Abstract: A cross arm for supporting live railway, trolley and subway lines comprising a cross-member (8) fitted with means of connection to an associable support structure (16), at least a first electric insulator (20) joined to the cross-member (8) and able to support a first live electric cable, electrically insulating it from said cross-member (8). Advantageously, the cross-member (8) comprises a body (24) having a mainly longitudinal (X-X) extension and at least a first attachment portion (28) fitted with a ‘U’ shaped profile able to form a shaped coupling with said first electric insulator (20). The first attachment portion (28) defines a first seat (32) for the continuous adjustment of the longitudinal position of the electric insulator (20) along the cross-member itself.