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?
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.
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.
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).
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'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. Source