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First off, I fully acknowledge that this may not be possible, CORRECTLY... but I have a 1939 house that has been through several generations of electrical wiring. The main panel and a lot of the wiring is from 2010 but there are remnants of every phase of wiring since original.

In a number of cases, I have old AC/BX cable (2 cloth covered conductors with ground as the metal cable jacket) going in to plastic junction, receptacle, or switch boxes. When this happens it is more common than not to have the ground completely unaccounted for, either when connecting to the receptacle or to outbound AC, or even worse, outbound NM/Romex cable where the grounding conductor is just connected to nothing and going to some other location.

What is the best way I can connect a grounding pigtail to the AC/BX cable and make it available to the receptacle/switch/outbound NM/Romex, etc? Is this even possible or do I need to replace the box with an old work metal box where I can install a proper AC grounding bushing and use the grounding lug?

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  • What type of connector is currently in place to secure the MC/BX jacket to the plastic box? Is it just the plastic "flap"? If there is a metal clamp or securing bar of some kind, you should be able to connect a grounding pigtail to that for each MC/BX, then use a wire nut to join those and any switch/outlet ground pigtails.
    – Armand
    Aug 27, 2022 at 3:50
  • As you suggest, if you can replace the box, then putting in a metal box would help solve this issue and perhaps provide more volume etc if needed. If you do put in a metal box, then using a squeeze/clamp metal connector to secure the MC to a box knockout should bond the armor to the metal box. All you have to worry about then is running a pigtail from the metal box grounding screw to the receptcle/switch.
    – Armand
    Aug 27, 2022 at 4:18
  • @Armand I'd be surprised if there is a single instance of any connector securing any MC to any plastic box in the house. With the receptacle box I was working on today that prompted this question, the MC seems to just be floating in there, I can freely pull slack in any out of the box. Aug 27, 2022 at 4:36
  • That's not good as obviously the cables/wires going into any box need to be secured. The question then becomes whether to ignore the clamping and jury-rig bonding to the armor or properly clamp the ends, which would involve accessing the outside of the box (so you might as well replace with a metal one at that point).
    – Armand
    Aug 27, 2022 at 9:59
  • Personally in these grounding situations I believe that imperfect but working ground bonding is better than leaving it untouched. Without a metal box, I would do something like using a small hose clamp to clamp the bare end of a grounding jumper to the outside of the armor on the end of each MC coming into the box. Then use a wire nut to connect any/all of these jumpers and a jumper from the switch/outlet ground screw. It would be clear to a later electrician what you are trying to do, and easily modified to a standard connection when a box is upgraded to metal.
    – Armand
    Aug 27, 2022 at 10:06

3 Answers 3

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Use an AC/BX fitting designed for the job

Fortunately, you're in luck, because there's a fitting out there designed precisely for your situation. The Arlington 38RASTG installs from the inside of the box (making it easy to use in old work) and provides a ground screw inside the box for the express purpose of attaching a grounding pigtail to. Problem solved!

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  • I've never seen one of these. Where do you find this stuff? :-)
    – JACK
    Aug 27, 2022 at 14:09
  • @JACK -- you'll need to ask your local electrical supply house for these, although a few supply house type vendors will let you order this sort of thing online. As to how I find this stuff? Manufacturer websites ;) Aug 27, 2022 at 15:57
  • Wow. I wish I knew about these earlier. I swapped an old light switch to a Z-Wave one that didn't like being in a metal box. The feed was MC so I used two boxes. This would have helped do it in one.
    – jay613
    Aug 28, 2022 at 10:53
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To correctly ground the BX cable or Romex you'd want to replace the non metal box with a metal one. The BX cable can be grounded to the metal box with a connector similar to the one shown below. https://www.supplyhouse.com/DiversiTech-640-845-3-8-BX-Set-Screw-Connector?utm_source=google_ad&utm_medium=Shopping_tm&utm_campaign=Shopping_TM_New_users&gclid=CjwKCAjwgaeYBhBAEiwAvMgp2m1O_17GQymkLCKGIDA7mg7X5jvvsTI76g8AkFbotYF3llyP4IhBIRoClWIQAvD_BwE

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The Romex cable ground wire can be screwed into a metal box with a 10-32 grounding screw readily available at your home stores. The screw will fit into the smaller hole on the box.

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Replace receptacles with GFCI. Just do that, it's easy and cheap and provides some benefit.

Replace boxes with metal? Maybe but with 1950s cloth MC subsequently poorly hacked to plastic boxes I'd consider taking that money and putting it in the full rewiring piggy bank.

Fully rewire .. not the advice you're looking for but it's old AND it's not done well.

Hack something with hose clamps? I don't see the point. Given all the plastic boxes you'll never know if you have ground path back from anywhere, you're just as likely to extend ground problems from one outlet to others, and you have the better cheaper alternative of installing GFCI outlets.

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