It's interesting how one's point of view can change one's understanding of a situation. buzz takes the point of view that the wire and its size is in place and must be dealt with. I think the actual point of view is that the loads and the cost of copper determine the fuse ratings and the sizes of the wires.
On September 19, 2017 at 22:23, buzz said...
The purpose of a fuse is fire prevention.
Well, yes, but it also protects the source of the power. You want to keep a power amp from blowing at current levels much below those that will cause a fire.
In your car the branch fuses (radio, fan, wipers, lights) are sized according to the current carrying capacity of each wire.
I don't think so. Thicker wire costs more than thinner wire. Since wire costs money, the wire is sized according to the amount of current it should be carrying -- it's not that the wires are some random size and the fuse is there to protect them. The fuse is rated to allow more than enough current for the load.
This keeps the wire from burning if there is some sort of fault along the way.
The wire is sized such that it can always provide the current needed by the load while not getting hot. This issue is way below burning level.
The source wire to the block of fuses might have its own fuse. The size of this fuse is determined by its own wire size (again for fire protection of that wire)
No, the wire is sized to deliver the current needed by the load, and the fuse is chosen to blow before the load will blow (though we know the jokes about transistors etc being devices that blow in order to protect fuses!).
and this fuse does not need to be the sum of the branch fuses -- unless there is a high probability that every branch will be drawing its maximum allowed current simultaneously. For example, a radio connected to a 20 Amp branch circuit might only draw a few amps while operating. Further, the dome lights are usually OFF while driving.
Exactly.
The primary wire connected to the battery terminal is usually not fused because it is sized to carry enough current to run the starter. As such, even if there is a large fault on that wire, the battery voltage will quickly collapse and limit the current.
Every car I've ever seen has two wires coming off the positive. One is very thick and goes directly to the starter solenoid. The other is what I'd call the primary wire, and it goes to a block of fuses. Yes, it's not fused at the battery, but each branch of it is fused.
