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The length of the vent stack or main vent shall be its developed length from the lowest connection of the vent system with the soil stack, waste stack, or house drain to the vent stack terminal, if it terminates separately in the open air, or to the connection of the vent stack with the stack-vent, plus the developed length of the stack-vent from the connection to the terminal in the open air, if the two vents are connected together with a single extension to the open air. The diameter of an individual vent shall be not less than 1 1/2 inches (40 mm) nor less than one-half the diameter of the drain to which it is connected. The diameter of a relief vent shall be not less than one-half times the diameter of the soil or waste branch to which it is connected. The nominal size of vent piping shall be determined from its length and the total of fixture units connected thereto, as provided in accordance with Table 18-29-916.1 in this section. Twenty percent (20%) of the total length may be installed in a horizontal position.