NEC Pull and Junction Box Sizing
The minimum pull or junction box dimension for large raceways under NEC 314.28 -- 8x the largest raceway for a straight pull, 6x plus the same-row others for an angle or U pull, and the 6x between-entries rule. box-fill is the separate 314.16 conductor-volume fill.
Formula and source
Straight pull: min = 8 x largest raceway. Angle/U pull: min = 6 x largest raceway + sum of same-row others. Between same-conductor entries: >= 6 x larger raceway.
NEC (NFPA 70) 314.28(A)(1) and (A)(2) (by name).
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