Electrical Wiring Question

[moc]

I have a 75 year-old house. The living room has a decorative brass cover over a ceiling box which presumably had a light fixture when the house was built. Or at least it was wired for one. The cover looks like it's been there for a long, long time (painted over many times).

There is a light switch on the wall which controlled nothing.

We decided to put in a ceiling light. I opened up the ceiling box and the light switch and the wiring is all consistent with the original house wiring. I was initially confused by what I found as the set up is a little different from most modern wiring I've seen. I think the difference is due to the fact that each circuit controls many rooms worth of lights and outlets. So there's no dead-end wiring.

Anyway, here's what I've found (as represented by an infantile sketch I made on my iphone while the power was out):





So essentially, the black and white wires come from somewhere and continue on somewhere and the red wire is used to complete the circuit to the light when the switch is thrown.

I tested the wires for power with no light fixture in place (i.e. the white, black, and red wires dangling free) but with the switch connected.

When testing power at the light fixture, there is always flow between the white and black wires regardless of the switch position. That makes sense as the white and black wires are continuing uninterrupted to the unknown next fixture or outlet.

When the switch is on, there is power between the red and the white. That makes sense because the red is carrying the flow from the black through the switch and to the white.

When the switch is off, there is no flow between the red and white (I understand) but there is flow between the red and black (that I don't understand).

There is also flow between the light switch prongs (connected to red and black) when the switch is off - which is consistent with the above, but again I don't entirely understand.

I realize I don't necessarily have to understand things in order for them to work, but I would like someone smarter than me to let me know whether my house is going to burn down tonight.

Any takers?

Here's my assumption of the overall circuit, in case that sheds light on my lack of understanding:



Thanks.

*moc

[Tom Slick]

what voltages do you get between black to red, black to white, and red to white?

[moc]

what voltages do you get between black to red, black to white, and red to white?

Good question... unfortunately, I just used the "two prongs and a light doo-hickey."

I'm pretty sure that's not the electricians' technical name for it.

*moc

[just started]

With the 3 colors it sounds like it might be a 3-way set up with 2 switches controlling the light from different doorways or entrances to the room.

[moc]

With the 3 colors it sounds like it might be a 3-way set up with 2 switches controlling the light from different doorways or entrances to the room.

That was my initial thought given the red wire, but there's no other switch anywhere near this room. It's a relatively small house (though probably big in 1932) - a 3-up / 3-down (living room, dining room, kitchen first floor and three bedrooms upstairs).

After thinking it through, I attributed the red wire to the fact that the circuit continues through the switch and fixture onto another unknown outlet or fixture.

Still, I don't have a reference to this use of the red wire in the few wiring books I have.

The good news, it's been 6 hours and the house isn't on fire yet...

*moc

[Uncle Cracker]

The red/white/black color code is used nowadays. Red is switched hot, white is neutral, black is full-time hot. This way, a ceiling fan can be hooked up to the full-time hot and neutral, while the light can be hooked up to the switched hot, and still share the same neutral. With an old home, however, assume nothing.

When testing voltage, get a load on it (so that your voltmeter, if digital, does not give a phantom reading). Remember that "hots" from 2 different lines on a fuse box may show voltage (a lot of it) to each other if out of phase. No neutral is needed. Remember that a slight voltage may show between two "hots" even from the same breaker, if wire length, switches, aging connections, etc. leave lesser voltage on one than the other. Example: One short wire may show a loaded voltage of 115V, while another longer one, or one passing through switches or connectors offering some resistance, might show a loaded voltage of 111V. So, many meters would read a differential voltage between them of 115-111, or 4V. This is enough to throw off many test lights or other lesser means of measurement.

[just started]

That was my initial thought given the red wire, but there's no other switch anywhere near this room. It's a relatively small house (though probably big in 1932) - a 3-up / 3-down (living room, dining room, kitchen first floor and three bedrooms upstairs).

After thinking it through, I attributed the red wire to the fact that the circuit continues through the switch and fixture onto another unknown outlet or fixture.

Still, I don't have a reference to this use of the red wire in the few wiring books I have.

The good news, it's been 6 hours and the house isn't on fire yet...

*moc

Since there was no light to turn on/off, the other switch (which may have been in the room at the other end from the existing switch or could have been in an other room) may have been thought to be dead and removed/covered over during a prior renovation. A close examination of the switch should give you a clue as to it being a 3-way or not.

[vaking]

May I suggest a simple test. Take a light bulb (some load) and connect it between red and white wires in the ceiling. Then test if light comes up with switch on and off. I also think you are getting phantom readings between red and black when switch is off. When the switch is on red wire is connected to black with the switch. When the switch is off red wire is expected to be isolated from both black and white. In that case you would have registered nothing on red wire. But presume that world is not ideal and there is some very high resistance (but not infinite) still connecting red to white. The tester you used is very sensitive and it showed flow between black and red over that high resistance. We know that resistance is high because when the switch is on you don't blow any fuses. If resistance was low turning on the switch would have shorted the circuit. And don't worry about burning the house - if the wiring had been very bad your house would have already burned.

[ragswl4]

Here's my assumption of the overall circuit, in case that sheds light on my lack of understanding:



Thanks.

*moc

If I am seeing this correctly it appears that the switch would have no function in this circuit as the black (hot) and white (neutral) go directly to the light. As the red wire is the switched black it would have no effect on the light as the black goes directly to the light and supplies constant current to the light.

As others have posted you may be looking at a 3-way circuit with a phantom switch located somewhere else.

[moc]

If I am seeing this correctly it appears that the switch would have no function in this circuit as the black (hot) and white (neutral) go directly to the light.

That's a poorly drawn schematic on my part. The red and white wires go to the light. The black is pigtailed to continue on (without being attached to the light fixture). I had drawn it this way to show how I tested the three wires at the fixture before they were connected to anything.

Here's a better (in a relative sense) representation:



----

I thank everyone for the input.

The wall switch is definitely not a 3-way switch (though it could have been changed over the years, of course).

A light fixture attached to the red and white wires is controlled appropriately by the switch.

I was primarily confused by the red/black flow when the switch is off. I had not considered a phantom reading due to the resistance - because I don't know enough to consider such things.

I will learn to use a voltmeter rather than my cheapie light tester.

Thanks everyone!

*moc