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I had a contractor come by my house recently to do AC maintenance for the unit located on my roof. While up there, he mentioned that the conduit pipes that carry the electrical wires were not rated for exterior use and could create problems at the joints. A solution he proposed was to take some silicone caulking and just cover the joints to help prevent water from getting into the pipe.

Does it matter what type of caulking I get, should I just look for a product that is 100% silicon and call it a day, or is there another type of sealant that I should look for?

I would prefer not to have to replace the conduit piping since there is a lot up there (there are 5 other units in my building) and prefer a DIY solution that will help mitigate any problems until ALL of the conduit on the roof can be replaced.

Here are pictures of the joints I am talking about: http://imgur.com/a/xYhhR

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From the Steel Tube Institute (an institute comprised of various steel conduit manufacturers):

Question: Can EMT be used on roof-tops?

Answer: There is no prohibition in the NEC. In some installations, there may be a concern about physical damage or corrosion protection. This is often a judgment call by the authority having jurisdiction. The NEC does not allow EMT to be used where, “during installation or afterward, it will be subject to severe physical damage.” The NEC also requires the EMT to be “securely fastened in place” and provides requirements for supports. The AHJ may determine more supports are necessary. If the AHJ approves the installation with EMT, consideration should be given to compliance with NEC 110.12, 300.6, 300.7(A) & (B), 358.10 and 358.30.

So, if your conduit was installed with fittings designed for a "wet location" (it looks like those are, but confirm) and it is properly supported, and provisions for expansion were provided (if required), then you should be OK. Most importantly, if the Authority Having Jurisdiction (AHJ) approved the installation, then you are fine.

In the end, it would not hurt to apply silicone. Use 100% silicone and clean the area(s) first so it sticks.

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I think the contractor was just trying to get some extra work. Your picture shows watertight compression connectors which have been used for years on exterior conduit, yes EMT is approved for such use. I agree with the @Jimmy Fix-it answer, other than I have never liked the idea of using any type of material, silicon included that wasn't approved by UL for such use and NEC approved for installation.

In short your conduit is fine, It does look a little rusty but is not unusual until a scheduled replacement. Just make sure there are no holes rusted through and the fittings have not pulled apart and you have a ground wire pulled and attached to throughout the circuit.

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