Piggybacking a new socket off of an existing socket is called wiring an socket in series. Doing this will allow you to not only wire the new socket to the same circuit but also avoid having to run a separate electrical line back to the service panel. Simply running a length of ROMEX® 12-2 cable from the new socket over to the existing socket and wiring it together will give you and additional power socket and avoid the need for extension cords.
Turn off the power to the existing socket that will have the new socket connected to it. The power should be turned off at the main service panel by locating the labelled breaker for the socket. Turn the breaker to the labelled off position.
Remove the screws holding the cover plate over the outlet, using the screwdriver. Set the cover plate and screws aside.
Remove the screws holding the existing socket in its junction box. Pull the socket out of the box, but leave the wiring attached. Punch out one of the knockout holes in the back of the junction box using the screwdriver.
Measure up from the floor 45 cm (18 inches) in the desired location for the new socket, and mark the measurement on the wall. Use the stud finder to locate any potential studs behind the wall. If there is a stud behind your mark on the wall, then move to the left or right 5 to 7.5 cm (2 or 3 inches) to clear the stud.
Hold the top of the new junction box up to the mark and trace its shape on the wall. Cut out a hole along the trace, just large enough for the junction box to fit into the wall. Punch out one of the knockout holes in the back of the junction box.
Drill a 1.8 cm (3/4 inch) hole (from inside the attic) into the top plate on the wall that is directly over the location where you will be installing the new socket. Drill a similar hole in the top plate of the wall directly over the existing socket.
Feed the fish tape down the hole and into the wall and out the hole for the new socket. Have an assistant attach one end of the ROMEX® cable to the fish tape, using the electrical tape. Pull the cable up through the hole as the assistant feeds it to you. Pull the cable over and feed it down through the second hole, over the existing socket, and have the assistant pull it out the junction box as you feed it down. Leave about 20 cm (8 inches) of cable extending out each socket location and cut away any excess cable using the cutters.
Strip back 7.5 cm (3 inches) of outer sheathing from one end of the cable. Inside the cable, strip back 2.5 cm (1 inch) of insulation off the two insulated wires (black and white). Repeat this step to remove the insulation off the other end of the ROMEX® cable.
Install the new junction box by running the end of the ROMEX® cable through the knockout hole in the back and out the front of the box. Slide the box up to the hole in the wall and insert it in place. Tighten down the screws to secure it to the wall.
Attach the new outlet to the wiring. Attach the black wire to the brass-coloured screw, the white wire to the silver-coloured screw and the bare wire to the green-coloured screw. Tighten down the screws on each wire. Push the wiring into the box, and insert a screw into the top and bottom holes on the socket and screw them into the matching holes on the junction box. Place the cover plate over the socket and attach it, tightening down the screws with the screwdriver.
Attach the new wiring to the bottom set of screws on the existing socket. Attach the black wire to the brass screw, the white wire to the silver screw and the bare wire to the green screw. Push the wiring into the box, and insert a screw into the top and bottom holes on the outlet and screw them into the matching holes on the junction box. Replace the cover plate over the socket and attach it using the screws and screwdriver to tighten them down.
Turn the power back on at the main circuit panel, plug a lamp into the new socket, and test for power.
It may be helpful to have a couple of assistants feed the wire up one hole as you feed the other end of the cable down the second hole to the other assistant. This will help prevent the cable from becoming kinked or too taut to pull.
Never work on or connect any electrical wiring before turning off the power at the main power source. Electricity can be very dangerous and even fatal in some instances if left on during a project.