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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?

manassehkatz-Moving 2 Codidact
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KJ7LNW
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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 07 '22 at 00:58
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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 07 '22 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 08 '22 at 08:42
  • @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 08 '22 at 13:34

4 Answers4

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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!

manassehkatz-Moving 2 Codidact
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  • Wonder how many years plastic boxes are go for, before deteriorating? Metal boxes almost forever if not in high humidly. – crip659 Sep 07 '22 at 01:08
  • @crip659 That's the one thing I would *not* worry about. The big problem with plastic waste (see # 10 - metal boxes can be easily recycled) is that it lasts "forever", particularly if protected from sunlight, UV radiation, etc. – manassehkatz-Moving 2 Codidact Sep 07 '22 at 01:15
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    @crip659 I have high humidify facilities and they do alright there too. Also with metal conduit, grounding *just happens*. Metal boxes *will* get snatched out of the waste stream because just about every garbage handler these days runs ferrous and non-ferrous separators to grab iron stuff and fling out other metals using eddy current something something. Even if exposed to UV, plastic boxes just degrade to microplastics and end up everywhere, even in our bloodstream. The only thing that reduces plastics is fire. – Harper - Reinstate Monica Sep 07 '22 at 02:58
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    This answer has convinced me! – Fattie Sep 07 '22 at 11:50
  • The plastic boxes can and should go into the recycling stream. That doesn't mean that they do. – FreeMan Sep 07 '22 at 13:06
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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. – manassehkatz-Moving 2 Codidact Sep 07 '22 at 13:10
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    @FreeMan going in the recycling stream also doesn't mean it is recycled! – user253751 Sep 07 '22 at 13:16
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    That is very true, @user253751, which is why I don't really bother too much with it. – FreeMan Sep 07 '22 at 13:18
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    @FreeMan if you don't put it in the recycling box then it's *definitely* not recycled, so you should put it in the recycling box. – user253751 Sep 07 '22 at 13:24
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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 07 '22 at 14:23
  • @Pelle True. But NEC/standard US/Canada practice is that metal boxes are always securely grounded with a wire to a screw or with metal conduit. To the extent that I have found several metal boxes in my house where the grounding was done improperly (at least by today's standards) but was still a good solid ground *despite other problems such as hot/neutral reverse* and *despite not having grounded (3-prong) receptacles*. – manassehkatz-Moving 2 Codidact Sep 07 '22 at 14:29
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    Not sure if I've ever seen a metal one in Germany... – Arsenal Sep 07 '22 at 14:54
  • I don't think I've seen metal old construction. The plastic ones that hook the drywall are very convenient. –  Sep 07 '22 at 18:52
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    #12 Depending on where you live, certain wildlife can easily chew through plastic. Very few creatures can chew through metal. – Darrel Hoffman Sep 07 '22 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. –  Sep 07 '22 at 20:53
  • @manassehkatz-Moving2Codidact location wasn't specified in the question, though. – SQB Sep 08 '22 at 13:50
  • @SQB Not sure what you are referring to. Question specified "outdoor" and I included that in my answer. I did not refer to wildlife, which is different in different locations - that was Darrel Hoffman. (Though I do agree with Darrel's comment.) – manassehkatz-Moving 2 Codidact Sep 08 '22 at 14:45
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    Many outdoor applications require wire to be run through conduit. Using metal conduit and boxes means you don't have to run a separate ground wire through, which can help *significantly* in such a space-constrained environment. – bta Sep 08 '22 at 16:43
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    @user253751 I hear you but if you throw a lot of things into recycling that aren't recyclable, you actually make recycling less effective e.g. reduce the value of your community's waste stream (yes, that's a thing) so it's important to understand what is recyclable in your area and put anything else into the garbage if there are no other options. – JimmyJames Sep 08 '22 at 21:26
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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.

Agent_L
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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.

chux - Reinstate Monica
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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.

RatTent
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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 '23 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 '23 at 14:26