Which of the following is NOT permissible as an equipment grounding conductor?

The reason 10 feet of 1 inch flexible metal conduit is not permissible as an equipment grounding conductor relates to the requirements set forth in the National Electrical Code (NEC). While flexible metal conduit (FMC) can provide a path for grounding, it does not meet the conductivity and reliability standards required for an effective grounding conductor. The NEC specifies that equipment grounding conductors must have a specific size and material to ensure a safe and effective grounding path that can carry fault currents.

In contrast, the other options listed are acceptable forms of equipment grounding conductors. A 4 AWG copper conductor is a strong and reliable choice due to copper's excellent conductivity. Rigid metal conduit is often utilized for grounding because it can adequately handle fault currents and is bonded connectorily to provide a reliable path. Green insulated wire is specifically designated for grounding purposes and is easily identifiable as a grounding conductor. These options meet the NEC's requirements and are designed to provide safety in electrical installations.