Yes, there are reasons why running MC cable or 3/4" conduit within a 1 5/8" space between CMU block and drywall might be problematic or non-compliant with electrical codes

1. Working Space and Accessibility:
Code Requirements

Electrical codes (like the NEC in the US) generally require adequate working space around electrical equipment for safe installation, maintenance, and inspection. Stuffing cables or conduit into a tight space makes it very difficult to work on the wiring later.

Accessibility for Maintenance

If there's a fault or need for modification in the future, accessing the wiring within that confined space becomes extremely challenging. This can lead to unsafe work practices or damage to the wiring.

2. Bending Radius



MC Cable

MC cable has a minimum bending radius. Forcing it into too tight a bend can damage the conductors within the cable sheath, leading to shorts or fires.

Conduit

Similarly, conduit has a minimum bending radius. If you try to bend it too sharply within that 1 5/8" space, you may damage the conduit or prevent the wires from being pulled through easily (or at all). Even using fittings like 90-degree elbows will take up more space than you have.

3. Securing and Supporting



Cable/Conduit Support

Electrical codes require cables and conduits to be properly supported and secured at specific intervals. With only 1 5/8" of space, it can be difficult to find a way to securely attach the MC cable or conduit to the CMU or the drywall framing. You'd likely need specialized fasteners, and even then, it might be difficult to meet the spacing requirements.

CMU Attachment

Attaching anything to CMU can be tricky and require specialized anchors, especially if the CMU is part of a load-bearing wall.

4. Heat Dissipation



Conductors Heating

Conductors in electrical cables and conduit generate heat when carrying current. If the cables or conduit are crammed into a tight space with limited airflow, heat can build up, potentially leading to premature insulation degradation and increased risk of fire. While this is less of a concern for standard residential circuits, it's something to consider.

5. Damage to Drywall/CMU



Pressure Points

Running rigid conduit in such a small space could create pressure points against the drywall. Over time, this could lead to cracks or bulges in the drywall.

Fastener Issues

If you are using screws/anchors to attach the cable or conduit supports in the cavity, you run the risk of damaging the drywall on the other side.

6. Code Violations



Improper Installation

Failing to meet the code requirements for working space, bending radius, support, and accessibility can result in code violations and potential safety hazards.

Alternatives and Solutions



Surface Mounting

Consider running the MC cable or conduit on the surface of the CMU or drywall. Use appropriate surface-mounted boxes and fittings.

Furring Strips

Install deeper furring strips on the CMU to create a larger cavity for the wiring. However, this will reduce the overall room size.

Horizontal Runs

Run the conduit horizontally, through the hollows of the CMU blocks. This is often done, but requires careful planning and coring the blocks.

Smaller Conduit

If permissible based on the number and size of conductors you need to run, consider using smaller diameter conduit (e.g., 1/2"). However, check that the conduit fill requirements are still met.

Before you proceed, consult with a qualified electrician and your local building codes to ensure that your electrical installation is safe, compliant, and meets all applicable regulations. They can assess the specific situation and recommend the best solution. It's always better to do it right the first time than to have to redo it later. Flag for review