Depending on where you want to add an electrical outlet can also determine the difficulty of the project. This is where a little research can possibly make the job a little easier to complete. If it is on an inside wall instead of an exterior wall, take a look on the opposite side of the wall to see if there is an existing receptacle somewhere on that side of the wall. If so, it is best to locate the new one between the same two wall studs. You will need a cut in or old work electrical box.
Grab a tape measure and get the distance from a door or the corner over to the center of the existing receptacle and then measure on the other side from the same point to see where the new electrical outlet would be. The existing outlet box will most likely be nailed to a wall stud and it is important to know which side of the box that the stud is on. You can find out with a stud finder or by removing the outlet cover and push a wire or a screwdriver beside the box and the edge of the sheetrock opening.
If you were standing and facing the existing outlet and the stud is on the left of the box then on the other side where your new outlet will be then that stud will be on the right of the measurement that you made earlier so you will need to move over to the left a few inches to cut your new box in the wall.
When you have clear area for the new electrical outlet just hold your old work box on the sheetrock and mark the perimeter with a pencil and saw the hole for the box. Kill the power to the existing outlet and push the 12-2 electrical wire through the knockout hole in the existing box and reach in the new hole and grab the wire. Stick the wire in the box and install the electrical box in the wall.
Strip the black and white back about 3/4" or so and put a curl in the end so you can slip the curl under the side lugs of the electrical outlet. White goes to the silver screw and black goes to brass screw. Tighten securely. Bare wire goes to the green screw which is the equipment ground. Same goes for the existing outlet and your new romex. Fasten the outlets into the boxes, install the outlet cover and turn the breaker back on.
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