How to isolate two lights controlled by three switches?
October 20, 2005 8:58 AM   Subscribe

I have two light fixtures in the entryway of my house, near the dining room. There are three light switches in the immediate vicinity that will turn both fixtures on or off. I would like to split the switches somehow so that I can turn on a fixture by itself. The problem is two-fold: impress my wife with household prowess and not to shock myself.

Here's the scenario:

2 light fixtures: A & B
3 light switches: X, Y, & Z

Light fixture A has a single black wire, white wire, and a ground wire attached to it.

Light fixture B has two black wires, two white wires, and two ground wires attached to it.

Light switches X & Y are (I believe) three-way switches. They each have two black wires and one red wire attaching to the switch.

Light switch Z is (again, I believe) a four-way switch. It has two black wires and two red wires attaching to the switch.

All light switches and fixtures currently work as they are supposed to -- they just don't work as I'd like them to. I don't care about which switch(es) controls which light (all switches are within six feet of each other) -- I just want to be able to turn on the nice light fixture near the dining room for a fancy dinner party. Any ideas?
posted by jmevius to Home & Garden (6 answers total)
Which light is closer to the switches?

Most likely, light A is at the end of the circuit. Light B is in the middle. One set of Black/White wires on B go to the switches, and one set go to light A.

Are there any unused wires in the cables? Often times cables going to fixtures with three way switches will be 3 wire (plus ground). If this is the case you might be able to do it easily, otherwise you're going to have to run a separate cable to one of the lights.

What do the switches currently do? Do they all control the lights in different combinations?

As for not shocking yourself, turn both lights on, find the circuit breaker to the switches and turn it off. Verify that the lights are now off. Then use a volt meter or circuit tester to CONFIRM that there is no current going to the switches. Experience has shown me that houses are often wired by Moe, Larry, and Curly and often make no sense. You can buy a cheap voltmeter at Radio Shack for $20.00. Small price to pay for not killing yourself.

I recommend getting The Black and Decker guide to home wiring. Lots of easy directions and tons of pretty pictures that explain all this stuff.

Basic home wiring is easy, fun, and safe as long as you check and double check everything. It's easy to kill yourself if you screw up, but not screwing up is easy if you check a couple of things.
posted by bondcliff at 9:13 AM on October 20, 2005

I just went through a similar experience with a ceiling fan. Fortunately my father-in-law is an electrician by training and was able to talk me through it.

WARNING: You may shock yourself severely if you try to do this yourself without knowing what you're doing.

You should probably pay a professional to do this because it will be cheaper than your expected medical bills.

Usually, a black wire is "hot" and an white wire is the "return", or neutral wire. Returns and neutrals may be hot even with the circuit breaker tirpped because they may be getting current from another circuit in the house that is still on. Treat all wires as if they are live all the time, because you never know whether the person who messed with those wires last had any clue what they were doing. (In every house I have owned, the previous owner was incompetent in this regard.)

The trick here is to figure out how the entire circuit is currently connected; once you can do that, you can reconfigure the circuit to be the way you want it.

Now, you haven't given enough information here for anyone to figure out which wire goes where. Which switch currently controls which light? You should probably chart out all possible combinations of switches and their results in order for anyone here to give you an idea of how to do this from start to finish.
posted by mikewas at 9:21 AM on October 20, 2005

You can buy a cheap voltmeter at Radio Shack for $20.00.

For that matter, buy a multimeter, which is not much more. (A multimeter is a voltmeter/ammeter/ohmmeter all in one.) Some of them also have a "connectivity" or some such setting, which is really just a subset of ohmmeters, which produce an audible tone when the leads are electrically connected, which is nice.

The reason for this is, after you've confirmed that the power is off per bondcliff, you can use the ohmmeter/connectivity setting to determine which wires are connected or disconnected for various settings of each switch, which helps immensely.
posted by DevilsAdvocate at 9:35 AM on October 20, 2005

I'd suggest hiring a pro. You would hate to impress your wife and end up burning down your home.
posted by leafwoman at 9:44 AM on October 20, 2005

Take a look at this 4-way wiring diagram (the first abstract drawing). You have the same situation. Don't believe me? Try to visualize sliding the light fixture along the top wire untill you reach the far right. Now the diagram matches exactly what you've described. The second light is simply in parallel with the first.

I don't see how you can break the lights apart without running at least one (maybe more) extra cable.
posted by sbutler at 10:16 AM on October 20, 2005

This is a job for X10! (Or Insteon, which appears to be the same thing...)
I've linked SmartHome cuz it's a well laid-out site, but you can save a few bucks if you shop around.

Rather than opening/closing a hard switch, an X10 controller sends a signal via the power line to a relay module that, in turns, opens/closes to turn the light on, off, or dim. The benefit, in this case, is that don't need a seperate wire pair running from each switch to each lamp.

Of course, X10 can also do funky things like run on timers, triggers, phone calls, computer control, etc., if you want to geek it to the hilt.

There are also similar solutions that work wirelessly from a battery-powered "switch" that you can mount anywhere. A trip to the home automation section of Home Depot, Lowe's, etc., will give you a better feel for what's available.
posted by LordSludge at 12:01 PM on October 20, 2005

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