How it's calculated

Star: V_phase = V_line ÷ √3, I_phase = I_line Delta: V_phase = V_line, I_phase = I_line ÷ √3

In star, each winding sees the line voltage divided by √3 (230 V on a 400 V supply). In delta, each winding sees the full line voltage, but carries the line current divided by √3.

This is why a motor marked 400 V Δ / 690 V Y must be connected in delta on a 400 V supply. In star-delta starting, the motor starts in star at about one third of the delta starting current and torque.