Wiring a 120-volt circuit off of a 240-volt circuit can be useful where there is primarily 240 volts available. This could be the case at a water well or a workshop. A 240-volt circuit is comprised of two hot wires, a common neutral wire, and a ground wire. It is actually two 120-volt circuits which share the neutral or common wire. If you measure the voltage between either hot wire and the common wire, you will get 110 to 120 volts. If you measure the voltage between the two hot wires, the voltage will be 220 to 240 volts.
Things You'll Need
ROMEX® 10-2 wire
Plastic twist caps
Plastic twist caps
Shut off power to the 240-volt circuit at the main power panel by turning off the circuit breaker for that circuit. Locate the junction box that will be used to supply power to the 120-volt circuit.
Open the junction box and push the ROMEX® 10-2 wire into the junction box through one of the knockout holes on the side of the junction box. The 240 circuit has four wires as follows: black (hot), red (hot), white (common aka neutral), green or bare (ground). A 120-volt circuit is most commonly black, white, and bare. It could also be red, white and bare.
Remove the plastic twist caps from the 240 volt wires in the junction box. Strip 1/2 inch of insulation off the three 120-volt wires. Connect the black wire of the 120-volt circuit to the black wire of the 240-volt circuit. Connect the white wire of the 120-volt circuit to the white wire of the 240-volt circuit. Connect the ground wire of the 120-volt circuit to the ground wire of the 240-volt circuit. Place new plastic twist caps on each of the wire splices.
This article assumes that the 240-volt circuit has a wire size of 10 AWG. If the wire size for your circuit is different, the proposed 120-volt circuit must use the same sized wire.
Use caution when working with electricity. It is very dangerous if mishandled.