A 20A power supply using a transformer/bridge/cap design is not really feasible. It's possible, but you will need a fork lift to carry the transformer and the cap will have to be HUGE, not to mention the heatsink for the regulator.
These type of high current supplies are almost always built using some kind of offline switcher. A 60 Hz transformer design is just not practical.
As an example: if you use a 60 Hz transformer fed to a bridge and capacitor, and you want the ripple voltage to be not more than about 3V at 20A current:
The filter capacitor would have to be about 60,000 uF.