Originally Posted by nigelmercier
Whoever wrote that (if indeed they did) is an idiot. The wire should be sized to provide the maximum device current at an acceptable voltage loss. The fuse or breaker should be sized so as to pass this current continously, this will protect both the wire and the device.
The panel breaker or fuse is sized to protect the branch circuit wire.
A 15 A panel breaker may be connected to 5 devices, each requiring current limiting of 3 A. (That the appropriate sized wire has to be run is a separate issue.)
In this case, the panel breaker does not protect any device in the circuit. It only protects the wire should for some reason, the sum of the device current or wiring
short circuit attempt to draw more than 15A.
It's sole purpose is to prevent the branch circuit wire from passing more current than it can safely carry without burning.
Wire size selection is a separate issue and varies with max current carrying capacity, max voltage drop, bundling, and ambient temp.
Some devices may
require current limiting.
For example, the panel may have a 15 A breaker marked "Instruments". This breaker may feed a fuse block to which a VHF
, and a depthsounder (each with there own off switch) are connected.
If the required current limiting device for each of these 3 devices is 5A, the 15A breaker is fine (assuming the wire is correctly sized). However, each instrument requires its own 5A fuse, so if one fails due to a wiring
short circuit, the others remain operational.
Sorry, no idiot here.