MATT--I apologize for the late reply. I just now came across your October 9, 2020 post with some questions.
...should [there] be a circuit breaker between the [battery switch] and the Blue Sea fuse box[?]
Let me answer this in a general way. Power distribution wiring is the method of distributing electrical power. In electrical power distribution there are three fundamental elements:
- the source of electrical power
- the loads that will consume the electrical power
- over-current protection devices, usually a fuse or a resettable circuit breaker, that will protect the wiring that connects the sources to the loads
In a small boat, the source of power can be considered to be the positive terminal of the boat battery. There are, of course, the negative terminal and all the negative circuits, but we can ignore them because of two reasons:
- there are never any fuses or circuit breakers in the negative circuit
- all loads share a common negative circuit
The wiring path from the battery positive looks like this:
BATTERY (+) ----unprotected short CONDUCTOR-A----main battery ON-OFF switch COMMON
A short unprotected conductor is allowed because the battery switch is usually within a foot or so of the battery terminal, the wire size is very large, and the conductor is often enclosed in a protective sheath. All of these factors tend to minimize the possibility that there could be a short circuit from this conductor to the battery negative. If the battery switch is more than about a foot or perhaps more than 18-inches away, there may be regulations or suggested practices that would prefer to see a fuse at the battery to protect this conductor. But in a small boat the battery switch is typically right next to the battery, so CONDUCTOR-A is not protected by any overcurrent device.
The next step in the wiring path will be
Battery Switch POLE--->unprotected CONDUCTOR-B ---> PRIMARY POSITIVE BUS
Again we have a wiring segment CONDUCTOR B that is not protected for over-current, and this is allowed for the same reasons as above for CONDUCTOR A. In some instances in a small boat there really is no separate BUS bar, and the terminal post on the battery switch itself performs the function of the PRIMARY POSITIVE BUS.
The next wiring path will be
PRIMARY POSITIVE BUS --->unprotected CONDUCTOR-C --->over-current device--->protected CONDUCTOR-D----> LOAD
The PRIMARY POSITIVE BUS typically will have a circuit breaker or fuse, located very close by, and unprotected CONDUCTOR-C connects them. The circuit breaker or fuse will protect the LOAD that is connected via protected CONDUCTOR-D.
Before going further, there is one important exemption from this practice: the propulsion engine electric starter motor. This load and the conductor connecting it to the PRIMARY POSITIVE BUS are not protected in most installations. The current anticipated to flow in this path is on the order of 300-Amperes or more at peak, so a fuse or circuit breaker is somewhat impractical. Also, the length of the conductor is anticipated to be short, a few feet at most, the conductor size is anticipated to be very large, and there is an expectation of some protection against damage to the conductor, such as very generous insulation covering the conductor and perhaps enclosure in a sheath. Finally, there is an expectation that there is a battery switch available to completely cut off current to this path if necessary in the event of a short circuit
All other LOADS should have over-current protection. A typical load on the PRIMARY POWER BUS will be a SECONDARY POWER DISTRIBUTION PANEL. (Now I am getting to your actual question.) This wiring path will be as mentioned above:
PRIMARY POSITIVE BUS --->unprotected CONDUCTOR-C --->over-current device--->protected CONDUCTOR-D----> SECONDARY POWER DISTRIBUTION PANEL
The length of protected CONDUCTOR-D may be quite long, perhaps more than 15-feet. The purpose of the over-current device is two-fold. First, the over-current device must protect against a short circuit of CONDUCTOR-D that might occur, so the value of the over-current device trip rating must be smaller than the rating of the conductor for maximum allowed current flow. For example, if an 8-AWG wire is used for CONDUCTOR-D, the wire itself is rated for a maximum current flow of 73-Amperes. (See the 'chassis wiring rating" in table at
https://www.powerstream.com/Wire_Size.htm.) The over-current protection device should be rated for a lower trip curent. The second purpose is protect the power distribution panel itself. The panel should have a rating for maximum current. A typical panel is a Blue Sea Systems fused panel model 5026, which is rated for a very generous 100-Amperes. So the over-current device should be less than 100-Amperes. The over-current device in this example would then be chosen to be some value less than 73-Amperes, but large enough that the normal current demand of the secondary power panel loads won't cause a nuisance-trip, that is, won't trip in normal operation. This can be estimated by adding up the total loads connected to the panel by adding up the fused circuit values, but with an eye to which ones will be in use simultaneously. A typical figure for total fused loads expected at any one time might be 50-Amperes. So the overcurrent device is chosen to be 50-Amperes. Because fuses and circuit breakers are only available in particularly values, you often have to choose a bit higher or even a bit lower than the value you might calculate.
So there is the answer to your original question: yes, you should have an overcurrent device between battery and the Blue Sea System power distribution panel.
But let me hedge a bit on that recommendation. If on a small boat there is no PRIMARY POSITIVE BUS, and the battery switch terminal performs that function, and if the SECONDARY POWER DISTRIBUTION PANEL is only a foot or so away from the battery switch, and if the conductor connecting them is large and has generous insulation and is protected against damage, you might be able to consider that the power distribution panel is really the primary positive bus and omit that circuit breaker. In the case of recreational boats, it is your boat, you are not a boat builder trying to comply with ABYC recommendations, there really are no USCG recommendations that apply directly to this situation (that I know of), and you can more or less do what you want. I would use a circuit breaker.