Wednesday, April 3, 2013

Problems with your light switch?

"I have a switch at the top of my stairs and one at the bottom that are supposed to turn on an overhead light, but the switch at the bottom only works if the switch at the top is on, what is wrong?"    

At least one student in every one of my electrical classes tells me of a problem like this.  It might be switches at opposite ends of a hall or at 2 entries to a room.  Regardless of the location the solution is the same. It is a very common problem and one that was likely caused by someone who knew just enough to replace a switch but didn't understand this particular set-up.

First some definitions:

When you have 1 switch that turns on one light (or set of lights) it is called a 2-way switching circuit and uses one (1) single pole switch.

When you have 2 switches that turn on one light (or set of lights) it is called a 3-way switching circuit and uses two (2) 3-way switches.

How to diagnose the problem:

Look closely at both of the switches where you have the problem.  In a 3-way switching set up BOTH switches must be 3-way switches.   How can you tell the difference?  A single pole switch will have the words OFF and ON imprinted on the switch.  A 3-way switch will be blank.  Both switches should be blank. 
  • If both of the switches are 3-way switches (blank).  Then one of the switches has been wired incorrectly and you will need to make changes to the connections.
  • If one of the switches is a single pole switch (has ON and OFF imprinted), then you will need to replace that switch with a 3-way switch and be sure the wiring connections are correct. 
 Removing the Single Pole Switch:
  1. Shut off the power to the circuit at your service panel.  If you don't know where your service panel is located or do not know how to shut off the power to your branch circuits  you SHOULD NOT be attempting this repair on your own.
  2. Remove the cover plate and and loosen the screws that hold the switch to the box.  Remove the switch if it was a single pole switch and replace it with a 3-way switch. See below for how to make the wiring connections on a 3-way switch.
Determining which 3-way switch is mis-wired:
  1. Shut off the power to the circuit at your service panel.  If you don't know where your service panel is located or do not know how to shut off the power to your branch circuits  you SHOULD NOT be attempting this repair on your own.
  2. Remove the cover plate and and loosen the screws that hold the switch to the box. Pull the switch from the box and look closely at the wiring connections.  Compare the connections to the drawing below.  If the connections are correct then it is the other switch that has been incorrectly wired.  Move on the the other switch and repeat.
Wiring connections for a 3-way switching circuit:


  1. Look closely at the 3-way switch and notice that you have one (1) black screw (aka: common terminal), two (2) brass screws and one green screw.
  2. You should have two (2) black wires, one (1) red wire and one (1) bare copper wire. 
  3. Connect the bare copper wire to the green or ground screw.
  4. Look into the back of the electrical box and locate the black and red wires (aka:travelers) that enter the box together.  Connect the red wire to one of the brass screws and the black wire to the other brass screw.  
  5. Find the remaining black wire and connect it to the black screw (aka: common terminal)
  6. Put everything back together and restore power to the circuit.
Congratulations You Fixed It!

Judy Browne, author of this blog, is a residential real estate consultant with more than 10 years of comprehensive experience as a broker, home inspector, teacher, landlord, homeowner, student and handywoman. Judy's range of knowledge and experience enables her to assist you in almost every area of home ownership.

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