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Thanks to Stackexchange's Hot Network Questions, I found out that my 3-prong dryer outlet was a safety hazard and I'm replacing it with a 4-conductor 14-30R receptacle and matching cord. I've replaced the dryer cord and now I'm working on the receptacle. The circuit feeding the dryer has a 4-wire cable, with the ground wire screwed to the old 3-prong surface-mount receptacle.

I had been thinking that in addition to connecting the ground wire to the new receptacle, I would also need to run a ground wire from the ground terminal on the new receptacle to a ground screw in the metal box I'm mounting the receptacle in. However, the new receptacle seems to have a built-in solution to this problem: the ground terminal has a metal strap connecting it to the mounting plate. So it seems to me that once the receptacle is screwed into the box cover, and the box cover to the box, the box is grounded and therefore I don't need a separate ground wire. Is that correct?

A few other details:

Here's the metal strap connecting the ground terminal to the outlet's built-in mounting plate, which in turn is screwed into the box cover:

enter image description here

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    What make and model is said dryer receptacle? Jan 30 at 23:55
  • @ThreePhaseEel The new receptacle is a Utilitech 2105S.
    – Dan C
    Jan 31 at 3:05
  • Is your receptacle box surface-mounted or set into the wall (flush-mounted)? Jan 31 at 3:11
  • @ThreePhaseEel I added that and a couple other details to the question.
    – Dan C
    Jan 31 at 3:20
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Anytime you have a metal box, you ground the metal box FIRST.

When the receptacle's yoke has hard-flush metal-metal contact with the junction box cover, and the cover has hard-flush bare metal-metal contact with the junction box, then it picks up grounding that way. There are other ways as well.

However, you NEVER connect the ground wire to the receptacle and fail to ground the metal box. In that case, the box's grounding will fail if you remove the receptacle.

"Grond the metal box first" works well for pushing the ground wires out of the way into the very bottom of the box. When using one of the allowed methods of grounding a recep via the box, it means you don't have to handle ground wires at all when trying to assemble the box. One less wire to wrestle!

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  • Thanks. So I suppose the metal strap in the outlet connecting ground to the mounting plate serves as a backup to the wired ground connection?
    – Dan C
    Jan 31 at 15:02
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    When used with metal box the strap would provide an alternate path, but more significantly it provides a path to ground the yoke and cover if mounted on a plastic box. Jan 31 at 15:58
  • @DanC It (or an internal path) provides the primary grounding path when mounted in a metal box. There is no need for a ground wire in a metal box. Do not land the cable's ground wire on the recep. Jan 31 at 17:15
  • @Harper-ReinstateMonica so in other words, I should connect the cable's ground wire to the box, and leave the receptacle's ground terminal unconnected to anything? The metal strap provides the only grounding that's required? I originally asked this question because I don't have any spare 10ga wire on hand to make a pigtail off the incoming cable for the ground (with one end going to the recep, the other to the box). But if the metal strap on the recep's ground terminal is the primary ground, I don't need to do that?
    – Dan C
    Jan 31 at 17:20
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    @DanC Yes, you've got it. I maintain 8 industrial buildings full of receps of every kind, and there's not a ground wire among them, and it's not my work, it's several electricians. It's just SOP in metal boxes. :) Jan 31 at 17:34
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Metal boxes must be always be grounded.

For a surface mounted metal device box NEC 250.146(A) allows omitting the jumper to the device if the yoke mounts flush and at least one paper washer is removed or omitting the jumper if using a cover mounted receptacle if the cover and box are "listed as providing a satisfactory ground...".

Really the Code paragraph is a little long and detailed, if you wish to omit the jumper between box and receptacle I suggest you find a quote of NEC 250.146(A) or read it directly on the NFPA website.

Edit: The Hubbell product page for the 810C indicates it meets the requirements of the NEC Article 250.146(A) that allows connecting the ground only to the box.

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