I’m working on updating the switch to an existing run. It’s a two way switch connected to an outside light. My wife took the receptacle apart and I’m left with the wiring. I see the 12/2 wire as expected but the previous installer apparently marked with a permanent marker in black one side of the white wire. I’m not sure what to make of this since it’s a two way switch. Any guidance would be appreciated.
Is there only 1 cable in the box? A cable is several wires wrapped in a sheath. Are you trying to replace a plain switch, with, a switch with a power socket?– Harper - Reinstate MonicaMar 28 at 23:54
It’s a single 12/2 wire with the expected white black and ground wires. Im just tasked to add a standard two way light switch to the wiring that exists in the wall. The light is already wired outside for it.– Zach SmithMar 29 at 13:30
Someone took the time to do it right.
On an older switch loop, white wire was used to carry power, making it live, since switches did not need a neutral wire to work. Today's smart switches usually do need the neutral wire.
The marked white wire should be connected to the black power wire at the light, to carry power to the switch.
The black wire at the switch then carries power back to the light and is connected to the light's black wire.
More recent code requires you to use 12/3(or 14/3 on 15 amp breaker circuits) from the light to the switch so you have an unused neural(white) wire in the switch box, with red and black wires for power.
Add picture to your question showing the wires at the switch to be sure. I have made a few assumption with the info you provided.
OK, that addresses a point I found confusing... My unjustified expectation would have been that hot and switched-hot would have used the pair the other way around. Good to have that corrected.– keshlamMar 29 at 2:39
1The comments on the other answer explained this much better, but by code(if people followed code) the marked(or unmarked) white should be used as the hot so people checking will find it has power and is not being used for neutral.– crip659Mar 29 at 9:42
Ah. That's actually a good argument. Tnx!– keshlamMar 29 at 12:20
@crip659 photo added! Mar 29 at 13:27
@ZachSmith My assumption was correct. That is your old style switch loop. Unless you are replacing with a smart switch, it is just a simple switch replacement. The white marked black should be your always hot with the black as switched hot. This is the way it should be, but people do use black as always hot. Checking at the light or with a meter will make sure(or the switch works backward).– crip659Mar 29 at 13:43