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Fuses are simply protection devices that melt before more expensive items break or cause fire. The ratings determine at what point they melt in the event of a short circuit, if a device requires 5A normal operation and suddenly tries to draw a lot more amperage then something is wrong so the fuse associated melts hopefully before something else does. Fuses don't restrict current flow in the manner you may be thinking in that post and the 15A fuse you installed between the battery and the accessory socket should be a good size.
If the USB adapter blew its internal fuse in both vehicles, as soon as you plugged it in with no accessories plugged into the adapter, it is probably damaged or defective so you probably need to replace or repair the USB adapter.
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