Why does my A/C unit ice over?

There are three reasons why the A/C unit will ice over. There is a restriction in the refrigerant line, the system is low on refrigerant and/or lack of air across the evaporator coil. The lack of air across the evaporator coil is caused by a dirty filter, a filter that is too restrictive, a dirty coil, a dirty blower wheel, incorrect blower motor and/or wheel, the inside unit is not matched properly to the outside unit, ductwork is not sized properly, too many supply registers are closed, the return are inlet is too small or restricted or the ductwork is restricted inside by a damper or ducting material that has come apart.