Duct Size	A	B	Max Dia.
6 inch	8"	3"	6"
8 inch	10"	3"	8"
10 inch	12"	3"	10"
12 inch	14"	3"	12"
14 inch	16"	3"	14"
16 inch	18"	3"	16"
18 inch	20"	3"	18"
20 inch	22"	3"	20"
24 inch	26"	3"	24"

 

Measure your specific application, then refer to the chart to select the appropriate wye branch size.  Branch can be any size up to trunk size.