No additional support is required for a conduit installation if the conduit length from the finish surface to the box is ___ inches or less.

For conduit installations, the National Electrical Code (NEC) provides guidelines on how much support is needed based on the length of the conduit run. When the conduit length from the finish surface to the box is 18 inches or less, it is permitted to be supported by the box itself without any additional bracing or support. This is a design feature that helps simplify installation procedures for shorter runs, ensuring that no extra hardware is needed while maintaining safety and stability.

When the length exceeds 18 inches, additional support becomes necessary to prevent sagging or stress on the connections, which can lead to failures or safety hazards. This requirement is critical for ensuring that the conduit maintains structural integrity, especially in environments where vibrations or other movements could occur.

Therefore, the stipulated 18 inches is the cut-off point where additional supports are no longer required, making it a crucial measurement in conduit installation practices.