You've hit upon the crux of the issue! Let's break this down: NEC 400.8: Uses Not Permitted

This section absolutely prohibits the use of flexible cords (like the "Type C lamp cord" you describe) in ways that are not specifically permitted. Concealing flexible cords is generally a big no-no.

NEC 411.6: Low-Voltage Lighting Systems

This is where the nuance comes in. 411.6 does allow for concealed wiring for low-voltage lighting systems if certain conditions are met. Critically, it references Chapter 3 wiring methods (like NM cable, conduit, etc.) or wiring supplied by a Class 2 power supply.

The Key Question: What about the input side of the Class 2 power supply?

This is where their argument falls apart (most likely). While 411.6 allows concealed wiring for the low-voltage output side of a Class 2 power supply, it doesn't grant blanket permission to ignore 400.8 on the input (line-voltage) side.

Here's the breakdown of why concealing the plug-in cord is likely a violation


The cord is a flexible cord

Your "Type C lamp cord" is a flexible cord.2.

NEC 400.8 Prohibits Concealment

Flexible cords cannot be run through holes in walls, ceilings, or floors; they cannot be covered by building materials; and they cannot be fastened to building surfaces. Concealing it in the "open air box" above the casework is essentially covering it by a building material (the box itself).3.

411.6 doesn't override 400.8 on the input side

411.6 pertains to the low-voltage wiring downstream of the Class 2 power supply. It doesn't grant an exception to the rules regarding the high-voltage supply to the power supply.

Your Argument to the Contractor

"While NEC 411.6 allows for concealed low-voltage wiring fed from a Class 2 power supply, this exception only applies to the output side of the power supply. The line-voltage input cord (the "Type C lamp cord") is still subject to the restrictions in NEC 400.8." "NEC 400.8 prohibits the concealment of flexible cords. The proposed installation, by placing the cord in a closed box above the casework, effectively conceals the cord, violating 400.
8." "The accessibility of the receptacle from an adjacent ceiling tile is not the primary issue. The problem is the concealed flexible cord."

Acceptable Alternatives

Here are some ways to do this in compliance with the NEC:

Hard-Wired Power Supply

The best solution is often to use a Class 2 power supply that is designed to be hard-wired. This allows you to use a Chapter 3 wiring method (like NM cable in a proper box) to connect the power supply to the building's wiring system. You can then run the low-voltage wiring to the LEDs, taking advantage of the concealed wiring provisions of 411.6.

Receptacle within Sight and Accessible

If you absolutely must use a plug-in power supply, the receptacle must be readily accessible and the cord must be visible and not concealed. The best practice is to have a power inlet in the casework itself.

Consider a Listed Assembly

If the entire casework/lighting system is a listed assembly, the listing might include specific instructions and allowances for the wiring. Carefully review the listing documentation.

Important Considerations



Authority Having Jurisdiction (AHJ)

Ultimately, the interpretation of the NEC lies with the AHJ (the electrical inspector). If there's any doubt, it's always best to consult with them directly.

Safety

Concealing flexible cords can create a fire hazard if the cord is damaged or overloaded. Proper wiring methods are essential for safety.

In summary, you are likely correct in your interpretation of the NEC. While 411.6 offers flexibility for the low-voltage output side, it doesn't override the fundamental restrictions of 400.8 on the line-voltage input side. Stick to your guns and insist on a compliant wiring method for the power supply input. Flag for review