Do I need outdoor-rated cable for an exterior receptacle?

Question

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?

Answer 1

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.

Answer 2

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).