Things You'll Need
Safety glasses or goggles
When a new receptacle, outlet or switch needs to be added to a room, the easiest source for providing electricity to the new outlet box is to tap into an existing junction box. This is a task that is easily accomplished at minimal expense, as long as safety precautions are taken.
Put on the safety glasses or goggles. Turn off the power to the junction box that will be worked on by flipping the breaker switch to the "Off" position. Unscrew the screws holding the cover plate on the junction box and remove the plate. Carefully pull the existing wires out of the box. To ensure the power is off, place the probes of the circuit tester into the wire nuts so they are touching the wires inside. If the tester indicates that the power is off, it is safe to proceed.
Remove a knockout hole in the box by sliding the screwdriver between the knockout cover and the box. Twist the screwdriver enough so that the knockout can be gripped with a pliers. Twist the pliers until the knockout comes off.
Unscrew the lock-nut on the cable connector and slid the connector over the end of the new cable. Pull the cable through the knockout opening, leaving 8 inches of cable sticking out of the box. Attach the lock-nut to the connector and tighten the screws on the connector.
Remove the outer sheathing on the cable with the cable ripper. Remove 3/4 inch of insulation from the ends of the black and white wires with the wire strippers. Untwist the wire nuts from the existing wires. With the lineman pliers, twist the new wires to the existing ones and screw the wire nuts back in place. If larger wire nuts are needed, based on recommendations on the side of the wire nut package, replace the old nuts. Carefully push the wires back into the box and screw the cover back on.