This isn't meant to serve as a comprehensive explanation for 12v vehicle electronics so I will spare some very important details.
However, there's power running to almost every connection point on the fusebox. The reason for actually is due to the design of the fusebox and the wiring on the back of the fusebox that supplies current. So yes, while there's live power running to it, that doesn't mean that the wire supplying that power, once again on the back of the box, is suitable for the load that you are introducing by adding fuses to it. Additionally, you do not know what vital system component (BCM, ECM, etc.) is also sharing current with that same circuit for which you are adding.
Splicing into existing wires is a long proven method od accurately integrating components into a vehicle where the electrical load is well established (i.e., closed system). Furthermore, you are not adding to the existing load, you are simply sharing the existing load (i.e., no additional current is being requested). Additionally, adding to the current system allows an installer to accurately assess the gauge of wire being used to determine if the circuit was designed for the load (i.e., the blown fuse will be easily located). Moreover, integrating with the current wiring allows an installer to know which critical vehicle systems are being interfaced with, which makes troubleshooting much easier as these are systems that were already designed with that particular trim level in mind (i.e., system load). Finally, you never want to increase the amperage of which a circuit was designed, which is why when interfacing with an existing circuit the load being drawn, as defined by the fused connection, is less than original design therefore never adding to the load and sharing current at a lesser amperage. When you add a circuit, you are adding amperage to a circuit that was not established and therefore run the risk of, at the very least blowing a fuse, or at the worst, if the fuse is faulty, damaging a critical component that will require a wire trace or something catching fire.
Hopefully, this is a good enough explanation from someone who did this for a living for a while.
FYI, fuses do not "have" to be rated for at least 5 amps.
Few problems with your explanation, I'll address them specifically.
However, there's power running to almost every connection point on the fusebox.
There isn't though, you can probe a few points in there and there's no current. That means power isn't being provided.
So yes, while there's live power running to it, that doesn't mean that the wire supplying that power, once again on the back of the box, is suitable for the load that you are introducing by adding fuses to it. Additionally, you do not know what vital system component (BCM, ECM, etc.) is also sharing current with that same circuit for which you are adding.
While I would agree with a power-hungry set up like a high-wattage speaker/amp, a
dashcam or
radar detector pulling less than 5-7W of power at full is
nothing to worry about, regardless of what you're pulling power off of. If something is pulling 12 amps (off of a 20A fuse), that's 240W of power at full load, 5W is well within tolerances for the system to have zero problems.
Splicing into existing wires is a long proven method od accurately integrating components into a vehicle where the electrical load is well established (i.e., closed system).
While it may be established, I believe this is due to a fundamental lack of understanding of how electricity works from the mechanics industry which I'll go into in a second.
Furthermore, you are not adding to the existing load, you are simply sharing the existing load (i.e., no additional current is being requested).
This is fundamentally wrong on many levels. When you add additional devices into a circuit, you are increasing the load on that circuit totally, from the battery. It's important to understand that electricity is
pulled, not pushed, which is caused by a difference in potential across a circuit from the positive to the negative side of the battery (which becomes your "voltage"). If you add additional devices into a circuit, you are increasing the voltage potential across the battery, which causes more current to run through the circuit.
For example: If your ignition pulls 10A of power over a 12V line, that's 120W of power at full load. If you splice in another device pulling 2A of power, that line's load from the battery will increase from 120W to 144W. You're not "taking" some of the 120W and giving it to another device,
both devices are pulling what they need to, and they're pulling it over the original line that was
expected to only pull 120W.
It's a bit confusing because you seem to understand this given your later quote:
Finally, you never want to increase the amperage of which a circuit was designed
Now, while I may not think this is overall dangerous in the grand scheme of things (the amount of current we're adding is negligible), I don't know of any mechanics that are trained and licensed electricians. Adding a very low wattage device to an open connection in the fuse box, coupled with a properly rated fuse (in this case, any USB device should almost certainly be paired with a 5A fuse) is almost objectively a better decision than cutting and splicing in additional wires,
especially to something as power-hungry and important as the ignition connection.
