Section 250.148 addresses the continuity of equipment grounding conductors and their attachment in boxes.

Not all boxes are metal or provide continuity. Some boxes are plastic and have no provisions to attach an equipment grounding conductor to the box. In a plastic box, continuity is maintained between the equipment grounding conductors by joining them together inside the box rather than connecting them “to” the box.

In a metal box, a wire type equipment grounding conductor can be attached to the box with a ground screw or clip and terminated to the switch or receptacle in the box. If metal raceways such as EMT are connected to a metal box, then in most cases, a wire type equipment grounding conductor is not required inside the raceways since EMT is recognized as an equipment grounding conductor according to 250.118(4).

Section 250.148 provides all of the methods permitted for ensuring proper continuity between the equipment grounding conductors when a box is installed, and circuit conductors are spliced within the box or terminated on equipment within or supported by the box.

Below is a preview of the NEC^®. See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2020 NEC^® edition of NFPA 70.

2020 Code Language:

250.148 Continuity of Equipment Grounding Conductors and Attachment in Boxes. If circuit conductors are spliced within a box or terminated on equipment within or supported by a box, all wire-type equipment grounding conductor(s) associated with any of those circuit conductors shall be connected within the box or to the box in accordance with 250.8 and 250.148(A) through (D)

(A) Connections and Splices. Connections and splices shall be made in accordance with 110.14(B) except that insulation shall not be required.

(B) Equipment Grounding Conductor Continuity. The arrangement of grounding connections shall be such that the disconnection or the removal of a luminaire, receptacle, or other device fed from the box does not interrupt the electrical continuity of the equipment grounding conductor(s) providing an effective ground-fault current path.

(C) Metal Boxes. A connection used for no other purpose shall be made between the metal box and the equipment grounding conductor(s) in accordance with 250.8.

(D) Nonmetallic Boxes. One or more equipment grounding conductors brought into a nonmetallic outlet box shall be arranged such that a connection can be made to any fitting or device in that box requiring connection to an equipment grounding conductor.