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)(1) provides the minimum required dimension for an enclosure containing a straight pull (see image). For a straight pull, the length of the box or conduit body shall not be less than eight times the trade size of the largest raceway.

The width of the box used for the straight pull is not specified. It just needs to be wide enough for the conduit entry in order to be able to spin the lock nut on.

For complete code sections, refer to the actual NEC^® text at NFPA.ORG. Once there, click on their link to free access to the 2017 NEC^® edition of NFPA 70.