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There are many outdoor electrical outlet boxes such that the entire box is mounted to the surface and then a wire comes in from the back.

... but what is the proper way to mount a box behind an external wall so the outward-facing plugs will be flush to the wall?

The exterior garage wall is T111 and the inside of that wall is unfinished, just studs, no insulation so easy to get to.

Are there boxes and plates specifically designed for this?

I've found waterproof outlet covers but they don't say anything about the box behind it. Is it just a normal old-work box and the plate somehow makes it rain-tight?

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  • I think, not sure it is code, that you seal the edges of the box to the outside siding/opening with the old standby silicone. One or the other.. The cover should seal the box and can seal the edge of the cover to the siding with silicone
    – crip659
    Commented Aug 21, 2022 at 21:26

4 Answers 4

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It all depends on the type of wall.

If you have brick or block wall then installing a flush mount receptacle is a lot of work, unless it was put in the wall when the wall was built. However, if you have wood, aluminum or vinyl siding, etc. then it should be possible to cut into the wall and install a metal box (don't use plastic, please).

The catch is that while with an inside wall you typically have a hollow wall cavity to work with, with an outside wall it will have multiple layers filling the space including insulation, vapor barrier, etc. A metal box surface mounted on the outside with a piece of metal conduit going through to a box on the inside (i.e., drill a 1" hole and you're done) is simply a whole lot easier in a lot of cases.

Based on comments, this is plywood siding (T111) with open stud wall behind it. In that case, I would cut from inside, next to a stud, using a metal box as a template. Mount the box next to the stud, with the front edge flush with the outside of the siding. Run wire to the box inside. Install a weatherproof receptacle with a proper in-use cover.

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The outdoor socket will require an in-use cover, which will be ugly and won't fit the large socket. Sockets are not the ideal way to connect EVs, anyway. But if you have to do it, framing in a little alcove with a door that matches your exterior may be the way to do it. Build the interior of the alcove to shed water, as it will be coming in.

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Now a little background info. Your car comes with a little travel EVSE that is intended to ride in the trunk and be used for opportunity charging "on the road". They give you two specific plugs for specific reasons: The usual plug that works anywhere, and a NEMA 14-50 because that's what RV campsites have. Capische? That travel EVSE and funny socket is for charging at KOA, Yogi Bear and other campgrounds. It was never imagined to be an at-home socket (unless you also own an RV, which early adoptors often did).

And of course now that DC fast chargers are everywhere, the idea of RV-park charging is a bit quaint.

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They typically offer other plug dongles. Tesla sells 8 (some other makers are still catching up). But if the travel unit is not used, it's preferable to hard-wire the unit with no plug or socket at all.

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Hardwired unit: NM or UF cable enters back of unit.

Separately, automakers generally expect you're going to get a wall-unit EVSE, and do your at-home charging that way, and that unit can be hard-wired, and thus avoid NEC 2020 GFCI requirements. This also allows the EVSE to be calibrated to the available power at your house, instead of being "50A or nothing", which frequently overloads electrical services.

90% of EV novices go off the rails here. They think "Well, 14-50 must be the standard EV plug" and this misconception has caught fire. In fact, the neutral in the 14-50 plug is entirely wasted (unless you also have an RV). EVs don't use neutral. As such, here's a NEMA 6-50 socket on an EVSE. And NEC 2020 requires a GFCI for a socket but not for hardwired EVSEs (they already include a better, smarter GFCI that self-resets and can alert you if there's a problem).

And most important, a Load Calculation must be done on the house, to determine the surplus ampacity available for EV charging. This is configured into the EVSE so it can tell the car what is safe to draw. If it's plug-in, the matching-amperage breaker and socket is installed, with the appropriate dongle plug for the travel EVSE. On those, the dongle tells the car what the safe amps are. There's a microchip molded into the plug.

  • 15A = 2880 W charging. NEMA 6-15 socket or hardwired.
  • 20A = 3840 W charging. NEMA 6-20 socket or hardwired.
  • 30A = 5760W charging. NEMA 6-30 or 14-30 socket or hardwired.
  • 40A = 7200-7680W charging, NEMA 6-50 or 14-50 socket or hardwired.
  • 50-100A = amps x 192 charging, hardwired only.

The large 30A-50A sockets require absolutely enormous insertion forces. That means typical "old work" boxes that use clips to grab the wall, are likely to tear right out. They're not intended for those forces. The box needs to be screwed solidly to a stud, and it's really better off being a welded or drawn metal box (the plastic boxes distort).

So while the wall EVSE's aren't free, they sidestep the cost and trouble of a socket, enclosure and often a useless GFCI... and result in a more elegant installation. The only Code requirement is for a locking kit on the breaker to allow a maintainer to lock it in the off position, for maintenance. (not needed if the breaker is nearby in line-of-sight to the EVSE).

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  • THANK YOU, I NEEDED THAT INFO
    – Traveler
    Commented Aug 22, 2022 at 0:42
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    um...what made you think this is an EV question? I have another unrelated EV question... but this question is just about a waterproof flush mount for a utility plug on our deck.
    – KJ7LNW
    Commented Aug 23, 2022 at 3:23
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    Anyway its a great answer, so would be a shame to delete it...but maybe it can be copy-pasted as an answer to a more appropriate location?
    – KJ7LNW
    Commented Aug 23, 2022 at 3:40
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They can be regular old or new boxes. The outdoor covers come with a foam or rubber gasket that makes it rain proof. Since this will be outdoor, you'll need a GFCI outlet rated for outdoor use. and a cover similar to the one shown below from Volt. the covers come in a variety of sizes.

enter image description here

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If you're doing new construction the easiest way I've found is to measure the height of the box from the inside. Then you mark it out and use a multi-tool to cut out the shape of the box. Make sure that one of the vertical sides of the box is directly touching a stud. Once you have the hole cut out stuff the box into the hole and use you hammer to hammer the nails into the stud. Run your wire and make it up. Then wait until the siders or whoever else gets their crap put up. Trim it out with a WR receptical and box.

This is using a plastic single gang nail on.

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