You're asking about a situation that requires careful consideration of both the National Electrical Code (NEC) and local jurisdictional interpretations. Here's a breakdown of the issues and what to look for

Key Considerations:
"In Ground" Definition

This is crucial. Are you talking about:

Direct Burial

The device box is directly buried in the earth with no enclosure. This is almost certainly a violation of the NEC for standard device boxes, regardless of GFCI protection.

Subsurface Enclosure

The device box is within a listed enclosure rated for subsurface/below-grade use (e.g., a handhole, vault, or weatherproof enclosure). This is possible but still has very specific requirements.

Grade Level or Slightly Below

The device box is mounted in a way that its bottom is at or slightly below ground level, but not buried. This is more common, but still requires careful attention to weatherproofing and potential for water ingress.

NEC Requirements



314.15 Damp, Wet, or Hazardous (Classified) Locations. This is the most relevant section. The NEC requires that boxes used in wet locations (which underground is considered) be listed for the purpose and installed so as to prevent moisture from entering the box. 314.27(E) Support of luminaires. If the power supplies are for lighting, this section would apply as well. 406.9 Receptacles in Damp or Wet Locations. If there are receptacles involved, this section is critical. It requires weatherproof enclosures for receptacles in wet locations. GFCI Protection (210.8)

While GFCI protection is required in many outdoor locations, it doesn't, by itself, make an installation compliant with the NEC for wet or underground locations. GFCI protection provides personnel protection, but it doesn't negate the need for proper enclosure, materials, and installation methods suitable for the environment.

Grounding (Article 250)

Proper grounding is always essential, but especially important in wet or underground environments. Make sure all equipment grounding conductors are properly sized and connected.

Listing and Labeling (110.3)

Equipment must be listed and labeled for its intended use. A standard indoor device box is NOT listed for direct burial or use in a wet location.

Water Ingress

This is the biggest concern. Even with GFCI protection, water entering the box can cause corrosion, equipment failure, and other hazards.

Corrosion

Underground environments are often corrosive. Materials must be chosen to withstand the soil conditions.

Accessibility

NEC 110.26 covers working space around electrical equipment. Consider how accessible the device box will be for maintenance and repairs.

General Guidelines and Best Practices


Avoid Direct Burial of Standard Device Boxes

This is almost certainly a code violation and a safety hazard.2.

Use Listed Enclosures

If the power supplies must be underground, use enclosures specifically listed and labeled for subsurface/below-grade use or for wet locations. These are typically made of fiberglass, high-density polyethylene, or other corrosion-resistant materials.3.

Proper Sealing

Ensure all conduit entries and other openings into the enclosure are properly sealed with watertight fittings and sealant to prevent water ingress.4.

GFCI Protection

Definitely use GFCI protection for all circuits supplying these devices.5.

Weatherproof Covers (if applicable)

If the device box is at or slightly below grade, use a weatherproof cover that is rated for wet locations (Type 3R or better).6.

Consider Drainage

If possible, provide a means for water to drain away from the enclosure.7.

Corrosion-Resistant Materials

Use corrosion-resistant conduit, fittings, and grounding conductors.8.

Local Codes and AHJ (Authority Having Jurisdiction)



This is the most important point. Local codes may have more stringent requirements than the NEC. Always consult with your local electrical inspector or AHJ before starting any underground electrical installation. They will have the final say on whether your proposed installation is compliant and safe.

In summary: Installing standard line voltage device boxes directly in the ground is almost certainly prohibited by the NEC. If you must have electrical equipment underground, use enclosures specifically listed for that purpose, follow all applicable NEC requirements (including GFCI protection), and always consult with your local electrical inspector to ensure compliance. If you can locate the power supplies above ground in a properly weatherproofed enclosure, that is generally the preferred solution. Flag for review