Originally Posted by JSchuricht
Every wire has a maximum capacity over a given distance before it heats up and the voltage drop becomes too great. The factory distribution block is fed by a wire sized to handle whatever Ford decided to connect to it. Most amplifiers can draw more current than that wire can handle, especially while other loads are present.
To put it another way, think of the battery as a water tank. The wire to the factory distribution block is a garden hose and the factory loads are drinking straws connected to the distribution block. The garden hose can flow enough water to supply any combination of the drinking straws with as much water as they can use. Now you come along and add an even larger garden hose to the distribution block that utilizes a variable amount of water (volume). As you turn this nozzle on this second hose, there is no longer enough water flow to supply all those little straws.
I understand what you're saying. If he was hooking up a 2,000 watt amp with 200 amps worth of fuses, I could see it being a problem. But for what he's doing (pretty sure he said he couldn't wait to hook up his "little" sub), he should be fine