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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.

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    What size is the conduit? What size conductors are the existing wires in the conduit? – longneck Dec 20 '19 at 19:44
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    Yes, we'll need to know the size of the conduit and the size of each of the existing conductors, and whether there is a junction box at the top of the conduit run. – Harper - Reinstate Monica Dec 20 '19 at 21:05
  • Yes, what size and type is the conduit, and what size and type are the existing wires in there? Also, hint: if you ever find yourself trying to jam NM or UF down a conduit, STOP, step back, and consider that maybe there might be a better way to tackle the problem instead. (Remember that folks in Chicago get along with conduit just fine!) – ThreePhaseEel Dec 20 '19 at 23:45
  • Is replacing the subpanel with a bigger subpanel an option, by the way? – ThreePhaseEel Dec 20 '19 at 23:45
  • Just to be sure - you are not talking about the service riser, the pipe with the weatherhead that the overhead wires from the pole come down, are you? – batsplatsterson Dec 21 '19 at 3:44
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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.

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  • De rating will not be a problem with nmb or thhn as you can use the 90 degree table if the conduit is large enough , but it would be silly to use Romex in my opinion harder to pull & will require larger conduit than thhn. – Ed Beal Dec 20 '19 at 20:37
  • I didn't suggest the NM rating would be a problem, but couldn't from the information given calculate every type of conductor and insulation, so I felt best to advise that the existing cable needs to be considered. – NoSparksPlease Dec 21 '19 at 3:32
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    Notice no down vote , not enough info provided I agree. But many don’t understand derates and that’s what you lead with other than fill and we had no info there. NM can be used in conduit but really 3/4” for a #12 if I remember correctly. – Ed Beal Dec 21 '19 at 3:45
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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.

Romex Outdoors

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.

Conduit Fill

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.

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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.

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