Controlling Amperage

Hiyas, I was wondering if you could point me in the right direction. I was wanting to build or buy something I could control Amp input. The system would be 12 -14 v dc and I want to be able to limit the input to say... 25 - 30 amps. So if a system would naturally draw more amps I want to be able to control the system be limiting the amp input...if that is making any sense. Got any thoughts ??? tks Huchleberry