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Today my TP Link Kasa smart switch started acting strange (turning on and off randomly) - it is broken now. When I hit reset button, a few drops of water dropped out. I removed the plate, pulled the switch out and noticed water inside.

I am in Illinois so my electrical wiring is running inside metal conduit (EMT). When my wife turned on hair dryer I noticed some extra water dropped from the EMT into electrical box. Water is dropping directly on the switch and through the ventilation vents (in red) going inside the switch and of course burns the switch.

I went to the attic and everything is dry there, so my guess is that it is condensation and when there was power consumption heat just increased the effect of that.

It is now around 30°F outside so not yet very cold. I had switches installed more than 1 year ago and they were working without any problems last winter when it was colder.

Do yo have any thoughts? Should I add some kind of insulation to conduit in the attic?

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  • Ahhh... formatting and capitalization...
    – FreeMan
    Jan 9 at 16:08
  • "I am in Illinois so my wiring is inside EMT"? Does Illinois have a state law or have state code requiring residental wiring to be in EMT? Jan 9 at 16:36
  • Have you tried drying out the switch with a vacuum cleaner and/or a hair dryer? Is the switch permanently inoperative? Jan 9 at 16:41
  • @JimStewart I don't think it is the entire state but rather some significant parts including Chicago. Jan 9 at 18:45

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Weep Holes

NEMA (as in "NEMA 5-15 receptacles", that NEMA) has some guidelines on this online. Basically they recognize that electricians drill drainage openings (weep holes) in conduit bodies and junction boxes and are officially sanctioning the practice:

  • Minimum 1/8" (I think smaller is ineffective), maximum 1/4" (bigger holes can cause other problems)
  • One per conduit body or box (exception: divided - e.g., to separate two circuits or to separate low voltage from mains - can have one hole per section)
  • Always in the bottom and not too close to an edge or corner
  • Only in boxes listed for damp or wet location - if it isn't listed for damp or wet location and it is getting wet then you have bigger problems

Insulation may help some, but keep in mind that condensation (or other water) can travel a long ways, as long as it keeps moving down. In addition to temperature, you also have to consider humidity. But some condensation may just be inevitable and in that case weep holes are the way to get rid of it.

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    Duct seal should work, but only at the right place. You don't want water collecting inside any junction box, so that means not sealing the box but sealing the entrance(s) to the box - i.e., the conduit where it meets the box. Depending on the amount of dripping, a plastic cup placed on top of the TP LINK under the drip may do just fine - i.e., if it takes several days to fill up and you periodically empty it. On the other hand, if it fills up every day then you really want to see if you can track down the source of condensation to reduce the amount of water. Jan 9 at 15:58
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    And now I see you're talking about a TP LINK switch, not an external TP LINK device (e.g., a TP LINK router). So yeah, a drop hole won't quite do the job. Except you might be able to get away with a strip of electrical tape across those holes so water can't get in and eventually makes its way down to the bottom to drip out of the box. But since the TP LINK itself is not designed for wet areas, better to see if you can block water from getting into the box - duct seal - or find the original source. Jan 9 at 16:02
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    What special switching does this TP LINK do? Jan 9 at 16:47
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    thanks. I am going to use duct seal. @JimStewart - it is just smart switch, it is a bit bigger and less robust for wet environment.
    – Arek
    Jan 9 at 18:44
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    Is this smart switch designed to allow separate switching of a light and an exhaust fan in a bathroom with wiring that only has a single switched hot? If so, given that this wiring is in conduit couldn't you just pull another wire and use a mechanical double switch? Jan 10 at 12:34
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Water will condense inside conduit in cold areas that have openings to heated living spaces that will have higher moisture content. In such cases, duct seal putty is in common use to seal the openings to conduit at boxes and cabinets.

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