Originally Posted by Grumpy-Hodge
Gil pulled the reference I had in mind. Read specifically the next to last sentence in Gil's AC43-13 reference beginning with "Wires...".
Just like the house wiring rv7charlie mentioned, 12-2 Romex is usually found on 20A branches and 14-2 Romex is usually found on 15A branches. If a wiring fault occurs, the CB should trip and arrest the flow of current in the wiring before the wiring overheats, catches fire and cause potentially catastrophic damage. An aircraft CB may not be able to prevent many fault conditions in an Line Replaceable Unit (LRU) from continuing unless it is of the magnitude of the CB rating or much greater as in a short condition, but the aircraft wiring should be protected if sized correctly not only for the load but for the circuit protection as well.
That's were I read it differently, in English not FAA-ese.
As an example I could wire my G5 with #18 wire and still have a 5 A breaker on it.
It would meet the requirement you mention of protecting the wire. You can always use a lower value breaker to protect the wire.
I think your reading of the requirement and using the highest value breaker to protect a wire is incorrect. The wire is protected with the lower value breaker.
To answer the Garmin Q. - it says #22 wire and 5A breaker. I might be extrapolating but I read that as minimum value for the wire (bigger wire still lets the G5 work), with the breaker being a recommendation to protect the device. In normal certified aircraft practice, a dedicated wire would be used with no sharing of other devices. This sort of removes the house analogy.