Why not use a plastic receptacle box for an outdoor outlet?

Question

This answer says "... it should be possible to cut into the wall and install a metal box (don't use plastic, please) [for an outdoor outlet]."

What are the problems associated with using a plastic box for an outdoor receptacle?

Answer 1

The problems are not specific to an outdoor location. They are for any use.

Top 10 reasons metal boxes are better than plastic boxes:

1. Metal completes a circuit. So if a hot wire comes loose, it will short and trip a breaker when it touches the side of the box, letting you know there is a problem and becoming safe in the process. With a plastic box, the wire sits loose waiting to bite someone who opens it up without flipping the breaker first. (Or maybe even thinks they flipped the breaker but since the receptacle is not providing any power, they get the wrong one and don't know it.)
2. Metal boxes are generally stronger than plastic boxes.
3. Metal boxes contain arcs, sparks and fires much better than plastic boxes.
4. Metal boxes generally have knockouts for attaching cables, which can be used with any type of cable (e.g., NM or BX) or conduit and with doorbell transformers and other devices. Plastic boxes generally have easy places to connect NM cable (a.k.a. Romex) but not much else.
5. Metal boxes can be mounted anywhere. If you don't have a screw hole in the place you need, drill one. Plastic boxes generally have to be mounted as designed - if you drill a hole in one you are likely to crack it.
6. While many people don't like the look of exposed metal boxes (I don't mind it myself), I don't know anyone who likes the look of exposed plastic boxes.
7. Some metal boxes are designed to be easily combined to create 2-gang, 3-gang, etc. boxes. I have never seen that with plastic boxes.
8. Metal boxes can be (actually must be) grounded directly to ground wires (unless metal conduit is used) which eliminates having a bundle of grounds floating around inside the box.
9. Metal boxes allow for grounding via the yoke for most switches and for better quality receptacles, eliminating the need for a ground wire.
10. Metal boxes can be easily reused (for a new circuit or whatever) and recycled (steel recycling is big business). Plastic boxes get thrown away.

And # 11: Because my electrician (who has been in the business for a long time) and Harper both say so!

Answer 2

UV light from the sun destroys plastic. For regular plastic, it's a matter of years or decades before sun will make it brittle and crumble into dust on a slight touch. You don't want your receptacle box to crumble into dust.

Also, that's more of an opinion than hard rule. There are outdoor receptacles made of UV-resistant plastic, but they're clearly marked (and priced) as so. As I understand this warning, it's aimed against repurposing common indoor boxes for a duty they're unfit for.

Answer 3

In addition to others: temperature.

Living in a climate where hot summer and cold winter temperatures are common and below -40 does happen once a decade, I do not see a plastic box coping with the thermal stress and so become brittle. Metal well handles those weather temperature far better.

Answer 4

If you're talking about a house as a service plug or simply just an outside outlet, plastic is perfectly acceptable. Just make sure you're using a weather-rated outlet and cover.

This is all related to residential work.

Despite what the other answers may suggest, you are going to be mounting them inside of a wall. This means that they are not exposed to UV Light and extreme temperatures. And wild animals will certainly not be chewing on your boxes.