# How to determine which wire in a light switch is live?

I am in Belgium and houses here have 230 V. I replaced a light switch recently and it had one spot for the live wire and one for the neutral. I used my multimeter to try to determine which wire was live. Both wires are black and have no markings on them.

First, I touched the black prong of the multimeter to one of the wires and the red prong to the other. My multimeter showed ~230 V. I swapped which prong was on which wire and it showed ~230 V again. I think this is to be expected as I was simply completing the circuit.

Next, I touched the black prong to ground on a nearby electrical outlet and the red prong to one of the light switch wires. It showed 110 V. I then touched the red prong to the other wire and it showed 190 V.

I was expecting one wire to show 230 V and the other to show 0 V.

I have very little electrical knowledge and barely know how to use my multimeter but like trying to learn things and fix things if it's within my abilities.

Do these results mean I have a faulty ground in my house and therefore need to call an electrician?

Or is the 110 V vs. 190 V normal in some way I don't understand? And if so, do I assume that 190 V is the live wire?

Edit: Here's a video of me measuring the voltages of each wire with my multimeter. I'm touching the black prong to a grounding prong on an electrical outlet out of view. I would assume that if my multimeter is of low quality or is defective, both wires wouldn't register a voltage. I also think that static emanating from the wires wouldn't register the 110 and 190 voltages consistently, even after I move the wires farther apart.

The switch appears to work the same regardless of which wire I put in which hole on the switch.

This is also the only switch that controls the lights in the kitchen (not the same lights illuminating me in the video).

Even though the light "works" now, I think I'm at the point where I should call an electrician as this has me concerned.

• Neither of them are neutral. One is permanently live, the other is switched live. The neutral should be in the ceiling, behind the rose. The live comes to the switch, then returns to the live side of your lamp. The neutral then returns from the other terminal after going 'through' the lamp. I can't set this as an answer because I don't know why you're getting 'odd' voltages. Feb 16, 2022 at 13:05
• Most switches just act to break(off) or connect(on) one wire(hot/live). Should have 230 between hot and ground, and 0v between switch hot and ground with switch off. Feb 16, 2022 at 13:09
• Adding pictures of the switch and switch box to your question will help us see what you see, and maybe help you. Feb 16, 2022 at 14:14

## 3 Answers

I just looked at your video.
You said both wires were black… but you failed to mention one was marked with red tape.
That's your live. The other is the switched live.

On a one-way light switch it actually makes no practical difference which goes to which terminal, but to do it properly, the taped wire goes to live.

• That may be a regional thing. If all wires are black & white, the abnormal - white as hot instead of neutral, black as switched hot instead of hot, any wire as a traveler, white as second hot (in a US 240V circuit) are the ones that get marked. If I saw two blacks, one with red tape, I would assume the one with the red (or the red wire if black/red/white wires) is the switched hot. Well, trust but verify. Feb 17, 2022 at 14:18
• In 'old colours' I'd expect red/black/green in there, 'new' colours, brown/blue/green. red/brown as permanent, black/blue as switched. I'd never expect to see a white in EU. A traveller should be yellow/grey, but for a single drop, no-one ever seems to use 'special' twin brown [or red] & earth, everybody just uses the same brown/blue/green. Lives should be taped. This is the best diag I can find, though it adds grey for 2-way - lightwiring.co.uk/… Feb 17, 2022 at 16:17
• @Tetsujin Apologies for not mentioning the tape. Since the wire itself was not marked, I did not want to blindly trust whoever put tape on this one and preferred to determine the live wire myself. There was a rubber band wrapped around the other wire, which I removed and can fathom no purpose for it. Feb 18, 2022 at 10:43

There is a wire between the switch and the lamp I call "switched-live". When the lamp is meant to be on, it is energized with 230V. When the lamp is meant to be off, it is disconnected altogether (if the lamp bulb is missing), and voltage between it and other wires is meaningless.

Or you are walking due east at 2 MPH while someone else walks due west at 2 mph on a boat. Do these things even compare? No, because you don't have a shared frame of reference.

A correct reading would be zero volts and infinity ohms. However, DVMs are quite sensitive and pick up tiny amounts of radiated energy "off the airwaves" as it were (like a crystal AM radio which can power an earpiece from the radio energy alone). Since the disconnected wire runs near an energized wire, "the airwaves" amounts to some capacitive coupling due to the wires being near.

So on a DVM, hoodoo numbers like that usually means "these wires are disconnected, but the disconnected one runs nearby some other wires".

• If the numbers were real, wouldn't that be wild-leg 3 phase? Feb 17, 2022 at 1:51

Normally the switch doesn't use the neutral wire, it interrupts the live wire.

There may or may not be a neutral wire in the switch box, that depends how the installation was wired.

If the switch is on the conduit route from the electrical panel to the light, the neutral wire will pass through the box on its way to the lamp without being connected to the swicth.

If the conduits make a detour to reach the switch (as shown above) then you'll just get one live from the panel, and the switched wire to the lamp. The neutral will be somewhere in a junction box instead.

On the drawing above, I just assumed the electrical panel was to the left and wires coming via the ceiling. If the panel is downstairs and the wires come from below, you could have this version:

...and in this case the neutral goes through the box where the switch is mounted.

In Europe the standard is blue for neutral. If the installation isn't too old, your black wires are therefore definitely not neutral.

I replaced a light switch recently and it had one spot for the live wire and one for the neutral.

The switch interrupts the live wire to the lamp. It doesn't connect to neutral, because if it did, then when the switch would be turned on, it would short live to neutral, which would make a bit of fireworks and blow the circuit breaker.

The only situation where you need neutral in a switch is if it's a radio controlled or "smart" switch that needs to be powered so it can receive signals. In this case it will be labeled on the switch, usually with blue and "N".

Now if you are installing a smart switch that needs neutral, and there is no neutral in the box on the wall, then you'll have to bring it from somewhere (ie, pull a wire through the conduits), or buy another kind of smart switch that doesn't need it (but those often cause problems with LEDs).

But if it's just a dumb switch, then you can forget about neutral.

I used my multimeter to try to determine which wire was live. Both wires are black and have no markings on them.

If you have the usual cheap multimeter with no "low impedance" voltage mode, then it will be sensitive enough to measure voltage on a wire that isn't live, but just happens to travel next to a live wire in a conduit and picks up a bit of electric field through stray capacitance. That's pretty confusing, basically the measurements are bogus.

You can use an electricians' multimeter with a lowZ mode, or the cheaper option which is the tester screwdriver, that lights up when it touches a live wire. It's a good habit to always test before messing with the stuff, just in case.

• Thank you for the diagram and explanation. This clears up some of my confusion and misunderstanding about how the wiring works. I'm going to open the switch back up in the coming days to see if I can get a better measurement. I'll try to get one of those tester screwdrivers before then. Feb 16, 2022 at 15:40
• That picture depicts the logical flow of power, but does not accurately represent the physical layout of the wires. Feb 16, 2022 at 15:57
• @FreeMan Yeah I didn't draw the conduits because that would have made a mess. For the physical layout of the wires, it depends where they come from (floor, ceiling, other side of wall, etc) Feb 16, 2022 at 16:50
• Fair enough, but it could be confusing/deceiving to someone who doesn't understand how wiring works. Feb 16, 2022 at 17:02
• Yeah I've added another drawing Feb 16, 2022 at 17:48