I haven't been inspected yet but most my rough in work is done. The problem is that I didn't realize until now that I wasn't supposed to remove or fully knockout the trap doors on the nonmetallic receptacle boxes. Can I fill in the hole with a fire retardant foam or other product and call it good, or do I have to completely remove all my boxes and reinstall new ones? If there is a product that can fix my goof, do you have any recommendations?

  • Did you remove the trap doors only on the openings filled with wire or the empty ones as well? Also are these single gang boxes or multigang?
    – bib
    Jul 24, 2015 at 11:46

2 Answers 2


The trap doors serve a few purposes in plastic boxes:

  1. They keep stuff in. Sparks, flames, and other rogue electrical gremlins.
  2. They keep stuff out. Fingers, pets, or anything else capable of getting electrocuted or causing a short.
  3. They act as the cable clamps for non-metallic cables entering the electrical box.

Fire proof foam or caulk would likely satisfy (1) and (2).

The plastic box conundrum.

On the cheaper plastic boxes (you know, those really flimsy ones), even if you don't try to break off the trap doors, they will, out of spite, break off on their own. Even if they don't break off, they typically end up so loose that there is no way they are an effective cable clamping mechanism.

This raises the question: If these boxes are manufactured this way, then shouldn't this be acceptable?

For certain single gang boxes where the cable is secured within 8 inches of the box, this is acceptable; otherwise, it is not acceptable.

2014 NEC

314.17 Conductors Entering Boxes, Conduit Bodies, or Fittings. Conductors entering boxes, conduit bodies, or fittings shall be protected from abrasion and shall comply with 314.17(A) through (D).

(C) Nonmetallic Boxes and Conduit Bodies. [...] Where nonmetallic-sheathed cable or multiconductor Type UF cable is used, the sheath shall extend not less than 6 mm (1/4 in.) inside the box and beyond any cable clamp. In all instances, all permitted wiring methods shall be secured to the boxes.

Exception: Where nonmetallic-sheathed cable or multiconductor Type UF cable is used with single gang boxes not larger than a nominal size 57 mm x 100 mm (2-1/4 in. x 4 in.) mounted in walls or ceilings, and where the cable is fastened within 200 mm (8 in.) of the box measured along the sheath and where the sheath extends through a cable knockout not less than 6 mm (1/4 in.), securing the cable to the box shall not be required. Multiple cable entries shall he permitted in a single cable knockout opening.


If you do not want to reinstall new boxes, you need to plug any holes that are not occupied by electrical cables. Then, be sure you have secured your cables to the stud or some other suitable surface within 8 inches of the box. Please note this exception does not extend to anything larger than a 1-gang box, so if you do have larger boxes you will need to make a judgement call.

If you do want to reinstall, I recommend using the heavier duty plastic boxes or metal boxes. From experience, the thicker blue boxes made by Carlon* are less susceptible to trap door failure.

*Other manufactures might make this style box as well, but Carlon is the only one I've seen in my area. This is not a Carlon endorsement.

  • 1
    The code (314.17) allows cables without clamps on smaller single gang boxes if the cables are stapled within 8 inches of the box. If knockouts are out only in single gang boxes, only in holes filled with cable and stapled, you should be OK.
    – bib
    Jul 24, 2015 at 19:32
  • @bib, updated answer using code. Thank you for the reference.
    – mjohns
    Jul 24, 2015 at 20:41
  • My thirty second search for square "knockout plugs" "plastic box", came up empty. They sell these?
    – Mazura
    Jul 24, 2015 at 22:24
  • @Mazura, not something I've ever seen because I've never had this issue. OP asked about using fire proofing foam or caulk, so I suggested this as plausible if unused knockouts needed plugging. Hopefully this is not the case and the point is moot.
    – mjohns
    Jul 25, 2015 at 0:40
  • Thanks for the advice. All but 1 or 2 are single gang. I think I will just double check to make sure I am within 8" on the singles and replace my double. Jul 25, 2015 at 18:27

Use some "duct seal"--it;s approved for sealing holes in use by electrical. Usually used where service entry cable enters your meter. It is very pliable, non toxic, not a fire hazard clay. Electrical supply stores and big box stores have it too in their electrical department.

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