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The main duct may be any shape or cross-sectional area, as long as the minimum cross section area is
provided. The illustration below shows the minimum cross section area for multiple dryer round or
square venting. These figures must be increased 10 square inches (64.52 square centimeters) when
rectangular main ducting is used, and the ratio of duct width to depth should not be greater than
3-1/2 to 1. These figures must be increased in proportion if the main duct run to the last dryer to where it
exhausts to the outdoors is unusually long (over 20 feet [6.1 meters]) or has numerous elbows (more than
one [1]) in it. In calculating ductwork size, the cross-sectional area of a square or rectangular duct must
be increased twenty percent (20%) for each additional 20 feet (6.1 meters). The diameter of a round
exhaust must be increased ten percent (10%) for each additional 20 feet (6.1 meters). Each 90° elbow is
equivalent to an additional 74 feet (22.55 meters) and each 45° elbow is equivalent to an additional 37 feet
(11.27 meters).
MULTIPLE DRYER VENTING WITH 20-INCHES (50.8 CM) DIAMETER
EXHAUST CONNECTIONS AT COMMON DUCT