Power to light switch turns off when light is on


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Old 06-13-16, 02:43 PM
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Power to light switch turns off when light is on

I have a complete mystery. When I turn a light on the light goes on, but both wires to the switch show no power. When the light is turned off, one of the wires to the switch becomes hot. I have tested this with both non-contact and contact connectors.

I have an older house with a small new back entry addition. This light switch for the ceiling lights is in the older part of the house. There are two lights and they both have live wires even when they are turned off. One light has the wiring exposed and has only one cable to it. The wiring for the other light is concealed in the ceiling. I can't figure out how turning the light on would cut off the power to a wire running to (or from) the switch.
 
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Old 06-13-16, 02:55 PM
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The switch is in the neutral leg of the circuit instead of in the hot leg. When the switch is on, both sides will show 0 volts (approx.) as they are both tied to neutral. When the switch is off, one side will go up to 120 as it is connected to the hot side through the light fixture.

This should be corrected.
 
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Old 06-13-16, 03:47 PM
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I second Paul's answer. I think he nailed it.

When I turn a light on the light goes on, but both wires to the switch show no power.
If you are using a non contact tester they are worthless for serious testing. In any event usually no useful information can be gathered measuring voltage at the switch. What were you trying to learn?

We will be happy to help you rewire this safely, just tell us all the wires and connections at the switch and the light.
 
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Old 06-13-16, 04:03 PM
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Shouldn't both wires be hot?

I must be missing something pretty basic. To clarify:

When the light is off, using a contact tester:
One wire is hot when tested with the other wire or with ground (or what passed for a ground the early 1940's).

When the light is on:
Neither wire tests as hot either across the other or when tested to ground.

That is using a contact tester. The non-contact tester also shows no power when the light switch turns the light on, but there is power when the light switch turns the light off.
 

Last edited by rosswilliams; 06-13-16 at 04:25 PM.
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Old 06-13-16, 05:14 PM
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When the light is off, using a contact tester:
One wire is hot when tested with the other wire or with ground (or what passed for a ground the early 1940's). When the light is on:
Neither wire tests as hot either across the other or when tested to ground.
With the switch on there is no voltage drop across the switch so there is no voltage. A switch turned on is the same as a wire. Touch that wire in two places an inch apart there is zero volts because there is no measurable voltage drop between the points due to the resistance being effectively zero. Neutral to ground is ~0 volts so if a neutral is connected to the switch you are measuring ground to neutral.
 
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Old 06-13-16, 05:28 PM
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"A switch turned on is the same as a wire"

I guess I am confused because I would expect a current in that wire when the light is turned on. My testers say there isn't any.
 
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Old 06-13-16, 05:36 PM
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"This should be corrected."

That is how I got started on this. But the situation at the switch made me think there was more to the problem than switching the neutral. It now appears I just started at the wrong end, the fix needs to be made at the light end of the circuit or somewhere in between, not the switch.
 
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Old 06-13-16, 06:13 PM
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Don't confuse voltage with current. Your testers are testing voltage, not current.

Maybe a diagram will help.

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I've shown a battery, but in your house it's really 120 VAC. I've shown a ground symbol because one side of your house wiring is connected to ground in the service panel.

In the top diagram (correct wiring) with the switch open, you will measure 120 volts from A to ground and 0 from B to ground. The light will be off. With the switch closed, you will measure 120 volts from A to ground and 120 volts from B to ground. The light will be on.

In the bottom diagram (incorrect wiring), with the switch open you will measure 0 volts from A to ground and 120 volts from B to ground. The light will be off. With the switch closed, you will measure 0 volts from A to ground and 0 volts from B to ground. The light will be on.

If you work it out, you'll see the description for the bottom diagram matches what you measured.
 
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Old 06-13-16, 07:05 PM
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First you need to determine where power comes in. If it is at the switch you will have at least two 2-conductor cables.

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If it comes in at the light you will have only one two conductor cable at the switch. So which is it?

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Old 06-13-16, 10:14 PM
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"Don't confuse voltage with current. Your testers are testing voltage, not current."

That does explain my misunderstanding if there is current there, but my testers can't detect it. But can you have a current with no voltage?

I understand the diagram - its what I thought was going on. What I don't understand is why the return (ground) after the light does not read hot when the light is on, but the feed before the light does. It would seem to be all one circuit with the switch closed.
 
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Old 06-13-16, 10:32 PM
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There is only one conductor cable attached to the switch, so it has a switch loop. Both wires look black, but that appears to be common with the old wiring in this house. I think they used something to color one white but it wore off. You can only see faint white on the neutral when you carefully examine it.
 
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Old 06-13-16, 11:48 PM
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I think they used something to color one white but it wore off.
Code requires that when a white wire is used as a hot it be recolored black, red, or any color but white, gray or green to indicate it is not a neutral. (See diagram.)
You can only see faint white on the neutral when you carefully examine it.
That is not supposed to be a neutral. It is supposed to be a white wire used as a hot. You will need to check the wiring at the light to correct this.
 
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Old 06-14-16, 07:51 AM
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As I said, they both look black and probably are in this case, but where there is a neutral wire its still hard to see the white coating any more. My next step is to open up the light and see what I have. I will be surprised if the switch loop wires are continuous between the lights and the switch. Thanks for your help.

I do have one last question. Is there any way to detect if a neutral has a current?
 
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Old 06-14-16, 01:01 PM
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So I opened up the light and there is a single modern three wire NM-B cable running to the light. Both the black and white wires test hot when the switch is off. Both test "cold" when the switch is on without the bulb in the light. With the bulb in the light, the white tests hot and the black cold.

In addition, the bare copper wire tests hot with a non-contact tester with the light switch on or off without the bulb, but cold with the light on and the bulb in the light. I assumed the bare copper was a ground wire until I took it off the ground screw and it was painted white. I can't tell whether it is paint from a painting accident or deliberate.

Of course, given the nature of non-contact testers this may all be meaningless. I am just not sure where to start. It does not appear that it can be properly fixed without tearing the ceiling and/or walls open to find the switch wire connection.
 
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Old 06-14-16, 01:27 PM
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The most practical way is with a clamp on ammeter like this one: Shop Southwire Digital Clamp Meter at Lowes.com

The jaws open and you slide the wire you want to measure into the hole inside the jaws. The meter measures the magnetic field generated by current flowing in the wire and displays how much current is flowing.

The other simple way involves cutting the wire and inserting a regular ammeter or a multimeter set to measure AC current. The clamp on lets you do it without opening the circuit.
 
 

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