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The fuse box only needs to have enough circuits of enough capacity to handle the loads anticipated. The fusing is to protect the wiring - not the devices. So, while a wire may handle 7A, the fusing on it should be fused to blow at the lowest amperage to allow device function, while also under the wire's capacity of 7A.

An important note is that automotive fuses are designed to not blow at the rated amperage, and very slowly as amps rise above rated; so don't oversize them for "extra protection" or thay cannot blow until total device failure. If a device requires 3A in it's specifications or on the label, then fuse it with 3A as a general rule.

Not quite sure what you are specifically asking for otherwise, but one response is to use a better grade wire for overall project cost savings. For example, OEMs today use cross-linked polyethylene (XLP) wire insulation as it has a higher temperature rating than the old standard PVC. While the gauge should be appropriate for the loads anticipated and within the temperature limits of the wire used; this means they can use lighter-gauge wire for each application.

Minimum gauge (for anything) was 18AWG for PVC insulation for durability and handling, which is now 20AWG with XLP wire. Less expensive due to less copper cost. For stuff like your horn button harness and the loads it would carry (3-7A without relay), 20AWG SXL grade wire would be suitable, or 18AWG PVC GPT.
 
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