I've got an electrical wiring question. I've got a 100 amp sub pannel close to the center of the house with an EXTERIOR 200 amp pannel of the west end near PSO FEED. I need to add a 30 amp recep in garage on the east end of the house. There isn't room in the sub pannel to add anything. Question is..if I have to run full length of house to 200 amp EXTERIOR main panel, can I run 10-2 w ground Romex in the same 10' of rigid conduit from the pannel to the attic, or do I have to add an additional conduit for this circuit.... Currently the only wiring in the exterior conduit is the feed to the sub panel.
It's a little tricky. You will get mixed interpretation about NM cable (including Romex brand) in a raceway, if you determine you can then you calculate the resulting percentage of conduit fill based of the size of the NM cable assembly as a single conductor, but bottom line just don't do it (unless you raceway is just a short protection sleeve), it's much easier to pull #10 stranded THWN wire than a large solid wire NM cable through a conduit that already has wire in it.
Then with 4-6 current carrying conductors get de-rate the ampacity of the wire to 80%, so your existing wire may become under-rated.
NEC 312.8(A) Restricts pulling conductors through a panelboard cabinet to a maximum 40% cross section fill for conductors, and 75% fill for conductors and splices. A label must be attached to indicate closest disconnecting means for through conductors.
Change the subpanel
A full panel is an intolerable situation. There is no way any house should ever be "out of breaker spaces". Normally that is a hard problem because it involves either changing a main panel (yikes) or adding a subpanel.
But simply upgrading a subpanel is well within the range of DIY. The supply breaker can be shut off in the main panel, which renders everything safe. You can snap "before" photos so you understand what's going on. You can post them here so we can catch legacy mistakes that you can now correct.
If you are clever and use a new subpanel compatible with the old, so you can reuse the breakers. A subpanel swap (for space) doesn't force you to install all GFCI/AFCI breakers or such upgrades.
Spaces are cheap. Be lavish in your oversizing, so you never have to do this again. I like to finish panel projects with 50% of spaces available.
Nope. Nope. Nope. Nope. You need to use outdoor rated cable such as UF... or outdoor rated wire such as THWN-2.
Cable (Romex or UF) in Conduit
It's not ... forbidden... it's just not how the pros do it. 99% of the time when people propose this, it's merely because they did not realize that other types of wire exist.
The preferred situation in conduit is individual 1-conductor wires - such as THWN-2 or XHHW.
You get two issues anytime you add wires to conduit.
The conduit is only allowed a certain physical fill. You're not allowed to overstuff. Flat cable (e.g. UF or most NM) counts as a circular cable of the largest dimension, because it typically twists (and has to be pulled through). This means it takes a lot of space in the conduit. It is quite likely there is not enough physical space in the conduit to accommodate 10/2 UF.
However, individual THWN-2 wires will probably work just fine. Probably ; you have to do the math. That's why we're waiting on numbers from you!
Conduit thermal derate
Too many wires in a conduit will make more heat than the conduit can get rid of. Because of that, wires are de-rated when 4-6 wires run together in conduit. Grounds don't count; nor do neutral in some situations. The derate is 80% of the maximum thermally allowed current for the wire. The 10 AWG THWN-2 wires will be fine. However we need to know about the wires that are presently in the conduit, what size and type of jacketing they have, as that decides the thermal limit.
There is no simple answer to this that we can give you from afar. Conduit has maximum "fill" values; the amount of annular space inside that can be occupied by wire. You must know the size of the conduit, the physical sizes of the existing wires in it now, and the physical sizes of the new cable that you want to install. Then look up the fill rate for that conduit and see if your total annular space left over from the existing wires is enough to allow the new one without violating that fill value.