You said you are using 4/0s
314.28 Pull and Junction Boxes and Conduit Bodies.
Boxes and conduit bodies used as pull or junction boxes shall comply with 314.28(A) through (D).
Exception: Terminal housings supplied with motors shall comply with the provisions of 430.12.
(A) Minimum Size. For raceways containing conductors of 4 AWG or larger, and for cables containing conductors of 4 AWG or larger, the minimum dimensions of pull or junction boxes installed in a raceway or cable run shall comply with the following. Where an enclosure dimension is to be calculated based on the diameter of entering raceways, the diameter shall be the metric designator (trade size) expressed in the units of measurement employed.
If you look at parts B, C and D of this you will see no mention of cubic inches.
