How is the formula for the base current value derived in a three-phase per-unit system?
Three phase circuits in power systems can be connected in two ways, delta connection or a wye connection. Luckily, both of these circuits have the same equation for apparent power.

\$S=\sqrt{3}VI\text{*}\$

It is this insight that allows for the current formula, despite the relation between line/phase voltage/current being different for the delta and the wye circuit.