Electrical Wiring in the Home/light switch problems


QUESTION: I have a light switch that is connected to operate 3 separate lights.  Originally, I only had 2 lights installed with the third blanked out since I bought the house 10 yrs ago.  My wife wanted more light where the blanked out box was so I bought a fixture and attached (I first shut off breaker - tested connections with test light and multi-meter to make sure it was dead with switch in both on and off positions).  There were 2 2-wire cables (plus ground) going into the junction box connected with wire nuts, so I attached the hot and neutral wires from the fixture to each of the connections in the box. I've installed new fixtures before without any problem.

Here's the problem: The newly installed light turns on when the switch is off and off when the switch is on.  The other lights operate normally.  I then shut off the breaker and took out the fixture, checking my connections along the way and didn't notice any issues. I then turned on the breaker and carefully tested the voltage with the multi-meter and was getting 120V with the switch in the off position and 10-12V with it in the on position - I'm assuming this was phantom voltage, but not positive.

I haven't looked at the switch yet, but do you have any idea what could be the problem to cause the behavior I'm seeing?

Thanks for your time.

ANSWER: Is there a 3-way switch involved? That's what it sounds like to me.

---------- FOLLOW-UP ----------

QUESTION: Good question. Sorry I wasn't specific, but it is a standard switch labeled on/off. Any other ideas? Thanks for your time.

Okay, this where some good old fashioned trouble shooting comes into play. We have to find out everything and write it down.

You need to look at the switch and see how many wires are at the switch.

If you are sure it is not a 3-way or double pole, then you need to take the wires at the box you added a new light and separate them (with the power off).

With the two blacks separated and the two whites separated you need to check for voltage. Be sure to check all combinations including to ground.

If you do have a regular single switch, I am thinking you wired directly to the hot leg.

Also, you did not mention in the new box if the blacks were connected together and the whites were connected together.

Once you have the power wires (black and white) identified, mark them.

With the power off, check continuity from the other black wire to the switch to see if it is the wire that feeds the switch.

It would not hurt to take a look at the fixture closest to this box to see how it is wired.

The other possibility is that the leg that got switched is from the last box, or it is simply wired wrong.  
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Electrical Wiring in the Home

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Greg Hughes


Troubleshooting electrical including appliances and HVAC.


34 years of experience.

Both college and tech schools in this field.

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