No, generally, it is not legal and is not a good practice to use a "masonry box" (designed for embedding in brick, block, or concrete) in a sheetrocked metal-framed wall. Here's why: Code Compliance (NEC - National Electrical Code)

Electrical installations must adhere to the NEC, which is widely adopted by local jurisdictions. While the NEC does not explicitly prohibit using a masonry box in a metal stud wall, it does require that electrical boxes be listed and labeled for their intended application. Masonry boxes are designed and tested for use within solid masonry and concrete structures, not for the thinner, hollow construction of metal studs and drywall.

Listing and Labeling

Electrical boxes must be listed by a recognized testing laboratory (UL, ETL, CSA, etc.) and labeled to show their intended use. Using a box outside of its listed application voids its listing, which is a code violation and could create safety hazards.

Secure Mounting

Masonry boxes are designed to be embedded in mortar or concrete, providing secure support. They typically lack the necessary mounting mechanisms to be firmly and safely attached to metal studs. You would likely have to jury-rig a mounting system, which could be unreliable and create a safety risk. The box could become loose over time, stressing the wiring connections.

Sharp Edges and Cable Protection

The inside of a masonry box is not designed with smooth edges. This can cause the wires to become pinched or frayed and eventually short. Also, the wires would not be properly secured inside the box.

Intended application: The "cubic inches" of a masonry box may be more than the specified box. However, the intended use of the box is to accommodate masonry or concrete. The code says a box has to be listed for its intended use.

Why is this important? Safety

Using an unapproved box in an incorrect application can create electrical hazards, including short circuits, fires, and electrocution.

Liability

If there is a fire or other incident related to the electrical system, your insurance company may deny coverage if the installation is not code-compliant. You could also face legal liability.

Inspection

If you have your electrical work inspected (which is usually required), the inspector will likely flag this as a violation.

Alternatives


Deep Metal Boxes

Instead of trying to force a masonry box, use deep metal boxes specifically designed for metal-framed walls. These provide more cubic inches than standard boxes and have integrated clamps or mounting holes for secure attachment to the studs.2.

Box Extensions (Cubic Inch Extenders)

If you need only a bit more space, consider using a cubic inch extender designed to fit onto a standard electrical box.3.

Two Boxes

Install two regular boxes next to each other and run a short piece of conduit between them.4.

Consider the Wire Fill

The amount of wiring in the box may not be NEC compliant. You may need to reduce the number of wires or devices you are trying to fit.

In summary: Do not use a masonry box in a sheetrocked metal-framed wall. It's a code violation, potentially unsafe, and there are suitable alternatives that are designed for that application. Always prioritize safety and code compliance when working with electricity. When in doubt, consult a qualified electrician. Flag for review