# Box fill calculation for isolated ground receptacles

After too much reading on what I thought would be straightforward, I'm a bit turned around on calculating box fill, and hoping I could get some help in doing so. Here is a diagram of the circuit: A few notes: Yellow lines are home runs for isolated grounds. Orange triangles are twist-caps, and I just grabbed a stock, non-IG outlet for the picture. All wire is 12awg stranded THWN run in LFMC or FMC, all boxes are metal, ideally for surface mount. Wiring in boxes B, C and D are the same as A, with one difference between each box - there is one less unbroken IG wire passing through the box compared to the one before it, so of course box D will have a smaller fill than A.

• I believe that code-wise you still get to count "one" for all grounds, but I'll leave an answer on that to one of our pros. Practicality-wise you might want to choose a "more-generously sized than minimum box" anyway. If this is still in the planning stage you might find a huge financial and practical benefit to using EMT rather than FMC... Feb 13, 2020 at 15:31
• I do agree with a larger box, but I know from working in a hospital in the past isolated grounds being a separate type of ground were counted so the total ground count was 2x the wire size where 1x is normal. Feb 13, 2020 at 16:05
• Assuming you're using raised steel covers, be sure to use crush corner style with IG receptacles to ground the metal cover. Feb 13, 2020 at 19:12

2 in, 2 out, 4 for 2 yoke devices, 1 ground, 20.25 cubic inches.

(12Ga, 2.25 cubic inches per "wire count")

If adding 4 isolated grounds to the count 29.25 cubic inches.

4 x 4 x 1-1/2 square box is 21 cubic inches.

Typical 4 x 4 exposed work cover for two outlets is 6.6 cubic inches.

4 x 4 x 2-1/8 square box is 30.3 cubic inches

So if you have to count the IGs Box A would need to be deep, or be a

4-11/16 x 4-11/16 x 1-1/2 is 29 cubic inches (plus exposed work cover - may be 8.8 cubic inches at this size, depending on design of the cover)

or get generous with yourself and use a

4-11/16 x 4-11/16 x 2-1/8 for 42 cubic inches (before adding the exposed work cover.)

Box B and beyond could be 4 x 4 x 1-1/2 + exposed work cover = 21+6.6 which is more than the 27 needed for B if IGs need to be counted.

If you can use EMT rather than FMC you can skip the (non-isolated) ground wire and they can all be 4 x 4 x 1-1/2 + exposed work cover. They will also be easier to pull. If you don't need to minimize the cost or footprint, a bigger box is hardly ever a bad idea.

• Thanks, I appreciate your showing the math and working through it. Agreed on largest box possible, cost isn't a huge concern, but footprint is. It looks like box A will be fine with a 4x4 x 2-1/8 (30.3 cu in.). I'll probably keep it the same curved edge profile for the remaining boxes but go down to 1-1/2 deep for B-D. We'll see. Conduit has already been placed and wires pulled within. Thanks again! Feb 13, 2020 at 18:53

I worked in a hospital back in the late 90’s early 2k. I still have notes in my code book pointing to 250.146.D with a note that states isolated grounds count as +1x the wire size, I jumped back and read it and today it is not clear. But since it is a separate means of grounding I would count the isolated grounds as grounds but separately from the required grounding. This would mean in that box a 2x total count for grounds where 1x the wire size is normal. An isolated ground can pass through a box or panel without being connected if not used in that box or panel. The isolated ground will be bonded at the service panel for the building.

Hope that helps , it is fuzzy but I know it was required so I would count it.