I am installing an additional potlight to an existing circuit in an older house. I wanted to control the potlight with an existing switch that controls other existing lights. I was able to fish a wire from the new potlight into the switch box controlling the existing lights.
The existing switch box only had one set of wires. I have determined that the existing hot wire is the white wire at the switch. When I connected the wires I could not get the lights working properly. I connected the hot wire to the new light's black wire then connected the two neutrals to the switch poles.
Only the new light comes on when I connected it this way. The existing lights don't work.
How do I wire this properly?
Wiring – How to wire an additional recessed light to an existing switched circuit
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Assuming this is the setup you have described:
You will need to pull a new 12/3 cable to your new 3-way switch. You will also have to replace your existing switch with a 3-way switch. Your wiring setup will look like this:
Because your lighting circuit's power is supplied by a conductor in a 12/3 cable, the figures above were modified as follows:
POWER SOURCE / 2-Wire Romex with Ground (i.e. 12-2) - I modified these to show your 12/3 wire with your RED supplying the power to your lighting circuit (since you described the BLACK in your 12/3 as being spliced into a bunch of other black conductors). From here, everything else should be one-to-one.
You can't have one switch operate light OR fan. Many jurisdictions absolutely require that a switch on the wall near a room entrance, control a lamp. That is to benefit house guests and mainly from the government's perspective, first responders. There's an exception to allow a switched outlet instead, presuming a person is going to plug a floor lamp into that. There is no exception to allow a fan/light locally switched at the fan. (such things can exist, they can't be the only light). That said:
They now make gadgets to solve the "I have 14/2 and wish I had 14/3" problem. Each has a smart switch (typically 1-gang) with 2 controls, and a control module which sits under the fan shroud. The switch communicates with the module either via powerline networking or via wireless, and some of those are compatible with smartphones or integrated home-automation systems as are now emerging in the market.
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Best Answer
There probably is no neutral at the switch box. A switch with "one set of wires" (I am guessing you mean one cable - pictures or a clear description would help me be accurate) is almost certainly a "switch loop." Under newer codes that should be a 3-wire (red-black-white-ground - ground is not counted in naming the cable, so "3-wire") cable to supply a neutral to the switch box, but under older codes a 2-wire black-white-ground cable was often used.
One wire (should be black) will be the hot to the switch. The other (should be red on the 3-wire, or on 2-wire the white wire which SHOULD have red tape or paint applied to it) will be the switched hot, from the switch, back to the first fixture. The neutral for the circuit will be at the fixture in this case.
(Pardon - North American color codes - other parts of the world do have other conventions, involving blue, brown, etc...)
Likely what you need to do is move the new light's wire to one of the other lights, not the switch.
Edit, Add - what you probably are doing is putting the one new light in series with all the other lights in parallel between it and neutral. If you remove the bulbs in all the other lights, odds are good that the new one will go out. If you remove most of them, you'll probably find that the new fixture gets dimmer, and the remaining lights on the circuit start to glow a bit...all the lights should be in parallel (from hot to neutral) but what you have now is hot, the one light, an intermediate connection from the one light to all the other lights, and through those other lights to neutral.