Its somewhere in article 250. It basically says any metal items need to be grounded if they are likely to become energized, and typically included metal duct systems, metal piping systems, metal siding, ....
"likely to become energized" would be an inspectors call. If it were me making the call, I'd consider how close it is to any 120V or other power wiring, and how long the conduit is.
If its a 10's stick all by its lonesome mounted exposed on a concrete wall, I would say not. But if it snakes through 3 floors and from one end of the building to the other with romex resting on it, then yes, I would say it needs grounded.
