THHN through EMT - then convert to Romex?

Question

I want to wire outlets and light in my unfinished attic. There is a light switch at the bottom of the stairs, cable (I think THHN?) that runs through EMT through the wall and comes out upstairs and then connects to the light.

Since the attic will eventually be finished, I don't want to add any more conduit (and will probably get rid of the EMT that is exposed upstairs). How do I convert the THHN to Romex? I'm confused about what to connect to what.

Also, I'm trying to wire a three way switch (one switch at the bottom and one at the top of the stairs). Is this going to be possible with the current wiring that's in the EMT or do I need to switch it out for something else?

Answer 1

You can transition wiring methods at a junction box. Use the EMT wiring method up to the junction box (whole nine yards with fittings and clamps). Then use the NM wiring method beyond it. The junction box must remain accessible without screws, nails or demolition (other than the ones on the junction box lid, of course). If you don't like the aesthetics of a blank junction box cover, then stick a receptacle there lol, or better, extend the conduit a little bit to a place you want a receptacle, and use an extra-large box so you have room for the splices. Then put a receptacle there fed off one of the circuits.

You simply splice wire to wire, except for grounds. Normally there isn't a ground in the EMT conduit, the conduit is the ground. Any junction box will have a #10-32 tapped hole and you fit a cute little green ground screw there with a pigtail. You tie the pigtail to the Romex grounds.

I get where you're a Romex guy and EMT/THHN feels alien to you. I strongly recommend you "get acquainted with" this new wiring method. It is actually rather versatile. For instance, a conduit can take up to 4 circuits - or 8 with multi-wire branch circuits! All circuits can share 1 ground wire, and if the route is entirely metal conduit and metal boxes, the conduit is the ground! It's a fantastic system.

That the attic will be finished is precisely why to think about extending conduit. Conduit lets you alter the wiring later depending on your usage. Find it's too hot and want a window air conditioner, but a 120V one would overload the circuit with your PC/laser, and you'd prefer a 240V unit anyway? Easy peasy - pop a few box covers off, pull two more wires, instant dedicated circuit. With stranded wire, pulling is easy. I do it all the time.

Answer 2

The advice to keep the EMT is good and makes sense, but I can think of a factor that might motivate its removal. If the EMT is surface mounted in the attic then it'll be in the way of any kind of wall finishing you might be planning for the future. It would make a lot of sense that you'd want to convert to NM cables installed inside the wood framing.

The transition between discrete conductors (THHN in EMT method) and NM cable is easy. Feed the NM cable into a junction box and connect the conductors with appropriate connectors. The NM ground would connect to the ground wire already in the box, if any, as well as to the box itself. The NM needs to go through an appropriate cable clamp and the box needs to be sized according to the number of conductors and other devices (light switch/receptacle) that might also share the box.

For the 3-way light keep in mind that all the conductors must be in the same cable or conduit -- you can't use a mix of the two methods for the one circuit. Pull more conductor(s) into the conduit, or if the conduit must be removed to accommodate wall finishes, you'd most likely replace it with a "/3 with ground" NM cable going between the switch locations.

Answer 3

I agree with the comments suggesting that you keep the conduit, but in general you'd do the conversion at a junction box. If there's a conveniently-located outlet, do it there to avoid having a blank box plate forever. (All junctions need to remain accessible.)

Just bring your conduit into a suitable box and convert to cable with wire nuts of the appropriate size. Use a strain relief for the cable and begin stapling it within a foot of the box.