Connecting a four-wire electrical line to a two-wire electrical line can be tricky without having some additional information. Depending on the voltage needed on the two-wire line, the wiring between the two wires will be different. A four-wire electrical line has a red hot wire, a black hot wire, a white common wire and a ground wire. A two-wire line is usually a black hot wire and a white common wire. The ground may be there, but it may not be connected.
Take the ends of both the 12-2 ROMEX® wire and the 12-3 ROMEX® wire and strip 1/2 inch of insulation off each wire. Push both ends into an electrical junction box. All electrical splices must be made inside a junction box per the National Electric Code.
Look at the four wires of the 12-3 cable. There are two ways to connect a 12-2 wire to this cable. The first option is to use the black wire of the 12-3 cable as the hot wire. Connect the black wires of both cables together, and connect the common white wires together. Cap the red wire and leave it untouched in the junction box. Place plastic twist caps on all splices. The ground wire may or may not be connected.
Alternatively, use the red wire as the hot wire. Connect the red wire of the 12-3 cable to the black wire of the 12-2 cable. Splice together the two white wires. Cap the black wire of the 12-3 cable and leave it untouched in the junction box.