Trying to choke NM cable down into EMT conduit is a "newbie idea" which I can understand if you are a rank novice to electrical, and NM is the only kind of cable you know.
Romex is not better, Romex is just cheap. It's legal because builders have a good lobby. The logic is a house fire is confined to one house, so one family, no big deal. Whereas an industrial plant fire can hurt dozens and does huge economic damage, so they must use metal conduit with THHN inside.
But you are absolutely correct that the oval Romex cable needs to be treated same as a round wire of the large dimension. To fit 1 wire or cable in conduit, the conduit inside diameter must be 138% of the widest width of the wire/cable. (3/4" for #12NM).
2 cables in 1 conduit needs a pipe 254% of cable width (so 1-1/4" for #12NM). 3 cables, pipe must be 273% of cable width (1-1/2").
Mind you this only applies when using conduit as a wiring method. Random sticks of EMT are often used as a damage shield in the NM cable wiring method, but they're just used as a random pice of metal - could as easily be gas pipe. Don't be confused when you see that.
THHN wire is absolutely the right stuff.
And better in every way. Once you get used to working with it, you're going to love it.
One thing you may be overlooking is that "2 THHN wires inside conduit" is NOT simply an alternate form of Romex. Conduit is conduit. You can put anything you want in it (up to fill and thermal limits). That means you probably have parts of your conduit network with 2-3 circuits in them. That's fine, up to 4 are allowed in 15-20A circuits. (but obviously that is not gonna work with Romex lol).
And, you are free to add additional circuits anytime you want.
#14 or #12 THHN?
#12 is allowed everywhere #14 is allowed.
#14 is slightly cheaper per foot - but that means the up-front cost of buying 2 extra spools of #14. Is the "savings per foot" worth the up-front cost of those extra spools? Absolutely not. When you're a "small project guy" like you or me, you'll never save enough on the per-foot cost to recoup the money sunk into those extra spools. You're better off just sticking to #12 for everything. That's what I do.
Neat side-effect: if all the wiring in a circuit is #12, you can breaker it at 20A instead of 15A. (15A is also allowed).
Solid or stranded?
That's a wobbler. Stranded wire is an absolute pleasure to pull. It's so great to work with, and stranded is the only stuff I own. However. Stranded is a royal pain in the butt to put on receptacle and switch side screws. I've mastered it, but it's VERY hard to get right and I don't recommend it. So that's a point in favor of solid.
Solid is very stiff and balky to pull, as you would imagine. It loves to kink and hang up, and must be carefully watched at the entry point so it doesn't get a tight kink that could crack and damage the wire. I use it a little, just to use up old scraps, and even one wire in the bundle makes pulling worse. UGH!
But here are a couple of workarounds that will allow you to use stranded anyway.
- You can "pigtail" all your receptacles and switches with solid wire. It's perfectly fine to join solid with stranded in a wire-nut or a Wago lever-nut. I use wire-nuts myself. Line them up even and push the stranded out another 1/16" or so.
- $3 spec-grade receptacles and switches, which have a "Screw-and-Clamp" feature where you insert the wire into a back hole, and tighten the screw to clamp. These play well with stranded wire. Also, spec-grade receptacles have Self-Grounding so no need for ground wires, ever, with them in metal boxes.
I own the following:
- #10 stranded THHN red, black, white
- #12 stranded THHN black, brown, red, orange, yellow, blue, purple, gray, white, and blue w/ red stripe (found it at a Habitat for Humanity store). Wishlist: pink.
- Colored electrical tape: black, brown, red, orange, yellow, green, blue, purple, gray, white.
So I walk my talk. I use the 10 colors to distinguish circuits from each other. My boxes get really busy, and "all browns together, all white-blues together, all purples together" is the only way it can be maintainable by humans.
What I recommend for you:
- #12 black, white (you pick solid or stranded)
- #10 and #8 bought by-the-foot as needed
- 10 colors of electrical tape or shrink tube (better)
Make liberal use of colored tape or shrink tube to identify circuits to make maintenance easier and to satisfy Code requirements of identifying which neutral goes with which hot. The rule is: is always mark both ends of the wire exactly the same.
Also, marking a wire a color does not change its basic function. Sorry, you can't re-mark white THHN to make it a hot wire :) So you will need to buy a spool of black lol. Upside: you can mark whites with colored tape to identify circuits. Yay!
If you want to use alternate native wire colors like I do, gray is alternate neutral, and hot is anything except white, gray or green.
Oh, and if you want ground wires, you'll be salvaging lots of ratty old wire. Strip the insulation off, instant ground wires. Copper does not degrade (it's one of the very few metals found in the earth in natural metal nuggets).
Pulling is straightforward, all boxes should be accessible
Code requires that with EMT conduit, all necessary "pulling points" remain accessible forever. So they should be
- the junction boxes that already have lights, switches or receptacles in them
- in unimproved space like basement or attic, junction boxes there
- "conduit bodies" at sharp corners which have removable lids.
So everywhere you need to access to do pulling should be accessible. Any corners not accessible need to be sweeps, and no more than 4 sweeps between pulling points.
The only problem is that sometimes people do improvements, and the remodeler does not respect the electrical code and buries boxes.
You can "test pull" any given wire to make sure it is able to move, while watching other boxes to see where it's moving from. If it's frozen, look for a conduit body undiscovered or a box it's passing through at an angle.
As far as pulling wires in using existing wires, either do 1 wire at a time or all the wires in the pipe. Trying to do 2 wires out of many isn't going to work since they probably weave with other wires.