Help me wire a light fixture.
December 7, 2008 10:18 AM   Subscribe

AC Wiring question!

I have a ceiling mounted light fixture. It does not work. Perhaps you can help me.


The wiring in my building is the old cloth covered wire. It is not marked for polarity.

The fixture itself has a black and white wire, as well as a ground wire. Only the black and white are hooked up to anything. They are connected to the cloth-covered wire with wire nuts.

One wire, when connected to ground, measures 120v. The other wire gives nothing.

I have tested the connection between the fixture and the switch. The switch is connected to the wire which measures 0v, and the switch works fine.

The wire which shows 120v when measured relative to ground is always on regardless of switch position.

How do I wire this thing properly, so that the switch actually turns on the light?
posted by fake to Home & Garden (18 answers total) 1 user marked this as a favorite
black to black white to white ground wire to the junction box. knob and tube wiring the black wire is always hot. Check your fixture if it still doesn't work doesn't check the switch again. same wiring set up with the switch black to the hot.
posted by pianomover at 10:44 AM on December 7, 2008

wait, i'm confused.

did this light previously work and now it doesn't? or are you installing a new light fixture?

are these controlled by a wall switch? or a turn-switch or pullchain-switch located on the fixture? if the latter, how do you know the switch is working fine?

One wire, when connected to ground, measures 120v. The other wire gives nothing.

these are the wires from the light fixture that are giving you these readings?
posted by rmd1023 at 11:01 AM on December 7, 2008

Response by poster: hi rmd1023,

here's a picture that should clarify things.

posted by fake at 11:11 AM on December 7, 2008

Response by poster: knob and tube wiring the black wire is always hot.

Does that mean that the other wire would be switched to ground? (in this case, ground is the box that the switch is wired into)

What I'm asking is: in this case, the hot wire seems to be continuously on. Does that mean that the other wire should be switched to ground to make the light turn on and off?
posted by fake at 11:13 AM on December 7, 2008

The green isn't connected because in old wiring there's no ground to connect to (sometimes you can tack into the armor jacketing if you've got something that "new" but it's a kludge at best).

Basic coding: Black is hot, white is neutral. So black lamp wire should connect to the wire showing 120V to ground (but see below).

What are you using to measure the voltage? Multimeters don't work reliably with household wiring; you need a voltage tester that connects a real load (typically a solenoid or a neon bulb) to the circuit. You can fake this by directly connecting an incandescent light bulb to the hot and neutral wires; if it doesn't turn on then the circuit is dead regardless of what your multimeter says (technical stuff below).

If the circuit is truly dead no matter where the switch is, and your breaker isn't tripped, the possibilities I can think of are:

1) Bad switch.
2) Bad splice in the wiring somewhere -- if someone didn't make a gas-tight fitting in one of the wire nuts, eventually the splice can oxidize and break the connection despite looking fine from the outside. These, speaking from experience, are a complete pain in the ass to find.
3) Crazy-ass wiring, in which this light is hanging off of a GFCI outlet somewhere upstream that's tripped, killing the light. This may seem extremely unlikely as it's a completely brain-damaged way to wire a house. That's exactly what I thought until I found it in my living room.

All three of the above might cause a multimeter to read 120V while delivering no current to a load. An oxidized splice (or busted switch, or tripped GFCI) can leave a pair of conductors in close enough proximity that the multimeter is reading through a capacitive coupling, which can transfer voltage but not current.
posted by range at 11:20 AM on December 7, 2008

So, looking at your picture, you really need a load tester, because that info doesn't scan. You've got:

Black - Green screw: 120V
White - Green screw: 0V
White - Black: 0V

I'm pretty sure it's not possible for all three of those numbers to be correct if none of them change w/ switch position; if white-green is 0V then white-black should equal green-black.

Just to talk about the topology of this for a second, the switch is doing one of two things:

Scenario 1: High-side switching. The switch is before the black wire, and the white wire goes right back to your breaker box.

Scenario 2: Low-side (aka, dangerous, but it happens anyway). The black is connected directly to your breaker box, and the white is interrupted by the switch on its way back. Very dangerous since you could turn the light on by connecting a wire from the case to your sink, but it happens sometimes.

Listing these just to help in troubleshooting, once you know what's actually going on with the connections.
posted by range at 11:29 AM on December 7, 2008

really incredibly stupid question: are you sure the light bulb isn't broken?

otherwise, i think your switch is broken, as a guess. you should have 120v from the black wire to ground and 0v from the white wire to ground. you should get 120v between the white wire and the black wire, however. if you can get to the wires on both sides of the switch, see if you get 120v across the switch.

also, to nitpick, the switch should not be connected to the white wire. that's dangerous. it should be connected to the black wire. also, on the light fixture itself, the lamp socket terminal that connects to the bottom of the light socket should be connected to the black and the terminal that connects to the screw shell of the light socket should be connected to the white wire.

the electricity comes in on the black wire, also termed the "hot leg". in normal operation, the electricity flows through the fixture, the light goes on, and the electricity flows out of the fixture through the white wire, which is the "grounded" conductor.

note that the "grounded" conductor (the white wire) is different from the "grounding" conductor, which is the ground wire probably connected to the green screw or the side of the junction box. in normal operation, current should flow through the white wire. since there's very little resistance along the wire, there should be virtually no voltage difference between the white wire and ground. under normal circumstances, NO current should flow through the grounding conductor. the purpose of the ground connection is set up to provide a better path to ground than "through you", so that if things break in a way that causes the normal metal parts to become energized, the circuit will short out rather than waiting to fuck you up when you happen to touch it and you're grounded.

caveat: be REALLY REALLY REALLY CAREFUL when moving those wires in the junction box. *particularly* if it's a kitchen light fixture. unless otherwise specified, most light fixtures can only dissipate the amount of heat from 60 watt incandescent light bulbs. of course, people tend to stuff 100w light bulbs into fixture, particularly kitchen fixtures, and then over the years, the overheating literally bakes the insulation on the wires. after a few decades of that, just touching the wires can make the brittle baked insulation flake away into powder.
posted by rmd1023 at 11:31 AM on December 7, 2008

The black wire being hot all the time is the easiest way to wire a light and is very common. Done properly the switch interrupts the hot side of the circuit. In canadian code the black(hot) supply line is wired to the white wire going to the switch, not to the bulb. The return wire, should be black, is then connected to the centre terminal of the bulb holder and the other terminal is wired to the neutral side of the feed line.

A problem is that you only seem to have two wires coming into the box (from the picture please correct if wrong). If so you need a lamp holder with integral switch (pull chain type) and the switch probably controls an outlet.

Normally you'll get continuity between any two neutral side wire or any neutral and any electrical ground point so that doesn't really mean anything. If you have continuity between neutral/ground and both sides of the switch when it's off, that indicates a problem with the switch wiring. However you should not connect any circuit to ground to pull a neutral, it can result in a very dangerous (IE: your kitchen tap could electrocute you) ground loop.

range writes "Crazy-ass wiring, in which this light is hanging off of a GFCI outlet somewhere upstream that's tripped, killing the light. This may seem extremely unlikely as it's a completely brain-damaged way to wire a house. That's exactly what I thought until I found it in my living room."

Actually that is not crazy. A GFCI outlet properly wired will provided short protection to all devices downstream of the outlet. Wiring one into knob and tube wiring is a common way to provide grounding protection (though not a ground) to the downstream K&T devices making them much safer.
posted by Mitheral at 11:36 AM on December 7, 2008

Best answer: I'd recommend getting at least one of those cheap neon voltage testers, since as range says, a modern multimeter puts so little load on the line that it's easy to get false readings. It's likely that the green screw and the metal bracket aren't electrically connected to anything.

As for switching the neutral wire instead of the hot wire: that "works", in the sense that the light will turn on and off, and it may be the way your light is wired, but it's dangerous and very much against code. (The problem is that when the light is off, the whole circuit including the apparently-inactive socket and the neutral wire going back to the switch is at "hot" potential, connected through the lightbulb. This means that elsewhere in the building there would be supposedly-neutral wires actually carrying dangerous voltages, which is obviously bad.) It is, however, a common mis-wiring in old and much-repaired buildings. I think maybe in the knob-and-tube days they didn't worry as much about it, perhaps.
posted by hattifattener at 11:37 AM on December 7, 2008

Your switch appears to be connected to the neutral wire (you say it is connected to the wire showing no voltage). This is not safe. Even when the switch is off you have 120 at the fixture, not good. I would hire an electrician to come in and make things right. Electricians are typically not that expensive and older wiring can be tricky.
posted by caddis at 11:56 AM on December 7, 2008

Response by poster: Alright, a little more investigation and I've found the source of the problem.

On the opposite side of the wall is my bathroom, which has a light switch exactly opposite the problematic switch. The backs of the two boxes have been cut out so that it is possible to see right through them.

Inside the box, there is a rusted screw that originally attached the bottom screw of the switch (where the wire attaches, not the mounting screw) to the outside of the box. This screw fell off, disconnecting the ground from the bottom of the switch and making my light inoperable.

I was just as confused as everybody here, as I have some electronics knowledge and some experience with AC wiring. The switch worked (as confirmed by my multimeter), the neutral was.. neutral... regardless of switch position and the hot was always hot, the opposite of the way I'd do things.

I don't like the way this is hooked up, but I am going to hook the bottom screw terminal of the switch to ground as it was originally done. An electrician *might* be able to reverse the way this thing is hooked up, but it is really doubtful because my building actually has concrete walls and is very old. The wire would have to be torn out and destroyed, and probably the walls as well.

I will pick up one of those little neon testers for future use; I had no idea about the load issue (though my meter seems to work properly given what I now know).

Thanks all.
posted by fake at 12:24 PM on December 7, 2008

Response by poster: Yeah, OK, so this is low-side switching, as outlined by range above.

Can someone tell me in non-OMG INTERNET terms what it will mean to hook it up the way I've described? The box that the switch is mounted in is actually connected to an earthed ground somewhere in my building. I want to connect the low side to that.
posted by fake at 12:29 PM on December 7, 2008

Response by poster: And now I see that hattifattener laid that out for me.

I will err on the side of not hooking it up, and see if my landlord will do anything about it.
posted by fake at 12:34 PM on December 7, 2008

The box that the switch is mounted in is actually connected to an earthed ground somewhere in my building. I want to connect the low side to that.

You don't want to connect the low side to that. The low side, neutral does not go into the ground, the ground goes into the ground. If your electrical boxes are metal with metal conduit, that is your ground. There will be a heavy copper wire that goes from your box to a spike that is driven into the earth, that's how your system is grounded. The low side, white wire neutral completes circuits, it's not the ground.
posted by lee at 12:41 PM on December 7, 2008

Response by poster: I understand now, thanks.
posted by fake at 12:51 PM on December 7, 2008

Best answer: I want to elaborate on lee, above, because it's more diabolical than that in a lot of places. In lots of installations the ground is established just like lee describes (conduit -> stake in the ground, or water main), and the neutrals are all connected to a copper bus bar in the breaker box -- but frequently that bus bar is also tied to ground. The reason this is trouble is that it means you can do exactly what you propose (connect the low side of the switch to the box ground instead of neutral) and it will work, which is exactly the wrong feedback for you to get.

It's a problem because the return path for the current is now something crazy like screw -> 100' of pipe & pipe fittings -> screw -> wire -> bus bar. Most of the elements in that path were never designed to carry sustained currents and you're running the risk of starting a fire somewhere in the conduit or other fittings. (Not to mention other potential delights -- in my personal electrical house of horrors I once cut a ground wire on a disconnected circuit and watched all the lights in my basement shut off. Someone had crossed up an upstream splice and was returning the basement circuit through another circuit's ground wire.)
posted by range at 7:38 PM on December 7, 2008

Best answer: what range says.

the neutral (also known as the 'grounded conductor') and the ground wire itself (known as the 'grounding conductor') should only connect at the service disconnect, where the wires come in from the street. the neutral is there to carry the return electricity.

the grounding conductor is there to provide a better path to ground than through your body if you happen to touch an energized piece of metal.

example: you have a refrigerator. a wire in the compressor motor comes loose and connects to the metal frame of the fridge. if you have a grounding conductor (that is to say, a grounded three prong outlet with a three prong plug stuck in it), there's a little PZZT noise as the electricity finds the fastest path to ground - along the grounding conductor, the circuit shorts out, and you have to unplug the fridge and replace the fuse or reset the circuit breaker.

where someone cut off the third prong on the plug, so it's ungrounded, however, what happens when the wire comes loose is ... nothing. except now the entire metal casing and frame of the fridge is dangerously energized with electricity. touch the fridge and a water fauce, for instance, and suddenly *you* provide the best path for the electricity to travel to ground. ouch.

not to sidetrack the askme, but...
range: i once worked in a house being gutted and renovated and discovered that the pipe carrying the underground service wires was hot. (gas workers nearby had cracked the pipe and it had nicked the conductor, but the pipe itself was corroded or painted enough that it didn't short to ground.)
posted by rmd1023 at 7:55 PM on December 8, 2008

Response by poster: I marked best answers not only because they were the best but also because you took the time to come back and clarify everything. As a result, I have a vastly improved understanding of AC wiring, and haven't made any dumb wiring decisions.

Thanks range, rmd1023 and everyone else.
posted by fake at 2:13 PM on December 9, 2008

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