You won't find that rule. This is the only rule I know of that applies:
110.26(C) Height of Working Space. The work space shall be clear and extend from the grade, floor, or platform to the height required by 110.26(E). Within the height requirements of this section, other equipment that is associated with the electrical installation and is located above or below the electrical equipment shall be permitted to extend not more than 152 mm (6 in.) beyond the front of the electrical equipment.