nfpa 54/common vent sizing

does anyone know which nfpa 54 table is used to size the common vent flue pipe size for category 1 furnaces? we have a job where we are doing an oil to gas conversion,and would be install five 120 mbh category 1 furnaces,relining the chimney and i need to find out how to size the common vent that the five furnaces would tie into.they give some examples in the book,but i did not see any that had five furnaces.thanks to all