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I'm going to install an exterior receptacle by running a non-metallic (NM-B) cable from an interior receptacle. I will be installing it just on the outside wall right behind the interior receptacle. The cable will exit through a hole in the exterior stucco and into a metal box made for outdoors. Then I will install an exterior rated receptacle with a receptacle box.

I will use duct seal (kind of like a gray putty) to seal around where the stucco hole meets the exterior box hole. I will also take the proper precautions to caulk the perimeter where the exterior box meets the stucco.

Will I need an outdoor/exterior (UF-B) cable to run to the outdoor receptacle?

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I have heard debates on this subject. It's very common to do just as you say, run indoor rated NM through the wall to a surface mounted exterior box.

In my opinion, when you come through the sheath, you're outdoors, and it's not compliant. Only for inches, and I would be really surprised if it ever amounted to anything, but still not compliant.

If you want to satisfy the sticklers, you could transition from NM to UF in a box inside before the cable goes through the wall. This also gives you some protection where the cable goes through the wall.

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    I did precisely this. In the "inside" box I had NM-B, then transitioned to UF-B for the 1 stud hop to the "outside" box. Odds are really good that I wouldn't have if I didn't have some UF-B leftover from wiring up the deck outlets, though. Hangs head sheepishly
    – FreeMan
    Sep 29, 2020 at 14:02
  • I keep THHN around since it has so many uses, like this one.
    – Jeff Wheeler
    Sep 29, 2020 at 16:09
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    Would you recommend drilling the hole so that it slopes down towards the outside, to help keep water out, and put some plastic pipe in the hole to protect the cable?
    – Andrew Morton
    Sep 30, 2020 at 11:48
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    @Andrew Morton, I slightly pitch holes through masonry that way, definitely, through wood framing, it can't hurt, but it seems less effective. Installing a short conduit between back to back boxes is definitely the deluxe solution, and no big deal if you have the materials on hand, but you may needlessly complicate the job if you're not practiced at it. If you don't get it just right, it won't set flush, and it's more prone to seep.
    – batsplatsterson
    Sep 30, 2020 at 18:04
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NEC is a bit vague here 334.12(B) says

Types NM and NMS. Types NM and NMS cables shall not be used under the following conditions or in the following locations:
(4)Where exposed or subject to excessive moisture or dampness

So batplasterson is right that it's not technically correct. The box is a wet location.

The rule mostly exists so you don't expose the whole cable to a wet location. I had an idiot once put NM cable in a buried conduit (which had standing water in it when I dug it up). NM sheathing is not designed for that and once the water penetrates it it could get to the bare ground (which was improperly being used as a neutral in a 240v setup).

You're not really exposing the cable to anything transitioning to the exterior box (and you're probably not running active voltage over the bare ground either). The simple fact is that you're going to strip the sheathing back either way, which means you'll have wires hanging out without that extra-thick UF sheath even if you go that extra mile. The only benefit is that if water penetrated all the way to the back of the box, it could penetrate the end of the NM cable. That might necessitate replacing the NM cable, but that chance is exceptionally small if you simply make sure there's no ingress points for water (which you should do regardless of the cable type).

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