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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?

  • Do you have a very old house? I would expect this for the old knob and tube wiring. – Ack Mar 15 at 22:15
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    No knob and tube. – Not_Einstein Mar 15 at 23:12
  • @Ack This is absolutely not a "knob and tube only" type of thing. It was pretty standard through most of the 20th century - my house (1950s) has plenty of switch loops. (Which confused me the first time I saw one.) – manassehkatz-Moving 2 Codidact Mar 16 at 0:55
  • @manassehkatz-Moving2Codidact You're mistaken, I did not use the word 'only' nor did I imply it, you're on your own there. K&T is the first thing to eliminate allowing us to move forward – Ack Mar 16 at 1:02
  • @Ack I know you didn't say "only" - sorry for any implication of that. I am just pointing out that 2-wire switch loops were extremely common until the new "neutral in (almost) every box" rule. – manassehkatz-Moving 2 Codidact Mar 16 at 1:11
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The outlet is not "further down". Electrical power is delivered to the outlet first, and the switch is "further down":

obsolescent standard switch loop

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.)

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    See NEC 200.7(C)(1) for widely accepted standard for feeding the hot to the switch with the re-identified white. – NoSparksPlease Mar 16 at 3:05
  • Thanks Harper - Reinstate Monica and @NoSparksPlease. I was going to ask for a NEC citation but I did not want to appear to be arguing the point. – A. I. Breveleri Mar 16 at 3:31
  • Brilliant! +1... – Harper - Reinstate Monica Mar 16 at 21:33
  • They say a picture is worth a thousand words. This makes it very clear. – Not_Einstein Mar 17 at 15:48
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Because in the case of a switched outlet you would then need 3 conductor to continue the hot to other receptacles or to a hot half of the receptacle, it's easier to use one type of cable. It also increases box fill.

The primary purpose of the rule is for light fixture wiring. If you were to use the black hot to the switch then you would have two whites connected to the fixture, it would be very easy to reverse the polarity and in the case of a basic Edison base the shell would be hot, and as you unscrewed a bulb you could contact hot threads of the light bulb.

Actually modern [2017] Code does require a neutral at the switch for most lights so 3 conductor cable is now required. But this new requirement doesn't apply to switched receptacles [NEC 404.2(C)(5)].

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  • If you were to use the black hot to the switch then you would have two whites connected to the fixture - Thank you so much for this remark. I knew there had to be a good reason for the rule but I was busting my brain trying to figure out what it is. – A. I. Breveleri Mar 17 at 18:06
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The mandatory colors are green, yellow/green or bare for ground. Then white or gray for neutral. All other colors are hot.

Where cables are concerned, you are stuck with the colors of wire that are in the cable. As such, there are some rules of precedence regarding neutral.

  • Where neutral is present, it must be on the white or gray wire.
  • If neutral is not present in the cable, the white wire can be re-marked for use as a hot.
    • If always-hot is in the cable, then the white wire must be used for always-hot.

I also prefer to mark always-hot wires black, and switched-hot wires red, using colored tape. This makes things clearer.

This thing is an old style switch loop.

Neutral must surely come into the receptacle box, since the receptacles need it. However it comes, white will be used for neutral there. The white always-hot to the switch will need to be marked black. It will be joined with other black(s) at the recep. I would mark the switch's black wire red, since it is switched-hot.

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