I'm trying to calculate the box fill for a box that has 3 12/2 NM-B cables going in, and 3 going out. The box is to extend circuits that don't reach into a new main panel. The box will not have any device (switch/outlet), just WAGO connectors to splice the three cables.

My question is about the NM clamp connectors. I know each clamp is counted as 1 conductor, but when I read 314.16(b)(2) it says "No allowance shall be required for a cable connector with its clamping mechanism outside the box". I'm planning to use 4 "3/8 in. Non-Metallic (NM) Twin-Screw Cable Clamp Connector" as shown in the picture below (2 clamps with 2 cables and 2 with just one). It looks to me that this could be considered "Clamping mechanism outside the box", or am I mis-interpreting this?

NM 2-screw connector

  • Calculation: 3 circuits, 2 conductors, entering and exiting the box
  • (3x2x2)=12
  • ECG: 1
  • #12 wire: 2.25 cubic inches / conductor 13x2.25=29.25 which would work in a 30.2 cubic inch 4 x 4 x 2 1/8 box.

If I have to count the 4 clamps as one conductor each, I get 17 * 2.25=38.25 cubic inches, which would require a 4 11/16 square box.

  • 2
    The 6 EGC's entering the box would be counted as 1 1/2 #12 wires, not just 1, under the new rules in the 2020 NEC. I wouldn't want to meet the person who would quibble over an excess .175 cubic inch allowance in a box.
    – kreemoweet
    Aug 21, 2023 at 18:25

3 Answers 3


I know each clamp is counted as 1 conductor

No. ALL clamps are counted as 1 conductor.

If you have 5 Romex cables coming in on 5 clamps, the wire count is 13 -

  • Five hot five neutral, so 10 counts
  • Grounds are 4 for the price of 1, so 2 counts
  • Cable clamps are 9999999999 for the price of 1, so 1 count.
  • Pigtails are free.

Here's what not to do: Don't over-cram a bunch of cables onto a cable clamp merely to save a wire count.

but when I read 314.16(b)(2) it says "No allowance shall be required for a cable connector with its clamping mechanism outside the box".

Everything I said above is notwithstanding that. So if all you cable clamps have the clampy bits outside the box, then you have 0 which takes 0 wire counts.

which would require a 4 11/16 square box.

I think a 4-11/16" box is a wiser choice anyway. Buy them at electrical supply - they're overpriced at big-box stores (surprisingly, they do NOT specialize in big boxes!) Remember too they make box extensions, or if you ever want to finish this space they make mud rings, and mud rings have cubic inches.

Not least, if you run metal conduit from the J-box to the new panel, and extend the circuits with THHN wires, you can put GFCI and/or AFCI receptacles and deadfronts two at a time in the 4-11/16" boxes, saving you the cost of the equivalent breaker.

  • 1
    "you can put GFCI and/or AFCI receptacles... saving you the cost of the equivalent breaker." - I went down that route once and the AHJ insisted on an AFCI breaker instead, claiming that the entire run needs to be protected, including the wire from the panel to the first box. Not sure if that's a code requirement, though. GFCI is probably okay not to use a breaker.
    – Eli Iser
    Aug 22, 2023 at 2:36
  • 1
    @EliIser -- Harper's referring to the NEC 210.12(A)(5) exception that lets you use metal conduit or MC/AC cables for the homerun to an OBC AFCI, as those aren't vulnerable to damage that'd lead to a cable breakdown-caused fire the way NM and UF are Aug 22, 2023 at 3:35
  • @EliIser Either the inspector was mistaken, or you failed to use metal conduit or MC cable to the first junction box. Romex, SE and other unarmored cable needs to be protected the first inch out of the box. Aug 23, 2023 at 14:59
  • 1
    @Harper-ReinstateMonica, I used NM, so that was a correct callout. The inspector did not suggest a conduit or MC, though
    – Eli Iser
    Aug 23, 2023 at 16:43

As normally/correctly installed, that clamp type is outside the box and does not add to box fill. Clamps that add to box fill are inside the box.

Not that it applies, here, but: Interior clamps add one wire, not one wire per clamp, as far as I recall.

  • Right, only internal clamps count towards fill. By my calcs, the OP has 12 #12 wires spliced in the box, plus a #12 ground wire, for a 29.3 sq in box.
    – Huesmann
    Aug 21, 2023 at 16:46

The box fill calculations will depend on the version of electrical code used. The 2020 NEC is a little different than earlier versions. Here's an online box fill calculator based on the 2020 NEC: https://www.omnicalculator.com/construction/box-fill . The clamps you show are external (if installed normally) and would not be counted.

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