"Auxiliary, or low point, drains are required in sprinkler systems where pipe is installed in a way that can't be drained back through the main drain. When the water capacity of this isolated 'trapped' pipe is more than 5 (18.9 L), but less than 50 gallons (189 L), the drain must be outfitted with a valve of 3/4-inch (19 mm) or larger and a plug, or a nipple and cap. The following table provides the water-carrying capacity of 1 foot (304.8 cm) of pipe, based on actual internal pipe diameters."
Coffee Break Training - Fire Protection Series No. FP-2006-52
United States Fire Administration: http://www.usfa.fema.gov/