Is the following installation method for NM cable clamps prohibited by the NEC for receptacle outlets?

Note: I know it would be more functional and less awkward to screw in the NM clamps had I used one of the knockouts on top of the box (indicated by the green arrow), right side, or bottom of the box. However, this was the only box I had laying around that already had a knockout punched out for an example and I was able to use a right angle driver to secure the NM clamps for this example setup. Also, since I know I will receive comments if I don't clarify: I know the metal box needs to be grounded (e.g. green ground screw and pigtail) even though it is not shown in the pictures below.

NM cable clamp screws inside box -- seen from outside box NM cable clamp screws inside box -- seen from outside box

This is what I have typically seen for instructions on installing NM cable clamps:

NM cable clamp screws outside box -- seen from outside box NM cable clamp screws outside box -- seen from outside box**

Reason for Question:

While I know the "reversed" method of installing cable clamps will take up additional space inside the box, if I use a 4"x4"x2+" box for a single gang outlet (using a single gang mud ring) then crowding the box is definitely not an issue. Also, if I leave a few inches of slack in the cable outside of the box, I can loosen the clamp to give me some "wiggle room" if I (or a future owner) is doing some work and needs extend the cable a little for any reason without having to tear open the drywall around the box or splice in pigtails.

  • I have had to do this on some inspected jobs and never received a write up, but will try to find a reference. It is similar when triple taping large split nut or crimped & bolted connectors. A very old electrician showed me a trick , put a layer of super 88 on sticky side out then Cambridge , liner less and a layer of 88. The backwards 88 holds the Cambridge and dosent leave all the goo on the wires (industrial motors get changed regularly). I asked both my local inspector and the state head inspector. The answer was code says a neat and workmanship manor. This was the cleanest method,
    – Ed Beal
    Jun 8, 2016 at 3:12
  • Ran out of space but similar to your issue. If in a gutter or panel, how are you going to tighten it inside a small box would be my only concern.
    – Ed Beal
    Jun 8, 2016 at 3:13
  • I wonder what the listing on the cable clamp says? Jun 8, 2016 at 3:14
  • 1
    If you want the clamp inside the box, why not just buy a box with internal clamps?
    – Tester101
    Jun 8, 2016 at 10:05
  • @Tester101 Good point and that is always an option. I have enough extra NM clamps like these lying around after other electrical work on my house where I would like to use them as part of remodeling one of my basement rooms one way or the other. Inside the box just seemed slightly more functional and I wondered why no one ever seemed to install them the way I showed based on Google searches. Jun 8, 2016 at 10:49

1 Answer 1


Approved Installations

The NEC often states "Approved method" of installation, which in other words is what the manufacture instructs. In the case of the non metallic NM wire connector, Halex© offers an online manual.

Other Items That Should Be Fixed

The metal stud looks to be 3.5" inch and Code requires no less than 1.25" from the edge of the stud. In other words, the wire needs to be moved to the center of the stud or use FHA straps to protect it from sheetmetal screws. See NEC 2014 300.4 Protection Against Physical Damage Cables and Raceways Parallel to Framing Members and Furring Strips.

Also NEC 2014 300.4 B Requires protection for NM cable passing through metal stud bored holes.

Excerpt....."where nonmetallic-sheathed cables pass through either factory- or field-punched, cut, or drilled slots or holes in metal members, the cable shall be protected by listed bushings or listed grommets covering all metal edges that are securely fastened in the opening prior to installation of the cable."

  • 1
    If only your link wasn't hidden behind a silly Flash applet.... Jun 8, 2016 at 3:24
  • Forgot to mention the back of the stud is a few inches out from a cinderblock wall, there are yellow grommets in stud through-holes, all wires "maintain spacing" or are appropriately derated in their ampacity, are appropriately sized conductors for the circuit, the correct breaker is attached and... well everything else about wiring bot directly related to the question. Nothing else to be fixed ;-). +1 and accept for a polite version of "search for and then RTFM". Jun 8, 2016 at 3:28
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    From the linked manual "3. Insert threaded portion of connector into knockout in outlet box so that the clamp portion is outside the box.". So it would appear that Halex© wants the clamp outside the box.
    – Tester101
    Jun 8, 2016 at 11:11
  • @Tester101 - I am pretty sure this has nothing to do with code or safety. It is probably Halex's tech writer, putting a doc together using common sense. Because why would you want to screw that clamp down in a tiny box? (clamp will work the same either way)
    – DMoore
    Jan 23, 2018 at 17:57
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    UL approves labeling and the official instructions as part of approving the device. UL does not approve instructions made in AV or interactive media. Therefore such instructions are mere opinions and have no force under 110.3(B). One of my great annoyances with UL is typing in a file number should instantly give you a web page with PDFs of the instructions and photos of device and labeling. UL is stuck in 1989 pretending that 3rd party marketplaces like Amazon and DealExtreme don't exist. Jan 22, 2022 at 2:15

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