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)


  • 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
  • 1
    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


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Browse other questions tagged or ask your own question.