Calculating box fill of NM

I have a couple of questions.

When calculating box fill for 12/2 w/grnd, 2-1/4"x4" plastic single gang box, the reference is to the number of ‘conductors’. Does 12/2 w/grnd NM count as ‘1’ or ‘3’ conductors?

The single gang box was one of several that had been attached after wall covering installed. There were two drywall screws securing to side stud. No threads exposed. Could this cause a problem with the wiring conductors?

Thanks for your help,

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You count each circuit conductor as it is (14ga=2" 12= 2.25").
All of the ground wires count as just one of the largest size in the box. Devices (switch/receptacle) count 2 of the largest size connected to that device. Conductors that simply pass through without a termination count one.

BTW that looks like a 16 cu/in box.
If you say
4 circuit conductors
1 for all the grounds
2 for the device

7 times 2.25 = 15.75
You just make it. That is why that is why they make that box :wink:

That box is installed in a non-compliant manner. Box fill is fine, but you have 110.3(B) issues.

Marc, my code book is in the truck. Are you refering to the way the box is mounted by the screws?

I avoided the mounting issue because different AHJs have different opinions on mounting issues in “old work”. I don’t like the flat head screws if they are not pulled up tight but if this was a round head or pan head I would not be as concerned. My issue would be the potential for wire damage on the sharp edge.

After reading in the IRC and NEC, I would expect the box to be mounted as per manufacturer intent - by flanges, nails through provided brackets, pre-drilled holes in back, etc. The side wall screw heads could inadvertently touch the conductors as the receptacles were installed.

I have another issue, also,

On the nonmetallic boxes where the flaps at the rear corners open for the entry of the conductor - several boxes had all 4 flaps cutout and only 2 entry conductors.

Should the flaps only be broken loose to ‘hold’ the wire as it enters, or can they be cut away completely leaving an open hole? These boxes were mounted into a wall cavity, and the wire was fastened to the stud within 8" of box.

Single gang non-metallic new work boxes do not have cable clamping means, and the little square is broken out completely. Unused knockouts must be sealed, but I am aware of any product that is acceptable for sealing a mistakenly removed knockout on a single gang new work non-metallic box. In this case, unless the box can be worked in someplace else in the construction, it’s basically trash. It is also legal and acceptable to run more than one cable through a single knockout in a single-gang non-metallic new work box.

Multi-gang non-metallic boxes and all old work non-metallic boxes must have a clamping means. If the cable clamp is completely broken out on such a box, the box is trash unless some acceptable product is utilized to seal the opening. I am unaware of such a product. It is also not permissable to run more than one cable through a built-in cable clamp on a non-metallic box

Be advised also that non-metallic boxes with built-in cable clamps have a “range” that this clamp may be used for. It is a common violation to see an 8-3 or 6-3 cable run into a 2-gang box for an electric range, and these cable clamps aren’t for cables this large. The installer usually hacks up the box by some means to make it accomodate that large cable Another type of box must be utilized in that case. Allied molded plastics, for instance, makes a box especially for this application.

That’s the short story on non-metallic boxes and cable clamping.

In the picture in the original post, a box such as this one should have been used for that application to make it a compliant install:

Marc, these are fairly new products, “old” old work may not have offered this option. I agree it is a technical violation but in the grand scheme of things I don’t think it is where I would be pulling the tag.


Warning heading off topic:

The reason most code inspectors don’t nit-pick everything is because they look at the ‘general’ overall condition of the installation. The fact they took a ‘short-cut’ would have me seriously concerned over the rest of the work. So if I, a layperson, were to look I would have fun. :smiley:

Ok, back on topic:

If there is a device, then the conductor count is 3 (first 12/2) + 2 (second same size 12/2) + 2 (one yoke) = 7. 7 * = The standard boxes I use can handle 15.75, but I would verify this.

Now off topic again, why? Was this box open for you view, or did you go pulling devices(even though there is nothing but jacket I see).


Just calm down about three notches. Nothing harmful about discussion and expanding one’s educational horizons.

EDIT: I just scaled the depth of that box in PhotoShop, which seems to be 2-1/4", and I believe it to be a Carlon B118A box, which is 18 cubic inches in capacity.

When I head off topic, people need warnings! For their own protection.


The box was as pictured. I didn’t remove anything to take the pic.

Several places around the exterior had wire run through a hole in the base stucco coat with a non-metallic blue box hanging on the wire (to be installed at a future point). Makes me wonder at the method of future installation. :mrgreen:

I am going to foward Marc’s picture to show the client the proper type of box (in case the electrician tries to snow him).

The box in the OP picture should never pass an inspection.

  1. Those sheet rock screws do potentionally create a hazard.
  2. The side wall of that box is not designed to support the box in such a manner.

Yes in ‘older days’, there were less ways to deal with this, but today there is no excuse for this installation.