# Characteristics of a transformer question

I'm trying to solve some problems and I'm confusing one thing . We have , let's say a 30MVA, 24.5/345 KV 3-phase transformer connected in wye-delta. I try to use one-phase analysis and want to find the voltage ratio and make use of the following equation: $$S=\sqrt 3VI$$.

The values V and I are phase or line voltage/current? And when I find the voltage ratio which of the following is right? $$a=\frac{24.5}{345}$$ $$a=\frac{\frac{24.5}{\sqrt3}}{345}$$

I divide by the root of 3 because of the Y connection.