I am trying to find a chart that gives standard for CFM in return air ducts verses air condition tonnage.

My house has a three ton a/c unit and there is one 14 inch flex return. Rooms and not as cool as I would like and a/c runs for long periods of time. Do I need another return duct?
For example a three ton unit requires "X" CFM".
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All A/C systems require 450 cfm per ton. 3ton system requires 1200 cfm. The return should be 16" flex with a 20x25 return grill. It sounds like you entire duct system, supply and return may be undersized but with out know the heat loss and availabile static pressure for sizeing the ducts I can not be sure. A 14" flex duct is rated for 900 cfm. 
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