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The minimum required pipe size for water closets or pedestal urinal branches and stacks shall be 4 inches (100 mm) except that any building three stories or less in height and used exclusively for family unit occupancy, may utilize a 3-inch (75 mm) soil stack when not exceeding 30 fixture units, with not more than two water closets per floor or two water closets per horizontal branch. The waste connection to any water closet shall be provided with an approved 4-inch (100 mm) closet flange. The minimum required sizes for underground branches from downspouts, floor drains, laundry trays and sink stacks shall be 3 inches (75 mm). (See Tables 18-29-710A and 18-29-710B.)