I don't believe the NEC has any restrictions, but the building code for commercial buildings does have a height requirement, of which I am not sure what the exact height is. It may be somewhere in the 6 foot minimum height range.
Per the IBC, "Means of Egress" 1003.2.5.3 Horizontal projections "Structural elements, fixtures or furnishings shall not project horizontally from either side more than 4 inches (102mm) over any walking surface between the heights of 27 (686mm) and 80 inches (2032mm) above the walking surface."
Well done! I've had to red-tag a few electricians for this. Also, notice how many fire extingiushers don't comply with this.
The intent is protection for the visually impaired. If someone needs a cane to assist their walking, the cane will hit the obstruction if less than 27" AFF. If not, their face may be what hits it.