GJ that is for sizing the box when using cables no raceways.

In that case you would find the smallest allowable raceway for the cable you are using and base the box size from that.

If you have 3" raceways entering the box you must size the box based on those.

I still will say 24" x 7.5" x 4" is the absolute smallest dimensions allowed by the NEC for this installation. [Linked Image]

Bob

Bob Badger
Construction & Maintenance Electrician
Massachusetts