Electrical – how to t-type splice power wires to add a new outlets

Something has to be different than what you say for this scenario to occur in a house that's wired in a manner even approaching code compliance.

The voltage drop on the outlet while the fan is on is normal. The fan is simply using some of the power of the circuit, decreasing the remaining potential elsewhere in the circuit.

You say that the voltage changes at the outlet when the fan is on, but the outlet does not work. This means you are getting voltage at the outlet even when the fan is on. Are you reading anything close to 120V? (anything between 110 and 125v is "normal" household voltage in the U.S.). Do you get significantly more voltage when the fan is switched off? If this is true, my hunch is that the branched connection at the J-box served by the panel is not as continuous as it should be.

With the circuit deactivated at the panel, take apart that junction (hot and neutral), make sure the wires all have about 3/4" of insulation stripped, the wires are all twisted together (you can't rely on the wire nut to secure an extra wire to an existing twisted wire junction), and the wire nut is securely screwed on to the twisted bundle. Tape around the entry to the wire nut for both the hot (black) and neutral (white) wires, then before you put everything back, turn the circuit on and try your multimeter again with the fan off and on.

If it's still cutting out, make sure all the connections in the switch and in the ceiling are kosher in much the same way.

Just a shot in the dark; are you sure the switch isn't a three-way? If it is, then the two terminals at the bottom of the switch are not continuous; you can't hook on to either of them with your new hot wire or with the wire from the panel to the J-box you're hooking into. You should instead hook into the top (Common) terminal (it will be the only one on that end of the switch body), which should be fed from the panel and will thus take the switch out of the circuit path to your outlet. If making that change means that something else is now always on, then you need to find or make a different branch of the circuit for your outlet that is fed directly from the panel.

As noted, this is not an entry level project. (And drywall is the enemy, but fear not.)

Safety notes: turn off circuits that you're messing with. Have a tester that tells you if a wire is hot. Don't assume that wire colors mean anything. If you think there's a chance that you'll hit plumbing/ gas/ other wires/ etc when drilling, then open up the drywall and fix it later. Read up on wiring best practices and safety.

My approach:

• pick your cans (size will be important). Bigger diameter will probably be easier to work with. Smaller height is probably better.
• lay out where you want the cans to ideally be
• make holes at each location that are big enough to get your hand into (but smaller than the eventual can). After the first hole, you'll know if there's anything really wrong with the cans you've purchased.
• feel around and see what obstructions are in your way. Adjust can locations accordingly.
• cut can holes to full size

• run wire between cans. Try to make small holes (3/4") in the middle of your joists. Open drywall as needed to enable drilling and passing wire. Watch out for other wires/plumbing/gas lines/etc while drilling.

• now, you need to get power to one point on the string of cans. Lots of ways to skin the proverbial cat, but if you're lucky, you found a permanently hot wire in the ceiling that is on a circuit that can bear the additional load of lighting. (LEDs are good here, as they draw very little.) If you pick up power from an existing source, the junction box has to be accessible.

• if you weren't lucky with that, you'll have to bring power from an outlet to a switch. Tear out the existing outlet box (without damaging any wires -- often easier said than done) and fish wire from the outlet to your new switch location. A long drill bit through the holes you're cutting for boxes will get you through the first stud, but you'll be making holes in the drywall to get past studs.

• now you need to get wire from the switch to the lights. Construction varies, but getting wire from the wall cavity into the ceiling cavity will almost certainly involve cutting away drywall at the top of the wall and the edge of the ceiling. With those holes, you can see what you're dealing with.

• if you want two switches controlling the same set of lights, run 3 conductor wire between the switches. Reference books will tell you how to hook up those wires.

• when patching drywall, less is more. If you put too much mud on, you'll have to sand it off and clean it up later.

• I'm sure I've missed something, but I hope this gives you a sense of what you're considering.