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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?

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    It sounds like an opinion. Metal boxes are usually more robust than plastic. Metal boxes tend to be easier to ground. Plastic I feel will tend to be easier to crack with the use of the weather cover. Do not know enough code to say if that plastic is not recommended for outdoor use.
    – crip659
    Sep 7, 2022 at 0:58
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    As a data point, all of the outdoor/weather rated boxes (surface mount, not flush mount) that I've seen at my local retailers are metal. The covers are often plastic, but those easily screw on/off so they are much easier to replace should they break (no messing with wiring).
    – FreeMan
    Sep 7, 2022 at 13:07
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    As a counterdatapoint, I have never seen a non-plastic outlet or box, either indoors or outdoors. Metal outlets seem to be a largely US phenomena. There should be no issues associated with using a plastic box for an outdoor receptacle.
    – BrtH
    Sep 8, 2022 at 8:42
  • @BrtH in domestic use in the UK there are loads of metal boxes, you just don't see them. The ones buried in the wall are usually metal. Metal faceplates are available - some people like the look of brass plated ones and surface-mounted all-metal is quite common in industrial settings. Our outdoor sockets, like yours, are tough plastic
    – Chris H
    Sep 8, 2022 at 13:34

4 Answers 4

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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!

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    Mixed plastic recycling is a joke. I do it because I think it is a good concept and helps a little. But aluminum and steel can be extremely well recycled into new products. Plastic can't (cost effectively) except in some single plastic types. Mix all the plastic together and it doesn't get back to the original fresh stock. It ends up as lesser quality products. Sep 7, 2022 at 13:10
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    @FreeMan going in the recycling stream also doesn't mean it is recycled!
    – user253751
    Sep 7, 2022 at 13:16
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    Point 1 is only true if the metal is securely grounded. If not, a loose hot wire touches the metal enclosure and makes the entire metal box hot, including any other metal parts further in the chain. A disconnected hot or neutral is easily noticed, a disconnected ground isn't. This is the reason metal enclosures are not allowed here in NL.
    – Pelle
    Sep 7, 2022 at 14:23
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    #12 Depending on where you live, certain wildlife can easily chew through plastic. Very few creatures can chew through metal. Sep 7, 2022 at 19:19
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    Point 1 is why I don't like metal boxes. I don't know if it's grounded correctly, so I don't even want to touch it. I know a loose wire can't shock me through plastic.
    – user19565
    Sep 7, 2022 at 20:53
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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.

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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.

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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.

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  • "wild animals will certainly not be chewing on your boxes" at least not until they've chewed through the plastic cover.
    – FreeMan
    Feb 16, 2023 at 13:32
  • Get a metal cover then you’re wildlife proof. Also boxes are inside of the walls, very hard for animal to get their jaws around.
    – RatTent
    Feb 16, 2023 at 14:26
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    Many exterior boxes are surface mounted. Particularly if they're added after the fact (as they are on many older homes).
    – FreeMan
    Jun 23, 2023 at 16:33

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