I have a 4 inch metal junction box with a 3/4" mud ring attached and recessed into the drywall in my garage. I want to connect an air compressor to this junction box directly without using a plug. What do I need in order to close up the junction box and have a hole for a 3/4" flexible conduit to run to the compressor?

Is it as simple as buying a blank metal 2-gang cover plate and drilling a hole in it? Is there something off the shelf that's actually made for this?

  • 1
    Don't. Wire in an outlet box and plug the compressor into that.
    – keshlam
    Jul 18, 2016 at 16:24
  • If the compressor is direct wired, it means (in most jurisdictions) that you cannot take it when you move. A plug and outlet is a little more expensive to install, but it adds serviceability, modularity, and you can easily move the compressor to a job site. If the compressor has no cord, just install a pigtail (as for a range or electric dryer) if it is 240 volt, or a standard cord for 120 volt.
    – wallyk
    Jul 18, 2016 at 16:31
  • The problem I'm finding with the plugs is that the highest rated one I can find is for 3 HP and this particular compress is 5 HP. I thought hard-wiring it was the only "legal" way to do it.
    – John Hodge
    Jul 18, 2016 at 16:39
  • 1
    @wallyK - if you disconnect it before the house is shown, (when selling) that should resolve that. John Hodge - a real 5HP or a marketing department's imagined 5 HP? Voltage and current from the nameplate?
    – Ecnerwal
    Jul 18, 2016 at 17:56
  • If the compressor has a plug, it can be plugged in; you just need to be sure the circuit is rated to handle it. 5HP is 2739 watts; at 120 volts that's 22 amps, at 240V it's half that. It isn't a small appliances, but it isn't all that huge either. "If it happens, it must be possible." (My electric dryer circuit is 30A on each of its two legs...)
    – keshlam
    Jul 18, 2016 at 18:47

2 Answers 2


I'm not sure for a specfically "2-gang" box. These are made for 4" and 4-11/16 and octagon boxes:

Box cover with knock-out - image from garvin industries

and there are 1-gang "handy-box" covers with a knock-out as well.

A 4" box will take 2 devices, but some 4" boxes are set up with tabs for the devices, while others are set up with corner screws like this plate uses and need a cover to hold devices in place.

  • Thank you. I've seen covers like the one above, but my issue is as that I used 4" boxes with corner screws then attached a 3/4" mud ring using the corner screws to bring the front of the box flush with the surface of the drywall. What I need now is something to attach to the front of the mud ring (which has tabs for the devices) and will accept a conduit connector.
    – John Hodge
    Jul 18, 2016 at 18:17
  • @JohnHodge: That's what that knockout in the middle of the plate is for. A conduit nipple goes through it.
    – wallyk
    Jul 18, 2016 at 18:20
  • @wallyk: Yes, but I need a plate that attaches to the mud ring using the device tabs and not directly to the box using the corner screws. The corner screws already have a mud ring attached to them and are now covered with drywall.
    – John Hodge
    Jul 18, 2016 at 18:22
  • @JohnHodge: What is the mud ring for?
    – wallyk
    Jul 18, 2016 at 18:23
  • @wally Bringing the front of the box flush with the drywall.
    – John Hodge
    Jul 18, 2016 at 18:29

I was aware of four-inch extension rings but the ones I found attached in place of the mud ring. Fortunately someone pointed me to the existence of extension rings that attach to the mud ring. They're listed as attaching directly to a switch box.

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