Ted, I would answer your question just a little differently.
Typically, breakers serve to protect the boat from the entire circuit by providing ultimate protection of larger loads - e.g. the port or starboard (or on a boat your size, perhaps the entire)
lighting circuit.
However, any piece of equipment that's rated to draw considerably less (let's say 2/3 or less) of the rating of the breaker probably needs to be protected. A good example is a
marine refrigeration unit. Typically, these are hooked up with heavier gauge wire (to avoid voltage drop which would affect the unit's efficiency) to a 15A breaker, and yet they will have an onboard fuse of perhaps 10A, to protect the control circuitry and onboard wiring. Another example that's perhaps applicable to your boat size is when you may have an 'Auxiliary' circuit, tied to a 10A or 15A breaker, but which supports a variety of
electrical items, none of which draw anywhere near that 10 or 15 amps. E.g. you may want to
power a tiller
autopilot, a
cockpit and/or
anchor light, and an AM/FM/CD
radio on this
single circuit. The amp rating of the breaker would easily support all these things when used concurrently but you'd want to protect them, individually, with fuses that are each sized appropriately to the
hardware. Usually, the owner's manual is the best guideline for choosing a fuse rating; sometimes I've been very surprised by what is recommended, and not always in the same direction.
BTW a great source of info for all of us 'learning' sailors/boatowners is Nigel Calder's _Boatowner's Mechanical and
Electrical Guide_, which is heavily illustrated, cleanly/clearly written, and provides all kinds of practical pointers that address questions like the one you asked. If you find it a bit pricey, remember you'll be using it for years...and you can buy it second-hand for a 'softer'
price. Be sure it's the 2nd Edition, however.
Jack
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