Voltage regulating without current drop
How can I get 8V 30A power supply from a 12V 30A DC supply?


In principle, a linear regulator would allow you to do this. Typically they only consume a few micro or milliamps themselves and pass the vast majority of input current to their load. 

In practice, 30 A through a linear regulator is a horrible idea due to the power inefficiency. 12 V x 30 A is 360 W while 8 V x 30 A is 240 W. So if you use a linear regulator that takes 360 W from the supply, and delivers 240 W to the load, where does the extra 120 W go? The linear regulator turns it into heat. Which means you need a truly massive heat sink, and a lot of careful design to avoid the regulator burning itself up.

This can be overcome by using a switching regulator. A switching regulator would be able to deliver 30 A to an 8 V load, while only drawing about 20 A (maybe 22-24 A in reality) from the 12 V input. 

But designing a 240 W switching regulator is not a trivial job, and not a good choice for a first design project. If your goal is to learn, start with something smaller like mentioned in the comments, and try this design later. If your goal is just to provide power to some device you want to use, you'd probably be best off just buying an off-the-shelf 8 V 30 A mains-powered supply.