three phase - Find impedence given per unit value-transformer

I really can't wrap my head around this. Three single phase transformers are connected to create a 3 phase transformer. They all have the same nominal values :

voltage conversion : 1.2kV/120 V (primary/secondary)

impedence: Z=0,05 pu

power: 7.2 kVA. They are connected in 4 different ways , star-star, star-delta, delta-star, delta-delta.

My problem is finding the actual value of the impedence when the primary side is connected in delta. I would normally find the base in the primary side like this :

$$Z_b=\frac{V_b^2}{S}$$ where Vb is the primary side voltage and S the nominal apparent power value given.

Having a look at the solution manual though the base is supposed to be this

$$Z_{b,old}=\frac{(V_b/\sqrt 3)^2}{S}$$

I've wasted too much time on this. Can somebody help? How can the connections matter since that 0.05 per unit value was assigned for the single phase transformers. Does a transformer's impedence change based on how it is connected?