Replacing switches


Old 06-08-04, 06:15 AM
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Replacing switches

I am trying to replace 2 switches in a bathroom. One switch is for a ceiling light and the other switch is for a wall light above the sink. There are 3 cables coming into the switch box. Two cables have 3 wires coming out of them (black, white, copper). The third cable has 4 wires coming out of it (black, white, red and copper). All of the white wires are pigtailed together and all of the copper wires are pigtailed together. I have tried many combinations of connecting the 2 switches but cannot get them to operate correctly. My main problem is that when I do get the switches to work the bathroom lights, one of the switches also turns on a hall light and a laundry room light located nearby. I cannot figure out how all of them are connected. Can you help?
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Old 06-08-04, 06:35 AM
Join Date: Sep 2003
Location: Central New York State
Posts: 13,973
Okay, first the lecture. When removing the old switches you should have made detailed drawings and/or notes about how everything was connected. Some people even use a digital camera and take pictures. Doing so would have made it easier to connect up the new switches. However, you didn't do that, so we have to start from scratch, or almost from scratch.

The bare copper wires are grounds. They should be connected together They should also have short pigtails or extensions that connect to the green grounding screw on each switch. if the junction box is metal these grounds should also connect to the metal box.

The white wires that are pigtailed together are the neutral or return wires. These are all attached together and you should not mess with these.

The black wires and the one red wire are for carrying 120 volts. These are referred to as the hot wires.

One of these hot wires carries 120 volts all the time (unswitched). This hot wire originates from the circuit breaker panel or fuse box and may pass through additional outlets on the way to the bathroom. This hot wire is most likely a black wire and most likely is from one the cables that does not contain the red wire. You need to figure out which of these wires is the hot wire. Use a two wire tester (available at a home supply store for a few dollars) and test each of these wires againat the collection of ground wires. One of the wires will light the test lamp. This is your incoming power wire.

Next you need to find the hot wire that leaves the box and goes on to the laundry room and hall. This is most likely the other cable that does not contain the red wire. Find this by process of elimination. Useing a wire nut join the incoming power wire to each of the other wires one at a time. When the hall and laundry lights work you have found that wire.

By process of elimination, the remaining two wires are the hot wires for the two bathroom lights. These are most likely the red and black wire in the same cable. You can verify this by wirenutting them one at a time with the incoming power wire and seeing what works.

Now you have identified the four hot wires. Your job now is to connect the wires in such a way that the wire going to the laundry and hall lights is always hot, and so that the wires going to the bathroom lights are each switched.

You will do this by connecting the incoming power wire with the wire going to the laundry/hall and two pigtails. The pigtails are each connected to one side of each switch. The two wires for the bathroom lights are connected to the other side of the switches.

Let us know how this all turns out.
Old 06-08-04, 06:41 AM
Join Date: Sep 2000
Location: United States
Posts: 18,497
Why are you replacing these switches? Are you replacing them with identical new ones, or are the new ones somehow different? Is there some reason you didn't just connect the new ones up the same way the old ones were (i.e., no guessing)?

It sounds like this box has feed-through power to the hall and laundry room. When this is true, the incoming power is connected to the outgoing power, and also to the switch. It sounds like you connected the outgoing power to the load wires rather than the incoming power wires.

Now you have the task of identifying the four cables. This can be done with a multimeter and a lot of testing. Chances are good that power is coming in on the black/red/white cable, with black supplying the unswitched hot and red returning switched hot to one of the two loads. One of the black/white cables is the switched cable to the other load, and the other black/white cable carries (hopefully) unswitched power to the hall and laundry rooms.

Try this:
  1. Leave all the white wires connected to each other and stuffed into the back of the box.
  2. Arbitrarily label one of the black/white cables "X", and the other "Y". Label the black/red/white cable "P". I'll use "b" and "r" to refer to black and red wires.
  3. Use a wire nut to connect Pb to Xb, with two pigtails (short pieces of black wire), one to a screw on each switch.
  4. Connect Yb and Pr to the oher screws on each switch.
  5. Turn the power back on and test. If all is well, you're done. If not, then reverse the Xb and Yb wires and test again.

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