The main rule in Section 250.86 is that all metal enclosures and raceways for other than service conductors must be connected to the equipment grounding conductor.

This rule applies to all metal enclosures and raceways, including metal pull elbows that are installed in a run of underground nonmetallic raceway.

Exception 3 specifies two situations where the main rule does not apply:

1. An isolated metal component may remain ungrounded if it has a minimum cover of 18 in. to all parts of the metal component.
2. Metal components may remain ungrounded if they are part of an installation of nonmetallic raceway(s) and are isolated from possible contact to any part of the metal components by being encased in at least 2 inches of concrete.

Similar language can be found in Section 250.80 pertaining to isolated metal components used in underground runs of nonmetallic service raceways.

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:

250.86 Other Conductor Enclosures and Raceways. Except as permitted by 250.112(I), metal enclosures and raceways for other than service conductors shall be connected to the equipment grounding conductor.

Exception No. 1: Metal enclosures and raceways for conductors added to existing installations of open wire, knob-and-tube wiring, and nonmetallic-sheathed cable shall not be required to be connected to the equipment grounding conductor where these enclosures or wiring methods comply with (1) through (4) as follows:

(1) Do not provide an equipment ground

(2) Are in runs of less than 7.5 m (25 ft)

(3) Are free from probable contact with ground, grounded metal, metal lath, or other conductive material

(4) Are guarded against contact by persons

Exception No. 2: Short sections of metal enclosures or raceways used to provide support or protection of cable assemblies from physical damage shall not be required to be connected to the equipment grounding conductor.

Exception No. 3: Metal components shall not be required to be connected to the equipment grounding conductor or supply-side bonding jumper where either of the following conditions exist:

(1) The metal components are installed in a run of nonmetallic raceway(s) and isolated from possible contact by a minimum cover of 450 mm (18 in.) to any part of the metal components.

(2) The metal components are part of an installation of nonmetallic raceway(s) and are isolated from possible contact to any part of the metal components by being encased in not less than 50 mm (2 in.) of concrete.