Electrical query

can't speak to rules and regs, but I've done it effectively many times...

silicone is your friend.

Yes you can run ROMEX in the conduit as long as it is sealed on both ends or like you said goes from dry to dry location and NO water gets into the pipe. Silicone isnt a good sealer because over time it will shrink or decay and the water will get in but I think in your case this isnt what will happen. As for the thermostat wire you can run that in the pipe with the Romex as long as the insulation on the thermostat wire is rated for 600Volts like the romex. Look on the jacket of the wire it will tell you what the rating is. Telephone wire and CATV Cable wire isnt allowed in the same raceway as electric wire because of the insulation rating. Also make sure you dont pull on that romex to fast and burn the jacket from friction while pulling the wire through the pipe. Anymore info just ask.

Why does it matter if the thermostat wire is rated, RSS? It's only ever carrying 18v or so.

That's what it states in the NATIONAL ELECTRIC CODE BOOK, The insulation has to be the same as the romex which is 600VOLTS. Just look at the jacket on the wire it is written right on it, plus at times thermostat wire is used for more than low voltage like motor controls and commercial installations. The 18 gauge wire that thermostat wire is is used in Florissant light fixtures, heating and cooling controls all kinds of stuff and can be used right up to the rated voltage of 600 Volts

Thanks for the info, RSS.

Too late. She's in the pipeline.

Would that be because of voltage surges? Does it have to do with the insulation's ability to withstand temperatures caused by the flow of higher voltage?

Higher voltage doesnt cause for higher temps, now if you put telephone wire in with the line voltage or cable wire you will pick up interference from one to the other and if its the phone line you can wind up with a HUMMM on the line.

Allowing that it is code, I still don't follow the reasoning. This particular wire will never see 600volts. It will never see 50v or 120v, unless there is a meltdown of the insulation surrounding the 12/2 and the insulation surrounding the 18ga wire.