Background
I'm in the middle of a bathroom remodel, and had to take off some drywall to get the tub in, then thought I may as well take all the drywall on that side off to give me an easier taping job. Also I wanted to replace another section of the polybutylene pipe while I had the chance.
The Goal
Then I thought now would be a good opportunity to add a few extra receptacles around our (new) vanity. We bought a 36" vanity, and would like to put one receptacle above the left side of the vanity, and another lower down for charging devices in the vanity (there's no back to the vanity).
The Details
I'd like to tap off the receptacle shown in the upper right. This receptacle is on the same circuit as three others in each of the three other bathrooms in the house, all four protected by one GFCI receptacle in the adjacent bathroom. I'd like to avoid cutting out the drywall around the receptacle itself, and instead cut out a section towards the bottom and tap off there so I don't have to be as concerned about doing a fantastic taping job since it will be hidden by the vanity (the strong glancing light from the vanity light would make the receptacle tape job very difficult I think).
If I tee off the wire towards the bottom of the floor, I believe I'd have to put a box in with a blank plate to make the connection accessible. Since the vanity would be covering it, would this not be acceptable? In that case I could put a plate on the other side of the wall in our bedroom.
Is there anything wrong with my plan?
Best Answer
I see a couple of issues. The first is that your three bathrooms are not wired properly, and you should take this opportunity to fix them! You should not have outlets in three different bathrooms all on one circuit. Each bathroom should have its own 20 amp circuit. The reason for this is that hair dryers, curling irons, etc. use a lot of juice.
Your second issue is going to be the ability to "T" off of an existing cable at some point below the existing outlet. Assuming that the cable actually does run down the wall below that outlet, I would imagine it runs in a fairly straight line without a bunch of slack, right? Unfortunately, in order to "T" off of it, you would have to cut that cable and then have enough slack to pull 6 inches or so from each side of the cut into a new box, where you would join those two ends with your new cable. It's highly unlikely that your wire has a foot of slack in it that you could use for this purpose. If it did have a foot of slack, then you would be better off just pulling it through the actual hole for the new outlet and connecting the two ends (from the top and bottom of the cut) to the screws on your new outlet - no need for a new cable or a secondary hole anywhere.
All that said, here's how I would go about accomplishing what you want to do without cutting any unnecessary drywall: