For space-saving reasons, I have outdoor breaker panels, and most of my branch circuits are wired through the attic down to individual outlets. Currently, the wires run through the wall from the breaker panel into the attic, but it is getting difficult to pull any more wires through.
My service wires actually enter from under the panel, and there is a ~2" entry at the top of the panel. What I would like to do is run my branch circuits through 2" SCH 40 PVC conduit up from the breaker panel on the outside of the building, and have one 90° bend to go through the wall and into the attic. The total length of the conduit run would be less than 10' and none of it is underground.
I would like to know if I can run NM-B (Romex) through this conduit from the breaker box legally. For one of the panels, this will consist of a maximum 12 individual 12/2 cables. For the other panel, this will consist of a maximum 4 10/3 and 2 12/3 cables.
I believe the conduit is appropriately large enough for the number of conductors, but I've seen other answers regarding running NM-B through conduit where conduit length is a concern.
NM-B cannot be in conduit that's outdoors. Outdoor conduit is considered a wet/damp location, and NM-B is not allowed in wet/damp locations. Aside from that, you'd have to do conduit fill calculations, to make sure the conduit is large enough.
This other answer might be helpful.
TLDR: Putting many wires in a conduit requires you vastly oversize the wire. Better to use many conduits, limiting each to four 14-12 AWG circuits or three 10AWG circuits.
In one box, you propose twelve 12/2 cables in a single conduit. That is 24 conductors (grounds don't count). Because you have so many, NEC rule 310.15(B)3 requires you de-rate the conductors at 45% (per table 310.15B3A)
and per whatever type of cable you are in fact using on table 310.15B16:
For instance if you aim to run 20A, and use THWN-2 or USE-2, 10 AWG wire is 40A x 45% = 18A -- not enough, you must go to 8 AWG wire.
If you're asking "why haven't I heard of this before", it's because the way the numbers work out on 310.15B, the de-rate rules generally don't bother 15-20A circuits with 9 or fewer conductors. It may be a factor for 30A circuits or larger. Grounds are not conductors, and neutrals are not counted in 120/240V or MWBC circuits, since they only carry imbalance current. So any circuit in single-phase or split-phase tends to count as 2 conductors, allowing 4 circuits.
Now that these wires are getting burly, you'll need to re-visit the normal conduit fill calculations appropriate to the type of cable you use (jacketed cable like USE-2 takes a lot more space than single wire THWN-2).
Since your boxes are outdoors, everything needs to be listed for wet locations. You cannot use NM cable. You would need cable listed for outdoor use, e.g. USE-2. If you use UF or USE cable, go back to 310.15B16 because there's an additional derate, for instance necessitating 6 AWG if using UF.
Since you can't stick 8 AWG cable on a receptacle terminal, you will need to transition to 12 AWG wire somewhere. It could happen in the receptacle box, but it'll take a lot less overall wire wrestling if you do it before that. However this must be a place where you've split to less than 4 circuits per conduit, bundle, or hole. Also, it must be inside an electrical box which remains accessible. You can't put an electrical box anywhere that disassembly of the building is required to access it. Behind a little door is fine; a panel screwed down is not.
The best way to avoid that derating is numerous conduits, so you are not exceeding the nine practical limit for #12 and #14, and the six practical limit for #10. For instance, five conduits will barely get you by: Two conduits for the four 10/3 and two 12/3 - and three more for the twelve 12/2. You will be full-up at that point, and would need to run more conduit to add anything. I would consider 8 conduits. Since you're already doing six, doing two more is no trouble. You're already there, have all the right tools and supplies, and by #5 you're getting pretty good at it.
Keep in mind conduits must have a certain spacing in order to count as separate conduits. That is for air circulation, as the name of the game is to cool the wires and conduit.
I would run it in the 2" pvc and not worry about it. This was standard practice for many many years without any problems. Now the earlier posts are correct about Nm not being wet rated, however all NM now has THHN wire inside the outer jacket and the IS wet rated. It is a stupid law some code panel decided on. That is my opinion from being in the trade 40 yrs.
I would consider opening a second unused bay in the Sheetrock wall up to the atti so that you can run additional cables easily.
Remove the SR, install an access panel where a 90 turn is required, replace, tape and paint the SR to original condition.