When installing insulated conductors of 4 AWG or larger, the minimum dimensions of pull or junction boxes installed in a raceway or cable run must comply with 314.28(A)(1) through (A)(3). The rules apply to “insulated” conductors for a reason. When installing large insulated conductors, care must be taken to ensure that the conductor insulation is not compromised during the wire pull because the box is too small.

Section 314.28(A)(2) provides the minimum required dimension for an enclosure containing an angle pull, U pull or even splices (see image). For angle pulls, U pulls or splices, the distance between each raceway entry inside the box or conduit body and the opposite wall of the box or conduit body shall not be less than six times the trade size of the largest raceway in a row (see dimension X and Y in the image).

This distance shall be increased for additional entries by the amount of the sum of the diameters of all other raceway entries in the same row on the same wall of the box. Each row shall be calculated individually, and the single row that provides the maximum distance shall be used.

Exception: Where a raceway or cable entry is in the wall of a box or conduit body opposite a removable cover, the distance from that wall to the cover shall be permitted to comply with the distance required for one wire per terminal in Table 312.6(A).

The distance between raceway entries enclosing the same conductor shall not be less than six times the trade size of the larger raceway (see dimension Z in the image).

When transposing cable size into raceway size in 314.28(A)(1) and (A)(2), the minimum trade size raceway required for the number and size of conductors in the cable shall be used.

See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to NFPA 70.