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Sometime I started (slow pace) the rewiring process of a large mansonry house. Overall I intend to use EMT conduit inside walls, and in the attic mostly bx/mc conduit. Will mostly use 12G THNN and sometimes I will use romex (and will derate the conduit whenever required: 100% filling rate) The house has a small basement (which is a breeze to do for what there is no ton of insulation there) and I plan on using mostly EMT, THNN, and sporadically nm-b.

I understand that romex is required to be affixed/clamped 12" from a box and also in the box's knockout. I have a situation whereby the romex travels a very short distance (about 4 feet) inside a partial wall which then makes it very difficult to clamp the wire at the box end or inside the wall cavity. The pic below depicts the situation:

The box below will contain a light switch and a dual receptacle, and the romex come from inside the wood joist and unless I drill a large bore there (or a special fitting exist) I don't see how to clamp the wire to the box itself:

enter image description here

The romex travels about 3 or 4 feet to the junction box (left of the switch box facing) inside the narrow wall cavity, thus making it difficult to clamp within the wall too.

enter image description here

By the way, most of the wire is thnn traveling in 1/2 emt conduit, as shown below: enter image description here

So, I was wondering whether or not I could use the exception present on NEC 334.30(B) [see below exception] and thus do not use clamp at the box, at 12" of the box, and within the short section inside the narrow wall cavity.

334.30 Securing and Supporting Nonmetallic-sheathed cable shall be supported and secured by staples, cable ties, straps, hangers, or similar fittings designed and installed so as not to damage the cable, at intervals not exceeding 1.4 m (4 1⁄2 ft) and within 300 mm (12 in.) of every outlet box. junction box, cabinet, or fitting. Flat cables shall not be stapled on edge. Sections of cable protected from physical damage by raceway shall not be required to be secured within the raceway. (A) Horizontal Runs Through Holes and Notches. In other than vertical runs, cables installed in accordance with 300.4 shall be considered to be supported and secured where such support does not exceed 1.4-m (4 1⁄2-ft) intervals and the nonmetallic-sheathed cable is securely fastened in place by an approved means within 300 mm (12 in.) of each box, cabinet, conduit body, or other nonmetallicsheathed cable termination. FPN: See 314.17(C) for support where nonmetallic boxes are used (B) Unsupported Cables. Nonmetallic-sheathed cable shall be permitted to be unsupported where the cable: (1) Is fished between access points through concealed spaces in finished buildings or structures and supporting is impracticable. (2) Is not more than 1.4 m (4 1 ⁄2 ft) from the last point of cable support to the point of connection to a luminaire or other piece of electrical equipment and the cable and point of connection are within an accessible ceiling

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  • When entering the box the clamp provides protection for the insulation. There are plastic snap in clamps that push into the hole and secure the wire. This way your wire is protected from the sharp edges and clamped.
    – Ed Beal
    Commented Feb 14, 2016 at 13:35
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    Exception or not, it's trivial to chisel out a little bit of wood behind the box to provide clearance for a cable clamp. @EdBeal had the suggestion of the plastic ones -- these are very low profile.
    – Aloysius Defenestrate
    Commented Feb 14, 2016 at 14:30
  • @AloysiusDefenestrate: yeah, on a second thought I plan on drilling an opening deep enough for a clamp at the box end and use some type of clamping at the there. But there would be no clamp inside the wall cavity, so I assume that it would be ok (local codes here do not deviate from NEC)
    – tk3000
    Commented Feb 14, 2016 at 20:58
  • @Ed Beal: are these the plastic snap in clamps you mentioned: link They are similar to the built-in clamps in some plastic electrical boxes, and in deed are very low profile; but certainly better than nothing. But I was wondering if I am really required to use them. I would probably use them anyway since the safer the better
    – tk3000
    Commented Feb 18, 2016 at 10:43
  • At a minimum I would use a plastic snap in bushing to protect the wires on a single gang box. A double gang box requires the clamp. I have seen a few times over the years where no bushing or clamp was used and the wires ended up shorting out to the box. These were older installs and had been in service for many years. The tough one was a grounded neutral causing GFCI problems in a concealed box another code violation that cost the home owner + 1K to repair. link
    – Ed Beal
    Commented Feb 18, 2016 at 14:07

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While your local code amendments may differ, the following codes are recognized in the National Electrical Code 2014

  1. Conductors entering boxes need to be closed and protected with approved means.

    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). (A) Openings to Be Closed. Openings through which conductors enter shall be closed in an approved manner.

  1. An exception is allowed when NM wire is used BUT ONLY when a single gang box is used not larger than 2 ¼ in. × 4 in. and mounted on walls or ceilings and the cable is fastened within 8".

    314.17 (C) Exception: Where nonmetallic-sheathed cable or multiconductor Type UF cable is used with single gang boxes not larger than a nominal size 57 mm × 100 mm (2 ¼ in. × 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 (¼ in.), securing the cable to the box shall not be required. Multiple cable entries shall be permitted in a single cable knockout opening.

  1. NM wire installed on A WALL of unfinished basements requires proper protection with conduit or similar tubing AND the conduit shall be provided with a insulated bushing or adapter at the point of entry.

    334.15 (C) In Unfinished Basements and Crawl Spaces. Nonmetallic-sheathed cable installed on the wall of an unfinished basement shall be permitted to be installed in a listed conduit or tubing or shall be protected in accordance with 300.4. Conduit or tubing shall be provided with a suitable insulating bushing or adapter at the point the cable enters the raceway. The sheath of the nonmetallic-sheathed cable shall extend through the conduit or tubing and into the outlet or device box not less than 6 mm (¼ in.). The cable shall be secured within 300 mm (12 in.) of the point where the cable enters the conduit or tubing. Metal conduit, tubing, and metal outlet boxes shall be connected to an equipment grounding conductor complying with the provisions of 250.86 and 250.148.

  1. Conductors entering building need protection.

    300.5 (D) (2) Conductors Entering Buildings. Conductors entering a building shall be protected to the point of entrance.

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  • I am aware of the points your made and mentioned. But the situation herein is mostly related to a short run of romex (4 feet run, thus complying with NEC 334.30(B)) travelling without clamping within a narrow wall cavity for which due to access and clearance it would very difficult to affix or clamp the romex. Also due to the romex coming out of a joist covering the wall (making it a concealed space and likely complying with NEC 334.30(B)) there is no clearance to install a box clamp unless a larger hole is bored (I'd rather not drill that large hole unless it is really required). thks!
    – tk3000
    Commented Feb 14, 2016 at 20:42

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