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My grandmother called me the other day to troubleshoot from half way accross the country. She installed a three way dimmer switch in her dinning room at the location most used. It turns the light on and off, but does not dim. It is an incandescent fixture and it has incandescent candelabra lamps. Even as a wireman I have been racking my brain to figure out what else it could be. After reading some other Q&As, I noticed some people suggested that a dimmer should be on the line side, doesn't the power need to go through both switches? And, therefore, shouldn't it dim regardless? I suggested she try to find dimmable bulbs as the first, easiest, and cheapest attempt to solve the issue. If it does matter which side the dimmer is on, would it help at all to have a dimmer on each end? TIA

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    Does she have a lifetime cache of incandescent bulbs squirreled away somewhere? Or will be swiching to advanced tech lamps at some point in the future? Those are the options available, there is no third. The dimmer choice needs to account for that, or it will need to be replaced eventually. Modern dimmers often need neutral, which may be the issue. Oct 27, 2017 at 18:20
  • My 2 cents: if the dimmer turns on/off but doesn’t dim, either the load is non-dimmable or it’s a bad dimmer.
    – Tyson
    Oct 27, 2017 at 19:05
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    Is it possible that she has a smart switch with some brains in it as the other 3 way switch? My other guess is if you add a dimmer to the lights on a ceiling fan, sometimes ceiling fans have their own dimming controllers installed to work with a remote control and that would make the same symptoms.
    – Dotes
    Oct 27, 2017 at 19:59
  • Can you have her send you photos of the insides of the light boxes? Oct 27, 2017 at 22:13

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I know it sounds pretentious, but it really does depend on what dimmer or set of dimmers or dimmer kit you buy. If you're a wireman you can probably go online to a manufacturer like Lutron and look up a set of specification sheets to help you select a proper dimmer.

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Replacing a 3-way switch with a dimmer should be an easy project. It doesn't matter which switch gets replaced as long as the three wires are hooked up correctly. If it's wired incorrectly, the lights will not turn on and off as expected (maybe only one switch will work, maybe nothing will work).

So, turn the dimmer up all the way, and turn each switch on an off. Do the lights turn on and off as expected? If that works, but the lights do not dim, it's either because the bulbs/light fixture are not dimmable, not compatible with the dimmer you selected, or in rare cases, the new dimmer could be bad.

If the light bulbs are not just plain incandescent bulbs, I'd bet they are not dimmable. Right now, dimmable LED and CF bulbs are common, but in the past they were hard to find or unavailable.

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    If you are installing a smart/home-automation dimmer, these are odd beasts and each manufacturer has their own constraints and quirks. There are ways to set them up as three-way, but some require another smart switch at the far end, some require the dimmer be the switch at the beginning of the branch circuit, and gods only know what else. Download and read installation manuals before sinking money into these; they're great once set up but setup is not usually a direct replacement.
    – keshlam
    Mar 7, 2023 at 23:58
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The problem could be an older wired home with incorrect neutral switching. She needs a professional electrician to troubleshoot the problem and not some handyman and make the necessary corrections. A good electrician will have the problem diagnosed the first ten minutes he is there.

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