A three-dimensional (3D) metal object manufacturing apparatus is equipped with a wire detector to determine a position of a top surface of melted metal contained in a receptacle of a heated vessel in the apparatus from time to time. The solid metal wire being fed into the heated vessel is retracted and the length of the retracted wire is determined using a signal generated by the wire detector. The determined length of the wire is used to identify the position of the top level of the melted metal in the receptacle so the receptacle can be replenished if the level has fallen below a predetermined capacity for the receptacle.