What is the smallest 4" square metal box required for a setup involving multiple #12 MC cables?

The smallest 4" square metal box for a setup involving multiple #12 MC cables must accommodate both the cables and their necessary fittings while ensuring compliance with code requirements for box fill. When calculating box fill, the NEC (National Electrical Code) specifies that each conductor (including insulated wires within MC cables) must have a certain volume allowance based on its size.

A 4" square box generally provides enough volume to handle the space required for multiple #12 wires. A box with a dimension of 1.5 inches would be inadequate as it lacks the necessary volume for the conductors and fittings.

While comparing the other dimensions, a 2.5-inch box might initially seem adequate, it is typically not sufficient for multiple #12 cables with their associated connectors, as it does not provide the volume needed for safe and practical wiring practices. A 3.5-inch or 4.5-inch box would offer larger volumes but also exceed the smallest requirement determined for accommodating the setup effectively.

In summary, for the described configurations, a box with a 1.5-inch dimension is indeed the smallest, appropriate size that meets the minimum code requirements for sufficient space, while still allowing practical use.