Usually you use the two outmost connection points for +/- 12VDC on the strip ... and usually + and - leads are marked by printed labels on the strips at each seperation mark.
For connection from power supply to the strip you either use the premounted socket (if any is mounted at the end of the strip) or you solder the cables coming from the transformer / DC source onto the connection pads.
With the shopping list of your OP I would do the following wiring
+-------------+ +-------------+ +---------.
~| |+ ---------- +| in out |+ ------------ +|
110/220VAC | Mains/12V | 12VDC | Dimmer | 12VDC PWM | Strip ....
~| converter |- ---------- -| |- ------------ -|
+-------------+ +-------------+ +---------.
The power rating of your ACDC converter is a function of the length of LED strip you want to use. If I am not mistaken - a single tripple of LEDs (the smallest separable unit) takes 60mA ... so all 300 LEDs (100 sections) would require as much a 6000 mA (6A)
The current limit of the dimmer is 12A which is fine for the whole strip.
If you are using more than -say- half of the strip I would consider more than 1 feed point to avoid voltage drops along the strip.
Your ohmmeter testing has established that the switch enclosure is NOT grounded. (or you were hitting it on a painted or rusted spot). That's not a surprise given the vintage of the home. Stop measuring voltages to it... or air. It's futile.
Most likely your house originally had gas lights - that's something to think about if you have a chandelier or ceiling fan, because they often hung those from the gas line. The active, never-disconnected gas line. Isn't old San Francisco housing stock fun?
And you know those 2 wires that were never connected to anything? They're not needed obviously, so please identify them and exclude them from the following test. God only knows where they go to, and energizing power onto them could be a mistake.
If feasible, put an AFCI (Arc Fault) breaker in there. Those prevent house fires. A GFCI (Ground Fault) breaker prevents electrocutions by making sure all current going down a "hot" comes back on the corresponding neutral. Distinguishing the difference is diagnostically useful here.
Time to make a test instrument. Buy a cheapie extension cord. Grab it by the prongs, cut the cord, and throw the prongy part away (or plan your cut and use it for another project.) Strip the cut end of the cord back to expose hot and neutral.
Plug a desk lamp with an Edison base into the socket on the cord. CFLs and LEDs won't work for this - get matched incandescent bulbs. Put one in every lamp under test (remove others), plus one in the desk lamp. Get spares.
Shut the breakers off, wire-nut the leads to any two wires, have a helper turn the breaker on and see what happens. Do the same matrix you did before (leaving out the two mystery wires for now).
- If the test lamp and a fixture both light dimly, you have found a "line" and a "load". This is guaranteed to happen at least twice.
- If they are of unequal brightness, different wattage bulbs, no big.
- If the test lamp lights full brightness, you have found a "line" and a "neutral" or "ground".
- If the lamp lights very brightly, once, you have found two opposing "line" legs of 120V (240V between them). That is special, possibly a MWBC.
- If an overcurrent trips, you did something wrong, or bad wire.
- If a ground fault trips, it means you have found a "line" and a "ground". Or (not likely) a "line" and the neutral from another circuit. Or bad wire.
- If an arc-fault trips, it means the wiring just tried to set your house on fire. There's a remote chance it succeeded - inside-wall fires can take hours to develop - so stay around for a few hours.
That's why I'd start with an arc-fault breaker, especially if you're messing with those two mystery wires. A dual-mode breaker is OK if it can tell you definitely that it tripped for arc-fault and not something else.
Then get back to us and tell us what you find.
Best Answer
Yes, absolutely. In fact it's a good idea to parallel if you have more than about 10 feet of strips - those internal pathways on the strips will cause a lot of voltage drop if they're too long.