This question wasn't brought up in my recent post. I thought it would've been asked already so I didn't think it was needed to be brought up. I apologize if it's been already asked but my searches have all struck out on this forum.

Are there alternatives to a damage shield for exposed NM-B romex inside unfinished out buildings ? The shed will never have finished walls. I was able to find a post that Harper commented on that he cited NEC 334.15 Exposed Work (C) for unfinished basements and crawl spaces.

I'll be bringing likely THWN-2 Cu into the shed for a 120V 20A branch ckt from a 200A service panel at the house. The run will be 10 AWG 2-wire underground in PVC conduit buried 18" and enter the shed through schedule 80.

I'd prefer to transition to cable and not have to run conduit for damage protection throughout the shed. I want to say it's non compliant to run Romex in conduit but I don't know if that's accurate. An authority having jurisdiction of performing an inspection is very unlikely (extremely remote location). Nevertheless, we'd like to have the work performed to be code compliant.

The place where the Romex could get hit by a tool or whatever is entering and exiting the gang boxes and then it would run vertically up to the light fixtures where it won't have any likelihood of getting banged.

Is a junction box required to transition from THWN to cable ? Or is wiring NM-B sheathed cable within gang boxes compliant ?

Note: The switches and GFCI will likely be mounted horizontally and not vertically.

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2 Answers 2


I can at least answer parts of this:

Is a junction box required to transition from THWN to cable?

Yes. But that junction box can be any of:

  • A junction box dedicated to this purpose
  • A box with the primary purpose of holding switch(es), receptacle(s), light fixtures, etc.

For example, based on the diagram, you could use PVC all the way up to the 2-gang metal box and connect the individual wires to the disconnect switch, connect the disconnect switch to the GFCI/receptacle and connect NM cable to the GFCI/receptacle ("load" screws) to exit the box.

I want to say it's non compliant to run Romex in conduit but I don't know if that's accurate.

It is perfectly OK to run Romex in conduit. It is just not generally advisable because you (a) need large conduit and (b) it is a hard thing to do. That being said, using conduit can be the easiest thing to do to provide protection from damage, in which case you would generally just be running it a short distance, in which case it is easy enough to do.

Are there alternatives to a damage shield for exposed NM-B romex inside unfinished out buildings?

There are really two separate issues here:

  • Where is damage shield needed?

I honestly don't know the answer to that. My house has plenty of (mostly old) Romex unprotected in the unfinished basement and laundry room. (I didn't put it there - electrician, builder, previous owner did.) I don't see it as a problem because it is on the walls and visible so no chance of damage unless you are really reckless - i.e., not hidden in a place where you could accidentally put a nail or screw into it. But it seems like code frowns upon that...citation needed.

But one update based on another answer/comments: rodents. While a shed is presumably dry enough that indoor (NM cable) wiring methods can be used, depending on how well it is built there may be more of an issue of critters of various types getting in than there would normally be in a house. If that's the case then you would really be better off with protecting all exposed cable, not for human damage but for animal damage. Which would mean either conduit (PVC or metal) or using armored cable.

  • What can you use as a damage shield?

Code (I just looked it up) lists a bunch of conduit types and, "or other approved means". My understanding is that generally speaking you can use metal (e.g., a steel plate) or wood (plywood or dimensional lumber) to provide a damage shield where needed.

  • 1
    As far as the questions I asked I believe you answered most. In regard to where the NM-B could need damage shield, my understanding from your comments are the 2-gang box can be used to transition THWN wires to Romex. I won't need a separate empty J-box to transition. Run the schedule 80 up to the 2-gang box, mount the boxes at whatever height they're supposed to be from the floor (horizontally) and then the NM-B exiting the 2-gang box and entering the 1-gang box, then exiting out to change in a vertical direction would be the only runs needed through short (4-6") pieces of conduit. May 8, 2023 at 19:21

Running NM/B in conduit is acceptable. It's subject to being confusing, though.

Where strictly servig as a damage shield, if it fits, you're good.

Where the wiring method is conduit, it has to be treated as a round cable of the largest dimension for fill calculations. Generally that means if you only need one cable, it's not unreasonqable, but if you need 2 or 3 it needs insanely large conduit. And you are still subject to the 60°C limit of the cable as a starting point for derating. And it's miserable to pull. That's several reasons why conduit as a wiring method is almost always done with THHN, THWN-2, etc...

Generally somewhere above 6 or 8 feet depending on the LAHJ it's considered "not exposed to damage."

A simple board can serve as "protection from damage" if it serves that function. Consider that drywall, which is not the strongest stuff, is typical for a that in a "finished" wall. You could cover the space between two studs with screw-attached plywood, (1/4" is plenty) on the "more robust" end of the scale, put your switches and outlets in that stud bay and run your cables up to the roof inside it.

Flexible metal armored cable (AC, MC or MC-lite) is a nice blend of protection (shed - think rodents gnawing) and no need to bend conduit.

Since junctions must be in junction boxes, transitioning from THWN in conduit to NM/B will happen in a junction box (which may also be a switch box/gang box.) Or we could corrupt you to the joy of "not-gang boxes" (4 x 4 or 4-11/16 x 4-11/16 boxes) and "exposed work covers" for holding switches and outlets, which are far more suited to unfinished (thus, exposed) work. No sharp corners to catch, no odd mounting to suit drywall that is never going to happen.

The box in question must be large enough for the wires and devices in it. That just takes some planning ahead to not buy inappropriately small boxes.

  • See diy.stackexchange.com/a/266404/18078 for examples of exposed work covers.
    – Ecnerwal
    May 8, 2023 at 19:06
  • Its going to be something we're going to have to find out regarding any account of rodents chewing exposed sheathing where the run changes to a vertical direction (6 to 8 ft) from the floor. I'll price AC, MC for the owner to decide on that section of the run. I like the idea of the "not-gang box exposed work covers". As far as NM/B in conduit for below 4 to 6 ft sections, it doesn't look like those sections would be too much added effort (short sections between boxes) May 8, 2023 at 19:59

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