One of our wall switches was acting a bit flaky so I replaced it but I was surprised by how the old switch was wired. I was expecting to see a continuous white wire going through the box and two black wires connected to the switch. What I saw was a black and white wire connected to the switch. I wired the new switch the same way and all is well but would like to understand more.
I found this old thread where the issue of the white and black wires was explained: Simple light switch wiring upgrade to new switch – NO NEUTRAL
"The wiring in your wall is called a "traditional switch loop". The two wires present (besides ground) are always-hot (we hope, the white) and switched-hot (we hope, the black). Note that neutral is NOT present in this box.
This is a case of white being used as a hot wire because the cable only has 2 conductors. Modern Code requires a re-tasked white wire be marked with paint or tape to indicate it is not a neutral. "
Okay, but I was wondering what it looked like further down, say at an outlet (which this wall switch actually controls). Assuming there's a black and white wire, is the white there neutral and the black hot? And why use this type of switch wiring instead of just breaking the black line with the switch and continuing it on to the outlet?
Best Answer
The outlet is not "further down". Electrical power is delivered to the outlet first, and the switch is "further down":
Constant-hot power is delivered from the outlet box to the switch, and switched-hot is returned to the outlet via the same cable.
(The NEC specifies that when a cable does not contain a neutral wire, the white wire can be re-marked for another use, and when a constant-hot is in the cable, it must be assigned to the white wire. This leaves the black wire to be re-marked as the switched-hot.)
Following Harper's recommendation, I have illustrated black tape for the constant-hot and red for the switched-hot.
(Switch loops are no longer wired like this, as the widespread use of smart switches has changed the standard so that a neutral connection is now required in every switch box.)