why line to line voltage is 400v?

 

Are you talking about 3-phase or about the peak-to-peak voltage of household mains?

 

If 3-phase, take a look in our E-book, here: http://www.allaboutcircuits.com/vol_2/chpt_10/2.html

~400vac phase to phase would be ~230vac per phase.

 

yes 3 phase, 4 wires system