How many outlets in 4 inch box?
Raised exposed work cover, or flush cover?
4 x 4 x 2-1/8 is 30.3 cubic inches. That fits 13 &quot;counts&quot; of 12 Gauge at 2.25 cubic inches per count. If using a raised cover they typically add 6.5 cubic inches or so.
What is a count? A wire terminating in the box, a wire passing though the box, 2 counts per yoke device (ie, a receptacle) 1 count for all grounds (unless using NEC 2020 where additional ground wires [edit: beyond 4] count as 1/4 of a wire...) Edit: right, I don't use NM/B, and if I do I use clamps that are external to the box. If you use clamps internal to the box they add 1 to the count.
If the outlets are on one circuit, you presumably have 3 wires in (hot neutral ground) 2 wires out, and 4 &quot;wires&quot; for the two yoke devices.
9 * 2.25 = 20.25 which is less than 30.3 so you are good. (even with clamps in the box)
If the two receptacles are on different circuits, 5 wires in, 4 wires out, 4 for devices. 29.25 inches which is less than 30.3 so you are good.
Edit: Ah, you said MWBC, so subtract a wire each way (one neutral in and out, not two) still good. And remember to pigtail your neutrals.
Pigtails (wires that do not leave the box) are, incidentally &quot;free&quot; (they don't add to wire count.)
Wires that pass though without connecting to anything in the box count 1 unless they are 12&quot; or longer, in which case they count 2. This effectively differentiates between &quot;just passing through&quot; and &quot;leaving enough to a possible future connection&quot; since you are supposed to have 6&quot; for a connection.
As with conduit fill, not being right up against maximum box fill is more comfortable to work in.