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Electrical boxes have nested knockouts so you can knock out the smallest ring that fits your locknut fitting. For example:

Panel knockout

During installation the correct 2" KO was removed, but the 2.5" ring was tweaked during installation and we want to make sure that there is still a good ground bond to the EMT. The 2" KO ring is still connected, but not sure how strong it is and we don't want to break/crack from vibration or heat fatigue over time.

Q: If the next-outer-ring cracks to form a (possibly) weak ground, then how should the connection be repaired?

Q: Is there some kind of panel-to-EMT-compression-fitting that has a bigger nut with the same size compression fitting?

It would be nice to avoid splicing or running a dedicated ground. In this case the conduit is 2" EMT with three 4/0 conductors (so it has room for a 2/0 ground, but its a long run and would rather not pull one).

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    Even Home Depot has some things that may do the trick. If not, go to an electrical supply house and I'm sure you can find the right thing. Aug 21, 2022 at 20:13
  • Here is what I understand. You mistakenly knocked out the larger knockout for your 2 inch EMT. Now you are looking for some kind Nut that screws on 2 inch EMT, but also large enough. to provides good grounding contact to you panel.
    – Traveler
    Aug 21, 2022 at 21:12
  • @Ruskes: we removed the correct KO, but the 2.5" ring was tweaked during installation and I want to make sure that there is still a good ground bond to the EMT. The 2" KO ring is still connected, but not sure how strong it is and we don't want to break/crack from vibration over time.
    – KJ7LNW
    Aug 21, 2022 at 21:18
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    You will have to use washers (on both sides. Recommend to use washes with teeth to provide better grounding. For example a 2.5 inch inner dia, washer (with teeth) has outside diameter of 3.5 inch
    – Traveler
    Aug 21, 2022 at 21:31
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    @AloysiusDefenestrate Based on the wire and conduit sizes, I don't think this is a typical little box - it is either a BIG box or a panel. Aug 22, 2022 at 17:27

2 Answers 2

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Whether or not just using a reducing bushing is code-compliant depends on the type of electrical box and use involved. If it is just a "metallic outlet box", then a reducing bushing also listed for bonding use (as apparently most are) is OK, even if all the concentric rings in the knockout have not been removed. If the box is some other enclosure than a metallic outlet box, you must remove all the concentric rings in the knockout in order to be able to use a reducing bushing there.

However, if service conductors are involved, you cannot use a reducing bushing but must instead use a specially-designed and listed bonding bushing. In these cases, according to my reading, you must use a special bonding bushing every place you have not removed all the concentric knockouts, as the connecting metal between rings has apparently not been tested and listed to handle a high-current fault situation as with service conductors.

From the size of the wires and conduit you mention, it sounds like you may have a service conductor situation (and certainly not an outlet box situation). Thus, not only for the damaged knockout you mention, but for all the other knockouts in use that have not had all their inserts removed, you may have to use a special bonding bushing.

The above info is based mostly on a QA in IAEI Magazine:

Question

If I have an installation that is 120 V or 208V and use a reducing washer in a concentric knock out in a panelboard or safety switch but do not remove all the concentric rings, do I need to install a bonding bushing?

Answer

Yes, you would need to use a bonding bushing that is certified (Listed) as Grounding and Bonding Equipment (KDER). The Guide Information for KDER can be located on page 262 in the 2014 UL White Book and can also be found on UL Product Spec at www.ul.com/productspec by entering KDER in the UL Product Category Code search field. The Guide Information states that grounding bushings for use with conduit fittings, tubing (EMT) fittings, threaded rigid metal and intermediate metal conduit, or unthreaded rigid metal and intermediate metal conduit have a provision for the connection of a bonding or grounding wire or have means for mounting a wire connector available from the manufacturer.

This is a NEC 110.3(B) issue. Reducing washers are certified (Listed) under Outlet Bushings and Fittings (QCRV), located on page 391 and 392 in the 2014 UL White Book and can also be found on UL Product Spec at www.ul.com/productspec and enter QCRV at the UL Product Category Code search field. The Guide Information for QCRV under the “Grounding” heading, states, “Metal reducing washers are considered suitable for grounding for use in circuits over and under 250 V and where installed in accordance with ANSI/NFPA 70, National Electrical Code. Reducing washers are intended for use with metal enclosures having a minimum thickness of 0.053 in. for non-service conductors only. Reducing washers may be installed in enclosures provided with concentric or eccentric knockouts, only after all of the concentric and eccentric rings have been removed. However, those enclosures containing concentric and eccentric knockouts that have been certified (Listed) for bonding purposes may be used with reducing washers without all knockouts being removed.” If the concentric or eccentric rings were not removed, you would need the bonding bushing to bond around those rings.

The only enclosures containing concentric and eccentric knockouts that have been certified (Listed) for bonding purposes are certified (Listed) under the product category Metallic Outlet Boxes (QCIT), located on page 389 in the 2014 UL White Book and also on UL Product Spec at www.ul.com/productspec and enter QCIT at the UL Product Category Code search field. The Guide Information for QCIT states under the heading CONCENTRIC OR ECCENTRIC KNOCKOUTS: “All boxes with concentric or eccentric knockouts have been investigated for bonding and are suitable for bonding without any additional bonding means around concentric (or eccentric) knockouts where used in circuits above or below 250 V, and may be marked as such.”

Edit: How to install a bonding/grounding bushing? First, the bushings I've seen are UL listed for use on a threaded end, so threaded rigid conduit or threaded IMC, or the threads of a male EMT end fitting. It goes on over any locknut securing the conduit end, and must engage at least 2 threads to have a good bond to the conduit. The lug is then used to bond the bushing via jumper or direct lay-in to appropriate ground.

A very helpful video showing this on YouTube is "Solar Snack - Grounding Bushing Wiring"

Still photo from linked video showing a grounding bushing installed. Note the lay-in lug at 6 o-clock and also partially obscured set-screw at 9 o-clock for making secure physical/electrical connection:

Still from "Solar Snack - Grounding Bushing Wiring"

Edit2: Remember that the lay-in wire or jumper connected to the bonding bushing must be sized to handle the maximum expected fault current, so don't just use a 12g grounding pigtail. Here's a diagram with jumpers sized according to the service and their function:

(source: https://www.ktrassociates.com/2020/05/13/whats-the-difference-between-the-main-and-system-bonding-jumper/ )

enter image description here

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  • A bonding bushing! Yes, thats it, I wasn't aware that such a thing existed. Thanks, this looks like the ticket: homedepot.com/p/…
    – KJ7LNW
    Aug 23, 2022 at 3:32
  • It sounds like the bonding bushing would replace the 2" locknut on the inside of the panel, is that correct?
    – KJ7LNW
    Aug 23, 2022 at 3:37
  • @KJ7LNW It actually goes on over any locknut - I edited the answer to add photo and video link on installation.
    – Armand
    Aug 23, 2022 at 4:43
  • So if I understand correctly: A reducing washer on both sides in conjunction with a bonding bushing solves the problem in the OP because the electrical integrity of the KO bond is moot when the conduit is directly grounded with a normal ground wire via the bushing?
    – KJ7LNW
    Aug 23, 2022 at 20:43
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    @KJ7LNW Yes in this case the pair of reducing washers are clamped onto inside/outside of bigger knockout opening by the conduit connector and its locknut; that provides mechanical security. It might also in practice provide enough of an electrical bond, but in the higher ampacity/special situations, adding a bonding bushing to the inside makes super duper sure the conduit tube is electrically bonded as desired, whether to a conductor directly via lay-in or via a grounding jumper that might just be connected to the box shell.
    – Armand
    Aug 24, 2022 at 1:50
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There are reducer washers made to solve this problem. They are available in all different size combinations. See picture below from Granger, https://www.grainger.com/search?cpnuser=undefined&searchBar=true&searchQuery=reducing+washer+3%2F4+to+1%2F2&suggestConfigId=6. I'm not affiliated with them or Raco in any way. You'll need two for each conduit. You'll want to emery away the paint from the panel where they come in contact to make a good ground connection.

enter image description here

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    After a bit of digging it looks like we would need the 52AW62 2.5" to 2" but I can't find a datasheet on it. Your image quotes "ground conduit or cables" and I want to make sure that whatever we use is spec'ed for grounding. Do you have a reference link for the image above?
    – KJ7LNW
    Aug 21, 2022 at 21:21
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    @JACK - Please provide the proper attribution link for your embedded image.
    – Michael Karas
    Aug 21, 2022 at 21:56
  • I found it on Grainger, but no mention of grounding. Of course would work, but I want to make sure it meets code for EMT conduit-as-ground. Here is the 52AW62: grainger.com/product/RACO-Reducing-Washer-2-1-2-x-2-52AW62
    – KJ7LNW
    Aug 21, 2022 at 22:09
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    @JACK - The part of avoiding links in answers relates to the Answer content detail such as link only answers. Attribution of used material from others is a different thing altogether.
    – Michael Karas
    Aug 21, 2022 at 22:19
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    As described in my answer, it is likely that the OP's situation involves special code considerations and a reducing bushing may not be allowed, or may be allowed only after removing all concentric rings in the knockout.
    – Armand
    Aug 22, 2022 at 17:54

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