Power comes into the hole in the ceiling for a light fixture, but by-passes that and the neutral wire goes down to the switch, and the black wire then comes back up from the switch. What side effects (to the light, or to up-/down-stream, or nearby connections) would there be if that switch's black wire is attached to the neutral wire of the light, and the incoming/ceiling power neutral wire is attached to the black wire of the light?

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    someone will die, because the light socket is always "hot" . At least, if what you describe is correct, rather than the suggested interpretations in the answers. Aug 25, 2020 at 15:25
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    @carl Witthoft the op is calling the switch leg white a neutral, in this configuration there is a hot and a switched hot and that is normal it’s not a switched neutral, many new to diy get switch legs confused I have probably been called several dozen times over the years for diy folks that wire the white to white and trip the breaker. That’s what I thought and the op responded reflects that it is a switch leg not a switched neutral.
    – Ed Beal
    Aug 25, 2020 at 16:46
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    Last week an electrician in Scranton, PA told me that many (maybe even most) houses in that city which got electricity around to 100 years ago are wired with the switch on the white wire. It was the accepted way to do it back then and it creates massive headaches for homeowners who want to replace things but can't figure out the wiring. Side effects - lots of shocked homeowners.
    – Moshe Katz
    Aug 25, 2020 at 18:14
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    @MosheKatz: I'm led to believe they didn't ground the fixtures back then.
    – Joshua
    Aug 25, 2020 at 22:49
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    @Joshua they didn't ground anything back then. Not on purpose anyway.
    – Moshe Katz
    Aug 25, 2020 at 23:03

4 Answers 4


I would say are you sure the neutral wire is being switched. What you describe sounds like a common “switch leg” where the white wire carries the hot from the fixture to the switch and black takes the switched hot back to the fixture.

This was code for many years the white was supposed to be reidentified as black with paint, tape as a hot most commonly overlooked.

If the wire was actually switching the neutral so the lamp had power all the time this would be a code violation and present a hazard to someone changing the lightbulb in the future.

I would verify hot vs Neutral with a meter to make sure as we have seen diy try to reword a switch leg that was properly wired. The white being hot all the time keeps it from being mistaken as a neutral and the reason code required this.

If they switch the neutral it is easy to fix and the colors at the light would be correct.

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    Ya I know there is a black and a white wire coming from the switch, and that white wire is attached to the power source's black hot wire. Is that what "switching the neutral" or "neutral wire being switched" means, and is that the same thing as a "switch leg"?
    – jordan
    Aug 25, 2020 at 15:01
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    Yes that sounds like a proper switch leg in that case it is called a switch leg “hot and switched hot”. the wire colors of white doesn’t mean neutral for this case.
    – Ed Beal
    Aug 25, 2020 at 16:39
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    @jordan -- that's not the same as "switching the neutral". Switching the neutral would mean putting a switch in the neutral leg. That's bad. Using a white wire to carry the hot leg to a switch is a "switch leg", and that's okay. Aug 26, 2020 at 17:37
  • Today’s code we need the neutral in the box so you will not see this in new construction. I am surprised they did not approve the grounded conductor from that circuit to be extended. Not that much different than allowing a grounding conductor to be tapped as long as the ground comes from the same panel(at least for electronic switches where the load is small and the reason for the neutral to be available at the switch now).
    – Ed Beal
    Aug 26, 2020 at 18:18

What you're describing - switching the neutral - is a good way to get someone hurt. Code is to switch the hot, so that if the switch is off, you can service the light safely. And on older lights, part of "servicing" was that they had these consumable items called "bulbs" that had to be changed every couple of months. The sockets they went into were easily touched.

Old-style switch loops

That thing you're talking about, where power comes to the lamp and a spur cable goes down to the switch, is a switch loop. And you are dealing with an old-style switch loop.

In an old-style switch loop, there are two wires (besides ground):

  • Always-hot which comes from supply to the switch.
  • Switched-hot, which goes from the switch to the lamp.

Both are hot wires. There is no neutral here. At all. Because old switches did not need it.

Wire colors

Both hot wires are allowed to be black, brown, red, orange, yellow, pink, blue or violet. Get any color of cable you like, as long as it's those. Now there's a mild preference for black for always-hot and red for switched-hot, but it's purely optional. So the ideal cable to use for an old switch loop is Red-Black.

What's that, you say? The only cables you can find have white wires you don't want? Well, we have a special rule that says if you need a cable with all hot wires, but all you can find is cables where one or more is a white or gray, then you are allowed to re-mark the white or gray to be a hot color. You can use paint, shrink tube, tape, etc.

Actual practice in the field

So it is SOP to run "switch loops" with black-white cable and re-mark the white.

It is also SOP for lazy production Romex-flinging electricians to blow off the "re-mark the white" step, and simply leave it white. Annoying, but true. Very confusing for newbies.

And partly because of that negligence, there's another rule that says -- when you convert a white/gray wire to a hot, and "always-hot" is present in the cable, you must use the white for always-hot. That is so that when Joe Lazy doesn't mark the white wire, the next person has a 100% chance of detecting hot voltage on the white wire, making it clear it is not a neutral. (if the white were the switched-hot, then it could measure cold, like neutral should be, and could be confused for neutral).

Fix it

When remarking wires always mark both ends. And do it around the same time so you don't forget.

Mark the white wire with black tape, since it is supposed to be hot, and specifically it must be always-hot. This mark is mandatory.

Mark the black wire with red tape, for some style bonus points, to show that it's a switched hot.

  • You note that this is the "old-style". What is the new style?
    – Michael J.
    Aug 26, 2020 at 17:23
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    When a bulb has blown, there is no obvious way to tell if the socket is live or not. I must have replaced dozens of light bulbs in live 240V sockets in the UK, and I've never heard of anybody accidentally touching anything. Maybe we rely more on "intelligent thought" than on "following codes" - i.e. always assume everything is live unless you can prove it isn't!
    – alephzero
    Aug 26, 2020 at 19:18
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    @MichaelJ., new-style would be using a /3 cable with black, red, and white, so no relabeling is required. To support smart switches, neutral is now required to be brought to the switch, so relabeling the white no longer makes sense.
    – Nate S.
    Aug 26, 2020 at 21:35
  • @aleph because your light sockets are wired so the tip is switched hot and the shell is always-neutral. As are ours. Hard to touch the tip. Aug 27, 2020 at 0:06

You're really asking two questions here--one about switched neutral and one about reverse polarity. Both can be easily resolved at the light box.

  • Connect the source hot to the black from the switch.
  • Mark the white from the switch with black tape, indicating that it's a switched hot.
  • Connect the marked white from the switch to the hot screw or wire on the fixture.
  • Connect the white from the source to the fixture's neutral screw or wire.

As to what can happen... not much. Since the switch loop only involves the light, it shouldn't affect other parts of the circuit. As far as they're concerned there's no difference in the light portion of the circuit.

However, this arrangement could be a shock hazard for those not aware of the situation. You should make the corrections.


(for single-phase installation)

For legacy lighting working with high current, switching the neutral instead of the hot wire has no practical impact, except the safety aspect mentioned: Someone can touch the hot wire in spite the switch is open. Anyway caution is required when working on a circuit, you can never be confident the switch has been actually turned off and both wires must be interrupted. However in many countries the legal liability of the professional could be sought if the switch is not correctly installed. Touching the phase rarely leads to death, but someone being electrocuted will likely fall from the stool they are standing on, and may be hurt or die from the impact.

In my country all home circuits are protected by a bipolar differential circuit breaker set at 30mA. The breaker monitors current which comes from the phase but doesn't return to the generator using the neutral. This difference exists as soon as some current travels from the phase to the ground through something, including a body. For a normal human being 30mA is a fair protection, cardiac arrest happens at 50mA in dry conditions (less with when moisture/water is involved as the skin impedance is lowered).

Some reactive current may circulate due to inductance and capacitance between phase/hot wire and ground (or long neutral wire), and a modern energy saving light (LED or fluorescent light) my still shine faintly (also this video but forget the explanation which is completely wrong). While this effect is exacerbated with neutral switching, it can happen even without connection at all.

Neon glowing without connection
Neon glowing without connection. Source

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