Rule 1: A resistance of any branch of a Y network is equal to the product of the two adjacent sides of a ? network that divided by the sum of the three ? resistances.
Figure: Y - Equivalent
Y to ? conversion:
R1= Ra Rb + Rb Rc + Rc Ra /Rc
R2= Ra Rb + Rb Rc + Rc Ra /Ra
R3= Ra Rb + Rb Rc + Rc Ra /Rb