If you use an extension cord, current travels from the 120 V outlet, along one wire inside the cord, through the appliance youve plugged into the cord, and back to the outlet through a second wire in the cord. The total resistance of the two wires in a light-duty extension cord is 0.4 ; the current through such a cord should be limited to 13 A. If a 13 A shop vacuum is powered using this cord: