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What type of cable clamp should be used with this box?

enter image description here

The diameter of those knockouts is roughly 5/8", which I think makes them 3/8 knockouts - does that sound right? If so, do they make cable clamps for NM or MC that fit in a 3/8 knockout? - I only see them for 1/2 knockouts or larger.

Bonus - do they make cable clamps for such a knockout that fit from the inside, retrofit style (ala Raco Insider)

thanks

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  • According to this Home Depot page a 3/8" clamp goes in a 1/2" knockout, which I suspect is the size you have in that box. Mar 4 at 3:51
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    The small knockouts in old metal boxes were intended for use with "loom" (flexible non-metallic tubing) used with old K&T wiring, and usually had integral internal clamps to press down on the loom, securing it. You can see the loom in the photo, but it appears the clamps have been removed.
    – kreemoweet
    Mar 4 at 5:56
  • There are plastic press-in clamps for this situation. Mar 5 at 11:25
  • Thanks @kreemoweet - the missing internal clamps explains it! Feel free to add this as the answer if you like.
    – tom
    Mar 10 at 22:45

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