Connecting a 4-wire and a 3-wire circuit at a junction box is used to connect two 120-volt circuits to one 240-volt line. This configuration is used most often for large lighting arrays. Using one 240-volt line to power multiple 120-volt circuits reduces the amount of wire needed and the number of circuit breakers needed in the main power panel. A 4-wire circuit will have a red wire, black wire, white wire and a ground wire. The ground wire can be green or bare (without insulation). A 3-wire circuit only has black, white and ground wires.
Pull the main power panel door open. Shut off power to the 4-wire circuit by switching off the breaker that controls that particular circuit. Then close the main power panel door. Be sure to tag the door with a note to let others know that you are working on that circuit. This will keep them from turning the circuit back on while you are working on it.
Go to the 4-wire junction box and remove the junction box cover by unscrewing the two or four screws which hold the access cover in place. Place the tip of the screwdriver on a knockout on the side of the panel and with a small hammer, punch it out creating a new entry point in the junction box. Knockouts are small access holes found in junction boxes which allow you to bring wire into the junction box.
Push a 3-wire cable into the junction box through the knockout hole. This type of wire is commonly referred to as ROMEX® 12/3, 10/3, etc. depending on the size of the wire. The first number indicates the wire size and the second number indicates the number of hot wires and common wires. Ground wires are not included in this number. Strip 1/2-inch of insulation off the end of the black, white and ground wires (if the ground wire has insulation) using the wire stripper. A wire stripper has holes the same size as the wire size.
Select the hole that matches your wire size. For example, stripping a 12-gauge wire requires using the 12-gauge hole. Place 1/2-inch of wire through the hole, tilt the stripper about 45 degrees towards the wire and pull hard on the wire.
Connect the white wire of the 3-wire cable to the white wire of the 4-wire cable using plastic twist caps. Twist caps make connection simple. Place the two stripped 1/2-inch pieces of wire into the twist cap and tighten the cap. Twist caps come in different sizes and each size is a different colour. Check the twist cap information on the twist cap box for the right size cap to use. Twist caps are the most common method used because they are safer than soldered splices which are hard to cover.
Connect the ground wire of the 3-wire cable to the ground wire of the 4-wire cable. Connect the black wire of the 3-wire circuit to either the red or the black wire of the 4-wire circuit. The red and black wires are the"hot" wires. Either wire can be used to power a circuit.
Check all connections to see that all twist caps are secure. Twist caps should not be able to be pulled off easily after they are tightened. Turn on the power at the main power panel.
Electricity is very dangerous! Contact a professional if you do not feel comfortable performing this task.