Balanced delta source to Wye source by considering magnitude only is ok?
If one doesn't care about the angle but only the magnitude then
\$ V_Y = \frac {1}{\sqrt 3} V_{\Delta} \$ is all you need.

For example, if I have a delta-star transformer feeding an induction motor nobody cares if the phases are 30&deg; out because it is completely isolated from the delta side. Only the voltage magnitude will matter.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Phasor representation of star (wye) and delta voltages in a three-phase system.

It is a fairly trivial exercise to prove the √3 relationship between the phase to phase and phase to neutral phasors.