Why is the three-phase voltage 440V, if single-phase alone is 230V?
Figure 1. General phasor diagram for three-phase voltages. The phase to phase voltage is &radic;3 times the line to neutral voltage.



Figure 2. In the case of a 230 V phase to neutral system the phase to phase voltage = 230&radic;3 = 400 V.

The &radic;3 term comes from simple trigonometry of the 120&deg; triangles.