Light Switch Wires

[petertay15]

Question about wiring on 120V light switch. This is an existing light switch in a recently rewired home. When I pull the plate off, what I see is all I know: two-pole, not 3-way. Seems to be on a dedicated circuit with just one other light.

Here’s what I’m wondering about: The black wire on the switch is always hot of course. The white wire has a bit of black tape wrapped on it. Is this some sort of electrician code I should understand?

Another question: When the switch is off, there is 120 volts between the two screws (meaning between black and white). When the light is switched on, there NO voltage between the screws. How does turning the switch on stop voltage between black and white screws?

[TDKPE]

Question about wiring on 120V light switch.  This is an existing light switch in a recently rewired home.  When I pull the plate off, what I see is all I know: two-pole, not 3-way. Seems to be on a dedicated circuit with just one other light.

Here’s what I’m wondering about: The black wire on the switch is always hot of course. The white wire has a bit of black tape wrapped on it.  Is this some sort of electrician code I should understand?

Another question: When the switch is off, there is 120 volts between the two screws (meaning between black and white). When the light is switched on, there NO voltage between the screws. How does turning the switch on stop voltage between black and white screws?

Actually, since this is a switch leg (unless there are other wires in there), the white is supposed to be always hot, and the black is the switched conductor.  That's counter to just about everything else you know about insulation color, but that's what it is.  Or was, since switch legs are no longer allowed.  

But regardless, marking a white conductor with any color other than green indicates that it's ungrounded ("hot" in the vernacular), and is code required in most instances.  Except for switch legs (only), in days gone by, when you didn't actually have to mark the white conductor.  
  When looking in the box where the first light fixture using that switch leg is, it's obvious that the white is joined with one or more black, so it's obvious that it's hot. Or so the reasoning went, back-when.

The reason you measure 120V across the switch is because the meter you're using is completing the circuit when the switch is open, albeit with an extremely high impedance.  Only a tiny current is flowing through it, and through the light bulb(s).  If you unscrew that bulb, you'll read 0V. With the bulb(s) there, you're measuring between the hot coming from the breaker, through the bulb(s) to the neutral bar in the panel. No different than sticking your probes on the hot and neutral bar directly; it's just a longer trip around.*

The switch itself has an extremely low impedance when closed, so measuring across the switch is measuring the voltage difference between the two lugs, which is far too small for your multimeter to register.  You're measuring the difference between 120.000000V at one lug to 119.999999V at the other (made up numbers, of course), with the difference coming from the impedance through the switch contacts.

*And do make sure the meter is set to "voltage", as if it's set for "current", you'll wipe out an expensive 11A fuse, and maybe the meter, too.

[petertay15]

Thank you for the extensive reply. That voltage is passing through the light bulb is helpful to know.
What do you mean by “switch leg”?
But let me tell you what I’m trying to do. The hallway outside the bathroom is dark, so I want to add a ceiling light and new switch for it. The closest source of power for the new light would be the bathroom light switch. Can I tap onto the hot black wire at the bathroom switch as a source of power? That would be the hot for both the existing switch and new switch. Neither of the whites would be connected to a switch, just twisted to the white from the source. Make sense?

[MstrCarpenter]

Tom was explaining why you saw voltage across the lugs. A "switch leg" was/is used when power is fed to the fixture box. Assume for a moment the fixture has a single pole switch on it, The neutral (white) is connected directly to the bulb (lamp "base" or "socket"). The black goes to the switch, then from the switch to the bulb. The "switch leg" is simply an extension of those two switch wires as a means to locate the switch away from the fixture (remember your white was marked as black); If there are only those two wires in the switch box there is no neutral (white) wire. Sorry but you'll need to find another source of power. If the bath light is a ceiling light, you might be able to get power from there to a ceiling light in the hall between the joists or strapping. There are wireless switches available, and if you decide to hard-wire a switch, you'll need to have both hot and neutral in the switch box to meet current code.

[srv52761]

May look like this (minus the bare equipment ground conductor), although they may have switched the black and white wires on the switch leg. You can check that when you open the light junction box.
If this is the case, as MstrCarpenter explained, your most likely source is the light junction box.

   

[Robert Adams]

Good picture there of it. The reason it isn't allowed anymore is because dimmers and timers and wifi switches require a neutral at the switch. Even if that weren't the case it's best practice to run your homerun to the switch.

[petertay15]

Thank you for the help, the explanations, and illustration. I have now run a new line up from the basement, so all is well.
—Peter

[carwashguy]

The reason it isn't allowed anymore is because dimmers and timers and wifi switches require a neutral at the switch. Even if that weren't the case it's best practice to run your homerun to the switch.

This thread probably saved me from failing my rough in inspection next week. Thank You!

[Bob10]

Good picture there of it. The reason it isn't allowed anymore is because dimmers and timers and wifi switches require a neutral at the switch. Even if that weren't the case it's best practice to run your homerun to the switch.

Funny you say that when I rewired my house probably 17 years ago there was tech called RadioRa or something similar that made every plug or switch a device that could be shut off remotely.  I thought it was something I wanted and wired accordingly yet by the time the job was done that new tech was out dated and I gave up on the idea for a while.  Today I can wire in any of the wifi switches being offered up for a fraction of what the old tech cost and with more features

[MarkSLSmith]

I'm still a bit unclear about how the switch should be wired according to today's code.  Is it possible to provide a diagram?

I don't have any immediate need, but I do learn a lot from these discussions.

Mark