Usually the generator set has single-phase and three-phase power.  Take domestic for example, domestic single-phase voltage is 220V, used for household electricity.  And three-phase 380V is used for industry.  

Single-phase current calculation formula: generator rated power/voltage.  For example, 10KW (10000W) single-phase generator, the current is 10000/220=45.5A.  

Three-phase current calculation formula: generator rated power /1.732/0.8/ voltage.  For example, a 10KW (10000W) three-phase generator has a current of 10000/1.732/0.8/380=19A.