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Light switches

jyang949's picture

Attached are instructions for a device to control two light switches. The wiring diagram and instructions look mismatched to me--or is it just me being unfamiliar with electrical stuff?

Janet

Yep, they've got "common (post #206948, reply #1 of 10)

Yep, they've got "common feed" and "separate feed" crossed.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Thanks, Dan, you saved me (post #206948, reply #2 of 10)

Thanks, Dan, you saved me some frustration. 

Janet

Mystery wire (post #206948, reply #3 of 10)

Any idea why the white wire isn't connected to anything? I thought it was an essential part of a circuit, but the lights work without it. 

Janet

There are several different (post #206948, reply #4 of 10)

There are several different ways to wire a light switch. 

The "classical" way is to put the switch in-line in the cable from breaker to light.  In that case the white wire just runs straight through the switch box, and the switch is connected to two blacks, one from the breaker and one going to the light.

The other common way is a "switch leg".  In that case the cable runs from breaker to light, where a second cable runs off to the switch.  Ideally the cable from light to switch is black/red instead of black/white, but sometimes a black/white cable is used.  In this case the black from the breaker is tied to the white wire of the switch leg cable (the only case where black and white should ever be tied together), the white from the breaker goes to the "shell" of the light, the black from the switch leg goes to the center contact of the light.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

Isn't the second example no (post #206948, reply #5 of 10)

Isn't the second example no longer allowed because the common would no longer be in the switch box?

Yeah, I think for new work (post #206948, reply #7 of 10)

Yeah, I think for new work you're supposed to put in a 3-wire cable, then cap off the white in the switch box.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

The switches lead to the same (post #206948, reply #6 of 10)

The switches lead to the same cable bundle, which has four wires:
white (unused--capped off with a wire nut)
black (switch 1 to light 1)
red (switch 2 to light 2)
bare copper (green screw on switch device)

Should the red wire be marked with black tape?

Janet

No, the red does not need to (post #206948, reply #8 of 10)

No, the red does not need to be marked with black tape -- that would be only if you were "redesignating" the white.

But the white should be connected -- it sounds like you have a jury-rig of some sort where the lights are getting their neutral connection from somewhere else, and that's not supposed to be.  The neutral and hot for a given load should always be in the same cable.


Of all the preposterous assumptions of humanity over humanity, nothing exceeds most of the criticisms made on the habits of the poor by the well-housed, well-warmed, and well-fed.  --Herman Melville

"it sounds like you have a (post #206948, reply #10 of 10)

"it sounds like you have a jury-rig of some sort"--Yes, we keep discovering evidence of short-cuts, some potentially dangerous.  :(

The switch device is actually paired with another in the same box. One has two light switches; the other has a switch and a three-prong outlet. One of them gave off sparks, so I replaced both of them, without making any changes to the wiring (see attached).

Janet

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The white or common never connects to the switch. (post #206948, reply #9 of 10)

You should always switch the "hot" feed from the breaker which is typically black on 115-volt circuits. 

These instructions are a good example of poor technical writing.  The term "Common" should not have been used.  The term "shared" would have been a better choice.  Because, the two switched circuits share the feed from the breaker. 

If you have to do a feed thorugh set up where the breaker runs to the device, and then to a switch you have to use a different color than white, or permanantely lable the white to indicate it is a hot feed.  It is also a requirement that the travelers on a three way or four way circuit not use a white wire. 

Under the current standards: white is the common, and not switched; green is always the ground; black and red should be used for the "hot" wires that are fed from the breaker.  If at all possible install switches between the breaker and device.