A single-phase transformer with 120/240V secondary has two separate 120V secondary windings and is usually connected into a 3-wire system. Care must be exercised in distributing the load on the two 120V windings evenly, so each winding is carrying about half of the total load.Similarly for a three-phase transformer, each phase should be considered as a single-phase transformer. When distributing single-phase loads between the three phases, each of the three windings should be evenly loaded.