what size fuse/ breaker

Welcome to the forum Breealee,

I edited your post into a list of what you have--Easier (at least for me) to follow.

The battery is usually the "heart" of your system. All wiring leaving the battery (the source of high surge current into loads and/or short circuits). Wiring and fuses/breaker are sized for your loads. And fuses/breakers are there to protect your wiring (from overheating/starting a fire). Fuses/Breakers are not there to "protect your equipment" from overcurrent (usually any electronics and other loads will fail before a fuse/breaker trips).

So, what are your DC loads... Maximum continuous current (like LED lighting, running a DC CPAP machine, HAM radios, or what?).

If I recall correctly, the Battle Born batteries will supply a maximum of 100 Amps before their internal Battery Management System (BMS) will shut them down.

Assuming that you would draw 100-200 Amps from your battery bank for your loads--Probably not what you are planning--It would take your battery bank "dead" in 1-2 hours--Not very useful.

To take a stab at what "reasonable" loads--Let's look at your solar panel. Say 360 Watts, in Tucson AZ, flat on roof:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Tucson
Average Solar Insolation figures

Measured in kWh/m2/day onto a horizontal surface:

Jan	Feb	Mar	Apr	May	Jun
3.30	4.20	5.64	6.89	7.59	7.52
Jul	Aug	Sep	Oct	Nov	Dec
6.43	5.87	5.55	4.57	3.62	3.05

And guess that most of your dry camping will be in months with >5 hours of sun per day (March-September). The daily harvest would be:

• 360 Watts * 0.77 panel+controller deratings * 5.0 hours minimum average sun per day = 1,386 WH per day (of 12 VDC energy)

And lets assume that you use ~5 hours of power (at night) for lighting, computer, charging cell phone, RV water pump:

• 1,386 Watt*Hours per day harvest / 5 hours of use per day = 277 Watt average load
• 277 Watt load / 12 volt battery bank = 23 amps
• 23 amps * 5 hours per day load = 115 AH @ 12 volts from battery bank

Those are all "reasonable" numbers to expect from your battery bank... You would use a little more than 1/2 of your battery bank capacity... I would suggest that plan on using between 50-80% of your battery capacity (for LFP batteries).

You can use the NEC table to size the cabling for your loads:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

You don't have too, but I like to derate the NEC wiring table by 80% (or 1.25x). So, for a 23 amp DC loading (assuming max continuous current):

• 23 Amps * 1/0.80 derating = 28.75 amps ~ 30 Amp branch wiring and fuse/breaker ratings

And for your solar charging--360 Watt panel and 30 amp MPPT controller... The max current expected from the solar panel:

• 360 Watt * 1/0.80 NEC NEC derating * 1/10.5 minimum battery bus voltage (worst case with MPPT controller) = 43 Amps

But MPPT controllers safely and normally limit their output current to their maximum rated output... So, in this case:

• 30 Amps * 1/0.80 NEC derating = 37.5 Amps = 40 amp rated wiring/breaker

And that is the minimum size wiring rating I would use for wiring your two batteries... From NEC, depending on insulation type, that is around 8 AWG minimum with a 40 amp breaker.

I don't know which 360 Watt solar panel you have, but to estimate its output current using Vmp~38 VDC:

• 360 Watts * 1/0.8 NEC derating * 1/38 volts = 11.8 Amps typical worst case current

So, your 15 amp rated switch/breaker is OK.

Please note that all switches/breaker/fuses need to be rated for DC usage. The solar panel switch need to be rated to >Voc (around 50 VDC), and the DC battery bus breakers need to be rated for 12 VDC or higher. AC current is "easier to break" vs DC current (DC "sustains arcs" much easier).

Each load/charging source should be "home run" back to the battery + and - bus (you have a Battery Monitor, so the + bus is the battery common connections. The - bus is not the battery but the "ground side" of the precision Battery Monitor power resistor.

There is one other major issue to take into account. And that is voltage drop for your wiring. For example, say you want to send 23 amps 20 feet (one way run for this Voltage Drop calculator), and you want no more than 0.5 VDC drop:

For 8 AWG cable the voltage drop would be:

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=2.061&voltage=12&phase=dc&noofconductor=1&distance=20&distanceunit=feet&amperes=23&x=0&y=0

Result

Voltage drop: 0.58
Voltage drop percentage: 4.82%
Voltage at the end: 11.42

And if you used 10 AWG (which is perfectly OK for a 30 amp circuit):

https://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=3.277&voltage=12&phase=dc&noofconductor=1&distance=20&distanceunit=feet&amperes=23&x=54&y=22

Result

Voltage drop: 0.92
Voltage drop percentage: 7.66%
Voltage at the end: 11.08

Which is a lot of voltage drop for a 12 volt circuit. Sending significant amounts of power any distance at 12 VDC generally needs lots of copper.

Anyway... Lots of typing and assumptions. Questions/clarifications/corrections?

-Bill