The fuse/ circuit breaker is required to first protect the wiring against overload (30 amps on a 20 amp circuit) and faults/ short circuits (500 amps on a 20 amp circuit). Typically the fault condition (starter wiring shorting on motor
mount) will be quickly cleared by a wide range of fusing sizes. Overload conditions are usually the more critical situation and there are tables that relate fuse sizes to wire size and also include a derating for higher ambient temp situations (wire running through the engine compartment).
As to protection of the load, sometimes you have it and sometimes you don't. Dedicated circuits (feeding a single
load) can usually be sized to "kill two birds with one stone" (protect the wiring and protect the load most typically motor circuits).
circuit can feed multiple loads and if they do not operate simultaneously the sum of their ampere draw can exceed the rating of the feeding conductor. If they all can operate at the same time then the feeding circuit must be sized to protect the total.
I have a single 20 amp circuit feeding my steering pedestal
with a 10 amp breaker feeding the AP, 10 amp feeding the radar
, 5 amp cigarette lighter, 2.5 amps feeding GPS
, and 2.5 amps feeding some remote
wireless stuff. All these breakers are mounted in a small box near the pedestal
. If I remember correctly there is an ABYC rule
on the sum of these breakers but don't remember off the top of my head
In 12 volt circuits often the wire is increased in size to reduce voltage drop if there is significant circuit length.
In the situation you asked about protecting the 2 amp load with a 5 amp fuse while not ideal better than protecting it with a 20 amp fuse. Protecting the 45 amp wire with a 50 amp fuse could be that for the fuse type in use it is not available in 45 amps. It is usually allowed to move up to the next larger standard size. In this case as 45 amps on a #12 is pushing it a little, think I would up the wire to #10.
On modern boats with all their electrics it is just not practical to protect every thing against every condition all the time (Hella Fan wires is one of my small wire irritants). There are some risks in all endeavors of life (assuming you get out of the bed
in the morning). You try to meet the concepts of the rules, but there will always be a few areas where it just don't work
out. Rules/ codes usually don't allow for judgement but good judgement is what makes the world work
(judgement based on good technical info).
Being a little more anal than most I have a number of 5 and 2.5 amp breakers scattered around my boat
, but bet that Garmin Chartplotter
wire and load are well below 2.5 amps. Also have several 1 and 2 amp fuses feeding instruments.