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4" metal face plates and box are very common for exposed wiring. Receptacles connect to the face plate instead of to the box, which is different than in a house. The face plate comes with a few screws and nuts for this purpose.

It's possible to attach the recep with 2 screws and nuts at the top and bottom (red arrow) or a single screw in the middle (blue arrow). Which is the best way?

(click for full-size)

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all three is the best way- the less screws used the higher the chance the receptacle screw holder will break from continous plugging in force. – ppumkin Aug 23 '11 at 10:11
use the tabs as in your last question and the center screw as well. – shirlock homes Aug 23 '11 at 11:00
@Shirlock, I think that's worth being an answer. – BMitch Aug 23 '11 at 11:55
up vote 2 down vote accepted

National Electrical Code (NEC), provides the answer in article 406.4 (C).

406.4 Receptacle Mounting.
(C) Receptacles Mounted on Covers. Receptacles mounted to and supported by a cover shall be held rigidly against the cover by more than one screw or shall be a device assembly or box cover listed and identified for securing by a single screw.

In your example image, the receptacle mounted using only one screw is a code violation.

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What you have there is called an RS (for raised steel) cover, which attaches to a 1900 or 11-B box. Since the 1900 box provides no support for the wiring devices, you really should be attaching them using the screws and nuts that come with the RS cover, in addition to the center screw. Don't forget to ground the box as well, using a green ground screw.

This opposes a similar raised cover called a mud ring, which provides the support that you would get from an ordinary rough in box. In that case, you'd be attaching a typical trim to the wiring device using just the center hole.

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