Two 20A circuits in a single kitchen receptacle - what size box do I need?

Question

I am wiring up my kitchen receptacles to have the top outlet on circuit A and the bottom outlet on circuit B. I found this question, which was helpful, but I was flipping through the 2017 Ugly's Residential wiring guide and I saw a section on Box Fill Calculations and I'm really doubting the boxes I'm using are big enough.
My best guess is that I need at least 22.5 cubic inch receptacle boxes, but I'm currently using 18 cubic inch boxes with Wago connectors and pig tails. I do plan to add a receptacle extender to these boxes, since I will be tiling the backsplash, but I don't think that will be enough.
| Item                   | Unit Volume     | Total Volume |
| ---------------------- | --------------- | ------------ |
| 8 conductors           |    2.25 ci each | 18.0 ci      |
| ? clamp                |    2.25 ci each |    ?         |
| 1 device               | 2x 2.25 ci each |  4.5 ci      |
| ? grounding conductors |    2.25 ci each |    ?         |
| ---------------------- | --------------- | ------------ |
| Total                  |                 | 22.5+ ci     |

I don't think I have a grounding conductor since I'm using PVC boxes, but I'm thinking the Wagos are considered clamps, in which case I'm using 5, so that would be another 11.25 cubic inches, 33.75 ci total. Is that the correct minimum box size required for two 20-amp circuits going to a single duplex outlet? Any suggestions for how to go about this?
Below is a picture of a test box I wired up for reference:

NoSparksPlease · Answer

Clamps are fittings that secure cable to box, which doesn't apply to the style of box you are using.
You count 1 total for all grounds. So 8+1+2=11, 11x2.25 = 24.75
The only one piece device box I have seen with enough capacity for four 12/2 NM cables and a device is an rd-30 .

Your other option is 4x4x2.125" square boxes and mudrings.

Harper - Reinstate Monica · Answer

Cable clamps are things which, well, clamp the cable to keep it from flopping around where it enters the box.  You must only count cable clamps for box fill if they exist, and if they intrude into the box.   If so, all of them together count as 1 of the largest wire.
Equipment safety grounds must be there. You need to ground stuff; using plastic boxes doesn't reduce the need for grounding - actually it makes grounding more important since the outlets can't ground to the box!  If you have any grounds and you darn well better, then all of them together count as 1 of the largest wire.
Pigtails are free.  At least in a statutory sense; they certainly take a lot of room.   However there is no need to use pigtails if you invest in the better, $3 "Screw-and-clamp" type receptacles, which are able to clamp 2 wires under each screw. Stuffing a recep with 8 stiff #12 solid wires into a box, that'll be fun. Seriously, I'd pigtail with stranded wire, and still use screw-and-clamps (since putting stranded wire on a screw is hard).
Your setup has 2 circuits serving the same yoke.  Those 2 circuits MUST be handle-tied. or on a 2-pole breaker.  As a practical matter, that will oblige you to put them on opposite poles, unless you do something weird with a GE panel.
If you are using GFCI breakers, you might want to acquaint yourself with the Multi-wire branch circuit. This provides a way to do this same thing with only 6 wires instead of 8.  (neutral must be pigtailed, and if you don't pigtail the others, you're only muscling 5 wires).  MWBCs are generally considered archaic because of the need for a 2-pole GFCI breaker, but you're pretty much there anyway, so why not?

Two 20A circuits in a single kitchen receptacle - what size box do I need?

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