07:02PM | 01/21/08
Member Since: 01/20/08
2 lifetime posts
I was preparing to install a ceiling fan in an existing ceiling box and ran upon something peculiar. The box did not have any light fixture in it when we moved in (obviously did at sometime in the past).

As there were 3 light switches in the room, I had the multimeter attached to the wires in the ceiling to see which switch (or switches) controlled the light. I found it, BUT, the voltage across the wires at the ceiling with the switch on was under 20 VAC.

I then opened up the wall switch to see what things were like in there. With the switch OFF, there was 80V across the switch terminals. 0V with it ON (as it should be). Weirder still is that there was 80V from the top terminal to ground, but 20V from the bottom terminal to ground (with switch OFF).

Is this just because of the open connections dangling in the ceiling? Before I hoist that ceiling fan up there and do all that work connecting everything, I wanted to double check.

fyi -- I am very careful around wires and only had the circuit breaker on because I was measuring voltages. Numbers may have been rounded slightly.


10:25AM | 01/22/08
Member Since: 04/25/05
1915 lifetime posts
Most likely you are using a digital voltmeter.

They have a very, very high input inpedance and will measure phantom voltage cause by capacitive coupling between open wires and other hot wires.

You might want to verify the circuit by getting a cheap keyless socket (basement ceiling type of light) and temporary connecting it and putting in a 40-100 watt bulb.


08:03AM | 01/27/08
Member Since: 01/20/08
2 lifetime posts
I do use a digital multimeter, but have never had an issue with it's accuracy in the past. At any rate, I did put a test socket up there. No visible light and all my readings are the same. Under 20V across the socket in the ceiling with the switch on, 80V from ground to the terminal side of the switch.

I guess I'll be getting one of those wire tracers to try to figure out where wires are coming from/to. There are two white and two black at the ceiling. I've tried all variations of hooking them up with the same results of no light ... using just one white/black pair -- each combo possible --- to both white together and both black together.

It could be an issue prior to the switch or past the light wherever the wires go next.


10:27AM | 01/27/08
Member Since: 04/25/05
1915 lifetime posts
This concerns me. With a working bulb in a socket you will never see any phantom voltage. It should either read 0 or 120 and the light would be on.

Now you might have a high reistance connection some place and you might be 20 volts, but that was the same as you where reading without the light.

First of all you need to ID the wires in the ceiling. It sounds like you have SWTICH LEG WIRING. One pair is the power source and the other the switch connections.

Note the color coding. And also the white wire to the switch should be remarked at each end with tape or marker to show that it is being used as a hot.

The supply should be 120 volts hot ot neutral if the wire is not broken. And 120 hot to ground and zero neutral to ground.

And you should be able to hook up a light to it and the light on when the breaker is on.

The switch leg will meausre zero hot to neutral if the switch is on. IF the switch is off and the other measurements might be phantom voltage 20 -100 volts.

BTW, if there is power in the ceiling box you can wire it so that the fan is "always hot" and is controlly by the pull string and the light is controlled by the wall switch.

Now you said that there was 3 switches in this room and that a fxiture had never been installed in that room. It is possible that power was never connect (correctly) as some other place upstream.

What room of the house is this?

What other kind of lighting is in this room?

Are the switches all in one box?

What do the other switches control?


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