I have a new workshop and I'm running 6/3 out to it. My run will need to go from my existing panel through the attic, down an external column (with normal housing siding) and then into an underground conduit.

I understand for the conduit portion of the run I should use THHN but do I need to change to romex for the attic part? Seems I should be able to just continue with the THHN, correct? I know THHN is unprotected but its not like its going through normal walls and it will basically be "inaccessible".

  • 1
    I don't think I'd run wires outside of some form of raceway. Whether it be a plastic jacket, conduit, cable tray, etc.
    – Tester101
    Commented Jan 19, 2014 at 13:42
  • Not sure how far the run is, but you'll want to make sure your wire is sized appropriately for the distance. It also sounds like you're feeding a separate building, so you'll want to make sure your grounding is appropriate.
    – Tester101
    Commented Jan 20, 2014 at 11:16

2 Answers 2


Run it in conduit. NEC 300.3(A) prohibits THHN (or any other type of single wire save for a few exceptions such as single conductor MI and SE/USE, overhead wires, or an EGC) from being run outside some sort of raceway (conduit, surface raceway, cable tray, etc):

(A) Single Conductors. Single conductors specified in Table 310.104(A) shall only be installed where part of a recognized wiring method of Chapter 3.

  • would a portion of thhn run as a feed to a sub panel need to be in conduit of it were entirely enclosed within a wall cavity?
    – jKlaus
    Commented Apr 22, 2023 at 19:47
  • @jKlaus yes, yes it would Commented Apr 22, 2023 at 23:01

Run it in conduit the whole way.

Attics are susceptible to rodents, especially the cute ones with fluffy tails. They chew on wires. Conduit is not that expensive, and can save a whole world of trouble.

I have seen way too much romex chewed down to the copper to trust romex where rodents could be - which is going to cause me some hassle and expense. But I hate burning buildings, especially if they are mine.

  • 1
    By that logic, all the wiring in the walls should also be in conduit.
    – Tester101
    Commented Jan 19, 2014 at 13:27
  • 2
    @Tester101 I agree conduit in an attic is overkill in most cases, but if there is a rodent problem, it is likly to be more sever in the attic than in the walls. Wall infestation is more commonly mice than squirrels or raccoons.
    – bib
    Commented Jan 19, 2014 at 16:46
  • 1
    Told you it was going to cost me some hassle and expense. I'm not in Chicago, but "Chicago code" makes more sense the more I see chewed-on cables.
    – Ecnerwal
    Commented Jan 19, 2014 at 19:02
  • Thanks everyone. Personally I think conduit in the attic is a bit overkill although it actually would save me money (at the expense of my time). I found 6 AWG THHN for 51 cents a foot and the 6AWG NM-B is $2.79 a foot! I'll probably go with the original plan - romex in the attic and THHN in the underground conduit. I believe this is what the code calls for anyhow
    – Jason
    Commented Jan 19, 2014 at 21:37
  • 3
    Only if the THHN is also TWN - anyway, one of the waterproof grades. If it's only THHN, you can't put it in an exterior conduit, which is a wet location. But most such cable is multi-graded.
    – Ecnerwal
    Commented Jan 19, 2014 at 22:43

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