When selecting the correct electrical box depth, consider: Wiring Complexity: More wires or larger wires require a deeper box. Conduit Entry: Multiple conduit entries require extra space for ease of installation. Covers wiring, placement, standards, and expert tips for a compliant setup. But it gives you 105 cubic inches, for the 3-gang size with the 3/4 raised mud ring. (They cost less at a real electric supply shop. ) I haven't done box fill math in years. The junction boxes I need to use are way deeper than that, (approx. Rule. I found a stud, drilled a hole beside it to see what surprises lay hidden in that wall, and noticed that there isn't enough depth in there for the utility box.