Q&A / 

Dimmer Switch

DEAR TIM: I would like to install several dimmer switches to control light output. Do I need special wiring to accomplish this? Older dimmers I have used scared me because the switch is very warm to the touch when they operate. Are these things a fire hazard? Are there specialty dimmers for individual lamps or fixtures? Jo Anne C., Edison, NJ

DEAR JO ANNE: Light dimming switches are not only very safe, but the variety of sleek and efficient dimmers you can purchase today is outstanding. In my opinion, the only fire hazard you might introduce to your home while installing one would be failure, on your part, of reading and following any enclosed instructions. I have installed dimmer switches for years and not one has ever caused an electrical fire.

The reason dimmer switches get hot is fairly simple. Some dimmers get hotter than others because of their design. Old dimmer switches typically were rheostats that varied the amount of voltage going to the light bulbs. As the rheostat was turn down to lower the voltage, the electricity was changed into heat. These old dimmers wasted lots of energy.

Modern dimmer switches use slick internal electronics to dim the lights. These components can turn the light on and off 120 times per second. This is too fast for you to see. By altering the amount of time the switch is off rather than on, you see a lower amount of light out of the bulb. This method of dimming is highly efficient. Well over 90 percent of the electricity that does flow into the switch gets used in the light bulb. The heat that you feel in modern dimmers simply is a result of the ordinary friction of the electricity flowing through the device.

Modern electronic dimmers have a heat sink or metal plate that directs this heat towards the room. This is why you feel the heat on the cover plate. This is done intentionally. Over time, excessive heat could damage the house wiring or the internal components of the dimmer switch.

You don't need any special wiring to connect a typical residential dimmer switch. If you have a standard toggle switch in place now that operates the light, a dimmer can replace this switch. You can even get three way dimmer switches that allow you to operate a light from two different switches.

Be aware that dimmer switches can only handle a specific amount of light load. You can typically purchase dimmers that are rated for 600 or up to 1,000 watts of lighting. Light bulbs are usually clearly marked as to their wattage. Determine the total wattage of the bulbs that are being controlled by any one dimmer. Purchase the correct size dimmer switch to suit your needs. But keep in mind that the metal heat sink plate on the front of many dimmers can be altered so that you can place multiple dimmer switches next to one another. If you break off the side tabs of the heat sink, as allowed in the instructions, you need to derate the capacity of the dimmer. If you snap off the tabs on both sides of the heat sink a 600 watt dimmer becomes a 400 watt dimmer.

Be prepared for tough decisions when you buy your dimmer switches. There are so many cool ones out there. You can buy one that has all of its controls in the tiny space that is used by a standard on and off toggle switch. Dimmer switches are made with tiny LED light level indicators, softly glowing night lights, and tap-on and tap-off capabilities. Perhaps my favorite dimmer switch is the one that has a hand held remote control. You sit on a couch or in a chair, point the remote towards the switch and you can dim the lights while still seated. It is the ultimate couch potato gift!

Point of use dimmer switches are also available. You can purchase a dimmer switch for a table lamp. This device allows you to stop buying three way light bulbs. You can buy a standard higher wattage bulb and use the dimmer to create an infinite amount of different light levels to suit the task. These dimmers are very handy.

Author's Notes:

June 2002


While the older light dimmers had a potentiometer (variable resistor- rotating or slide type) as the method of controlling the semiconductors within them, they never were truly a 'rheostat' as you explain. The power to the light was not directly handled by the potentiometer, but by a semiconductor switch called a triac, which had a control signal provided by the potentiometer.

A true rheostat to work the common load of 600 watts handled by the standard light dimmer would be about half the size of a toaster (which is a 1200 watt resistance load typically) and generate as much heat...really think that would have worked well in a wall box?

It is possible that the writer to the column experiencing the warm dimmers had several dimmers ganged in the same enclosure, and that the proper derating (typically a 600 watt max load for the first dimmer in a box, derated to 500 if there are two, 400 watts each if there are three) was not followed during installation OR someone installed larger or more lamps than the original installation intended.... seen both of these happen in my career....

While some newer dimmers have eliminated the potentiometer in favor of touch plates, and the working components are about the same, the efficiency of the semiconductor devices have improved tremendously, and improved efficiency leads to less loss on the switching which results in lower heat dissipation.

I hope this helps you for future reference. As a former electrician and now engineer, I am alarmed sometimes at what I read from everyone who is an expert on electricity, and am alarmed at what I see on TV in some of these home improvement shows, etc. EVERYONE is an expert when it comes to electrical work it seems. I hope you take this constructively. If I can be of any help in the future, let me know... I love teaching ....


Walt Flasinski, PE

West Chester


3 Responses to Dimmer Switch

  1. I was curious about my dimmer problem and it led me here. While very insightful, I am now wondering if my dimmer switch problem is due to its age. It's an old dimmer attached to a chandelier with around 6 lights. It dims really dim, but suddenly it went bright on its own, like regular brightness. I went to turn it down and the switch was dimmed all the way. I tried increasing it and the lights got abnormally bright. The switch is old but not sure how old. There's a good chance it's 20 years old. Can an old dimmer switch act this way?

  2. Hello. I just replaced six hi-hat bulbs with LED inserts that provide more light. Then I also replaced the light switch/dimmer switch combo (for incandescents lights) with a similar LED switch. The problem was the new switch had an extra terminal. I placed the 2 blacks onto the 2 terminals which were side by side and grounded the green wire.
    The switch had to be inverted to be in the down position to be off; and the dimmer slide is opposite the switch. so the slide is in a down position for full power.What did I do wrong? Or did I replace a single with the wrong switch?

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