Wiring an overhead light in an old house?

Question

I have the following setup:
I have two separate pieces of flex conduit; one goes to breaker box, the other goes to the switch that controls the light.
Flex conduit coming from breaker box has two wires, black and white.
Flex conduit from switch has three wires, black, black, and white.
Fixture has three wires, black, black w/ ridges, bare.

How can I tell which of the two black wires on switch side are which and once I do, what goes where?

I took the switch out and it looks like both black wires connect to the switch while the white continues on to the receptacle beneath.

I tried William B's solution and got no power at either light or receptacle.

Answer 1

If you have power running from the breaker directly to the fixture, and then on to the switch, you've got what's called a "switch loop".

-The neutral (white) from the breaker panel connects to one side of the fixture, and may continue on to other locations (does in your case)
-The hot (black) from the breaker panel does NOT connect directly to the fixture. Instead it goes down to the switch, and a second black comes back up from the switch and connects to the fixture, completing the circuit for the fixture.

In a switch loop circuit, all that's needed is two "blacks" going to the switch junction box. Since you also have a neutral going to the switch box, power most likely continues on from there to another location (nearby receptacle or another switch). In that case there should be a second conduit into the switch location, with a black and white wire. The two whites simply connect together. The black coming from the light location *that's connected to the black from the breaker panel* will connect to the other black (that doesn't go to the light), plus a "pigtail" (a 6" length of wire) that connects to the switch, thus providing power to the light circuit.

Answer 2

Let me make sure I got this right: you have flex conduit from the breaker box to the light and a separate flex conduit from the light to the switch.

First disconnect power at the fuse panel.

If so, connect the black wire from the breaker box to one of the black wires going to the switch. At the switch, connect one black wire to each terminal of the switch. The second black wire from the switch needs to go to the HOT connection of your light fixture. And the white wire from the breaker box should be connected to the NEUTRAL connection of your fixture.

To figure out which is which, you will need an ohmmeter or a continuity tester. Remove the light bulb and look inside the socket. There is a tab or button at the bottom of the socket and a threaded metal ring. The tab or button is the HOT; the metal ring is NEUTRAL. The idea is that if you screw in a bulb with your finger on the metal part, it will not contact the HOT connection until it is completely screwed in, a safety feature.
With your continuity tester, determine which wire is HOT, and which is NEUTRAL.

There should also be a bare wire coming from the breaker box. Sometimes they used the metal jacket of the flex conduit instead. With your continuity tester, check where the bare wire of your fixture is connected; I’m hoping it is to the housing. Connect the bare wire to the junction box, there should be a screw for just for that purpose.

There should also be a bare wire going from the light to the switch to connect the two boxes together. If not, you can use the white wire. The National Electrical Code requires you to put green tape on the white wire at both ends if you use it for a ground.

Answer 3

It sounds like you have the power running to the light first "from the breaker box" and a wire from the light to the switch and then one to the outlet box.

You have the beginning of the circuit at the light so you need the 3 wires to the switch to give you your common white, a "hot " black to the switch, the outlet (and it will carry on.). The other black will be from the switch to turn the light on and off (switched). At the light you will connect all whites plus one wire from the light together, the black from the power panel and one black to the switch together, and the other black to the switch (switched) to one side of the light.

At the switch you will tie all whites together, the unswitched black from the light to the black to the outlet and to one side of the switch. the other black that goes to the light you connect to the other side of the switch.

Simple, but if you don't see, then call an electrician.

Answer 4

Best to assemble the switch the way you took it off. Ditto for the light. On the other hand if you've already disassembled it and have lost the proper locations... because of the complexity old house wiring can have, I can't even begin to tell you all the possibilities you might have. You might have to call an electrician.

Answer 5

pull the switch out look at wires, if black wires are on the termanals and white is on green or ground lug ,its a ground,
start by turning the power off, useing wire nuts, twist the black wire from the breaker to A black one comming from switch, twist one black wire from light to other black wire to switch, twist the other black wire from light to white wire comming from breaker, if white wire from switch is on green or ground lug twist it to the bare wire on light,

Answer 6

you may have a 3 way switch there. go to the library and get a book with some diagrams of basic switching to guide you.

Answer 7

Hire a licensed electrician.