Step 1: Calculate power in a star system

Power per phase: $P_{ph}=V_{ph} I_{ph} \ \cos⁡ϕ$

Total power PT :

$PT =3P_{ph} \\[2ex] PT=3 V_{ph} I_{ph} \ cos⁡ϕ \\[2ex] PT=3\times\dfrac{V_L}{\sqrt{3}}\times\dfrac{V_L}{\sqrt3 Z}=\dfrac{(V_L^2)}Z cos⁡ϕ \ \ \ ...(1)$

Step 2: Total power in a delta circuit