You're right to be cautious and to question this. While those slotted octagon boxes might seem like a convenient solution, there are a few things to consider regarding NEC (National Electrical Code) compliance

Potential Problems and NEC Concerns:

1. Enclosure Requirements

The NEC generally requires that electrical splices and connections be made inside an approved enclosure. Octagon boxes serve this purpose. The key issue here is whether that "slotted" box still qualifies as an approved enclosure when it's resting on the grid ceiling, with the grid potentially inside the box.2.

Foreign Objects in Enclosures

You've already highlighted the main concern: The metal grid ceiling itself is not intended to be part of an electrical enclosure. It's a "foreign object" within the box. The NEC generally prohibits mixing electrical and non-electrical systems within the same enclosure (for safety and fire prevention).3.

Accessibility of Connections

Splices need to be accessible for inspection, maintenance, and future repairs. Depending on the installation, the grid ceiling might obstruct direct access to the octagon box and its contents.4.

Support of the Box

Octagon boxes need to be securely supported. Simply resting them on the grid is unlikely to provide the necessary support, especially considering the weight of the wiring, splices, and potentially some of the light fixture itself.5.

Grounding

The metal grid ceiling might be considered a "conductive surface" that could become energized under fault conditions. How this interacts with the grounding of the electrical system needs careful consideration. The grounding of the light fixture, the grid, and the electrical system must be properly bonded to avoid a shock hazard.6.

Listing and Labeling

The octagon box should be UL-listed (or equivalent) for its intended use. Using it in a way that puts part of the grid inside the box might void the listing.

Why This is Likely Not NEC Compliant (as you suspect)



Grid Inside the Enclosure

The biggest issue is the grid being inside the electrical enclosure. That violates the spirit of keeping electrical and non-electrical systems separate. It could compromise the safety of the electrical system and create a potential shock hazard.

Support and Accessibility

Resting the box on the grid and potential access issues both contribute to non-compliance.

Better Alternatives & Recommendations


Independent Support for the Octagon Box

Securely mount a standard octagon box to the building structure above the grid ceiling, independent of the grid itself. You can use mounting brackets, straps, or other approved methods. Run your flexible cord through an appropriate strain relief connector into the box. Make your splices within the box using approved connectors (wire nuts or other listed connectors). The grid ceiling then has nothing to do with supporting the box or its contents. Use a long pendant or flexible conduit (Liquidtight Flexible Metal Conduit - LFMC) to drop down from the box to the fixture if the fixture is not directly below.2.

Use a Listed Fixture Designed for Grid Ceiling Mounting

Many manufacturers make linear light fixtures specifically designed for grid ceilings. These often have integrated junction boxes and mounting hardware that comply with the NEC.3.

Consult with a Licensed Electrician

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This is the BEST and SAFEST course of action. A qualified electrician can assess your specific situation, understand the NEC requirements in your local jurisdiction, and recommend the most appropriate and code-compliant solution. They can also ensure that the installation is done safely and correctly.

In summary: While the slotted boxes might seem like a shortcut, they likely create code violations. Prioritize safety and compliance by following the NEC guidelines and seeking professional advice. Flag for review