There are two types of dimmer switches - single pole (or S1) and three way (or S3). You can determine which type you have without even removing a screw. Here's how: How many switches can control the light fixture now? One or more than one? If you answered one, you need a single pole dimmer switch. If you answered 2 or more (it doesn't matter how many at this point), you need a three way dimmer. Ok, we're done with step one almost.. make sure that the one switch that controls the light has ON and OFF on the toggle handle. Likewise, none of the multi-switch arrangements that control a light should have ON or OFF on the toggle handle. None of the wide rocker or Decora style switches have ON or OFF on them.
Next, do the light switches that control the fixture only control an incandescent light? If the switch operates a light AND a fan - like a paddle fan fixture - you can't simply replace the switch with a dimmer. Dimmers are for use with a 120 volt incandescent lamps (tungsten, quartz, halogen, etc.). They are not designed to work on motors circuits - such as fans - nor are they designed to work for lights that have a "ballast" like neon or fluorescent types or a "transformer" like low voltage track lights, etc.. Check the bulb's voltage rating if unsure if a low voltage type or not. The only exception to the above that I am aware of would be for lamps that SPECIFICALLY state on the package that they can be used with a dimmer. You can find dimmable compact fluorescent lamps that will work, but as far as I know, there are no other types of lamps or motors for that matter that will work WITHOUT OVERHEATING or DANGER OF FIRE.
A retail package of dimmable CFLs.
That means, yes; if you install a dimmer on a motor, it will adjust the
motor speed BUT, the motor WILL over heat and can easily start a fire.
The same holds true for lamps that have a ballast. If it doesn't specifically
state it is for use with a dimmer, don't try to use a dimmer to control it.
Check the wattage rating of the fixture. There is a sticker affixed to every fixture that indicates the maximum wattage of each lamp socket. Add these values together. Most fixtures are well under 600 watts. This 600 value is significant, as this is the base rating for dimmer switches. Nearly all unmarked dimmers are rated for 600W - but if you look closely, you should find this wattage rating along with the voltage rating on it somewhere. If your fixture is capable of more than 600W, you should select a dimmer that is rated to at least handle this wattage. The next higher wattage rating for most dimmers is 1000W, and costs about twice as much as the 600W dimmer. It only gets more expensive from here. Fortunately, not many residential applications need 1000W+ dimmers for the loads they will control. The need for 100W+ dimmers comes into play when there is a dimmer switch in the same box as the old switch. When converting a dimmer & switch to a dimmer and dimmer in the same box, the wattage rating is derated to disapate the heat created by the dimmers. Please, consult the chart below to see how to properly derate: http://www.residential-landscape-lighting-design.com/lighting_dimmer_derating_ganging.htm
Ok, the preliminary work is complete. Shut off the power to the circuit. Remove the wall plate and remove the mounting screws that secure the switch from the switch box. Gently pull the switch out of the box. If it is a single pole switch, it will have two (2) terminals. If it has 3 or more terminals, (not counting the green ground screw) skip ahead to the three way section. If there are more than 2 wires connected to the switch (again, not counting the green ground screw), mark the wires so that you know which wire(s) go to which terminals. Use masking tape and a pen to write or colored tape to identify all wires that connect to the same terminal. Turn power back on. Test the all the wires for the presence of 120 volts. Once this is learned, shut the power off again. Test to make sure power is indeed off. Remove the wires from the switch terminal screws. Remove any bare or green ground wire from the green screw. The new dimmer should be connected so that the the "power" or "line" wire or terminal is connected to the wire(s) that you found to be powered in the previous step. If there was 2 or more powered wires that had the same identification or mark on them when testing - make sure all those wires get connected to this terminal of the dimmer. Connect the remaining wire(s) (that should also ALL have the same marking) to the remaining "load" or "light" wire or terminal of the dimmer. Make sure the wires that connect with a supplied wirenut have no exposed metal to short out. Check your work. Make sure none of the marked wires have been mixed with the other marked wires. Connect any bare or green ground wire to the green or bare wire of the dimmer or green screw on the dimmer. Gently fold the wires to the rear of the box or sides and insert the dimmer. Secure to the box with screws and install the wall plate. Turn on power and test.
If you have a multi-switch or three way installation, it's a little more complicated. Firstly, only ONE dimmer is used in the circuit. Installing more than one dimmer will prevent the light from being made brighter than the current brightness setting of lowest switch. If there are 3 or more switches that control the light, two are three way types; and are the only ones that can be changed to a dimmer. The third, and subsequent switches in multi-switch installations are "four way switches". Dimmers are not made to replace them. Three way switches have 3 terminal screws and four way switches have four screws (not counting the green ground terminal screw). This is the only way to determine one from the other. Start by shutting off the power. Go to the switch that you would like to replace with the dimmer. Remove the wall plate and switch mounting screws. Gently pull the switch out of the switch box. Count the number screws on the switch body (do not count any green ground screw). If it has 3 screws, it's a 3 way. If it has four, its a four way and can not be changed to a dimmer. If it has 4 screws, resecure the switch into the switch box and reinstall the wall plate. You will need to find a three way switch that controls this light. If none of the other locations is not desirable, you can not install the dimmer. Otherwise, remove any bare or green wire from the green ground screw. Next, locate the dark colored screw. This is called the "common" or "shunt" screw. Sometimes it is painted black; and other times it is gold when all the non-ground screws are silver colored. In any case, it will be the "odd colored" screw. Mark the wire(s) that connect to this screw with a number "1". Mark the wire under one of the other screws (it doesn't matter which) with a "2"; and mark the wire under the remaining screw with a "3". The wires that are connected to these terminals and marked as number 2 & 3 are called "traveler wires". Traveler wires run between switches and common (or shunt) wires that connected to the terminal with the same name and marked as number 1 does one of two things: it either comes from the power source or it supplies power to the light fixture. Make sure each wire that was connected to the 3 way switch has a number on it, then remove all the wires from the switch terminals. You will connect your traveler wires numbered 2 & 3 to the dimmer's traveler wires or terminals (it does not matter which wire is connected to which terminal; and the common or shunt wire numbered 1 to the dimmer's common or shunt wire or terminal. Many dimmers use black as the common or shunt and red & white for travelers. You'll need to consult the wiring information that comes with the dimmer to find out the colors used for common or shunt and the traveler wires on your switch. Check your work. Make sure no exposed wire can short when it is powered up, later. Carefully fold the wires into the switch box. Insert the dimmer into and secure to the switch box. Reinstall the wall plate and turn power on & test.
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