The unit melting energy for a certain sheet metal is 9.5 J/mm. The thickness of each of the two sheets to be spot welded is 3.5 mm. To achieve required strength, it is desired to form a weld nugget that is 5.5 mm in diameter and 4.0 mm thick. The weld duration will be set at 0.3 sec. If the electrical resistance between the surfaces is 140 micro-ohms, and only one-third of the electrical energy generated will be used to form the weld nugget (the rest being dissipated), determine the minimum current level required in this operation.