In article , Sam Takoy wrote:

I'm technically not allowed to do this because that would be too many
wires/devices. 20 cu in is the deepest single gang box available. And
with two sets of 14-2 wires spliced, I used up 17 cu in of nominal space
(2x4x2 + 1).

Where does that "+ 1" come from?

Here's what your calculation *should* look like:
4 14ga cables @ 2 conductors each = 8 conductors
add 1 for one or more equipment groundi ng conductors
add 1 for each cable clamp in use (presumably at least 2)

That's what you have *already*: the equivalent of 11 conductors, at 2 in^3
each, which means you're at 22 in^3 *now*. Not 17.

Then add 2 more for the device yoke you're planning to add. That brings the
total to 13 conductors x 2 in^3 each = 26 in^3 minimum.