turned On.
- Possible Cause: A Neutral Wire has been connected to a cable that is used for Switching Circuit Power to a light fixture.
- Solution: Test the existing white wire(s) of the cables to make sure they are actually a Neutral Wire. NOTE: It has been a common practice where a 2-wire cable has been installed for looping the circuit power through a light switch, therefore the white wire is in reality a wire used for power. With this type of wiring method the white wire is not a neutral wire. In many older homes this method of wiring did not include marking the white wire with black electrical tape to indicate that it is not a neutral wire, therefore this can cause confusion. Electrical Code Compliance: The electrical code now requires marking white wires used for switching applications.
Problem: The New Light Switch and Light Fixture Only Work When Another Switch is On.
- Possible Cause: The power source for the new switch is connected to a wire that is connected to an existing light switch.
- Solution: Identify the primary circuit power source. Most likely, the new switch is wired to the secondary side of an existing light switch, therefore this problem can be solved by splicing the primary wire for the new switch to the circuit power source.
Problem: The New Light Switch and Light Fixture Works When Other Light Switches are Turned On or Off.
- Possible Cause: The power source for the new switch is connected to a wire that is used for three way or four way light switches.
- Solution: Identify the circuit power source. Most likely, the new switch is wired to the wires that travel between two three way or four way switches, therefore this problem can be solved by splicing the primary wire for the new switch to the circuit power source. NOTE: When three way and four way switches are installed the circuit power containing the circuit wiring necessary for a new light fixture is typically found at either one switch box, or at one of the light fixtures due to several wiring configurations that can be used for three way and four way switch applications.
Problem: The New Light Switch and Light Fixture Does Not Work At All.
- Possible Cause: The power source for the new switch is connected to a wire that is not a circuit power source.
- Solution: Identify the wiring to see if there is circuit power available. NOTE: The full circuit power source containing all of the required wires for a light fixture may not be located at the switch box or junction box where the new wiring has been installed. The best practice is to positively identify the required circuit power before installing additional wiring.
Conclusion and Summary
The Key to Light Switch Wiring is to identify all the circuit wires involved and understand what their function is within the circuit and light fixture switching process.
I have provided the following resources that lead to fully detailed information that will further assist you with wiring light switches.
More about Light Switch Wiring
Light Switch Wiring
Wiring Diagrams for Light Switches
Fully explained pictures and wiring diagrams about wiring light switches describing the most common switches starting with photo diagram 1.
This link is helpful as a Homeowner
Home Wiring Diagrams
Consult your Local Building Department about Permits and Inspections for all Electric Wiring Projects.
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