Designing a space-constrained PV system for a boat. Advice needed.

The question is "what is the average/peak current expected", size the wire to manage those currents (and size to minimize voltage drop--if needed), and then size the fuses/breakers to protect the installed wiring. Details matter (exposed wiring, vs wiring in conduit, ambient temperatures--on deck under sun, vs inside/shaded, temperature rating of insulation, etc.).

In general, the wiring from the charge controller to the battery bank should be sized for low voltage drop (say 0.05 to 0.10 maximum for a 12 volt battery bank) to insure that the charge controller "reads" the voltage of the battery bank accurately. That will probably suggest a larger AWG cable--And would allow you to use a larger rated fuse/circuit breaker.

For example, say you use 40 Amps. And using the NEC table and deratings:
https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

• 40 amps * 1.25 NEC derating for wiring and fuses = 50 Amps
• From table, 8-6 AWG would be indicated
• Choose 6 AWG and based on insulation, you can use 55-75 Amp rated breaker

Now--Lets say you have 5 feet from charge controller to battery bank and want at most 0.1 volt drop at 40 amps, using a simple voltage drop calculator:
http://www.calculator.net/voltage-drop-calculator.html?material=copper&wiresize=1.296&voltage=12&phase=dc&noofconductor=1&distance=5&distanceunit=feet&amperes=40&x=73&y=7
6 AWG:
Voltage drop: 0.16
Voltage drop percentage: 1.33%
Voltage at the end: 11.84

Which is "too high" (in my humble opinion) for voltage drop... So try another AWG:

4 AWG:
Voltage drop: 0.099
Voltage drop percentage: 0.83%
Voltage at the end: 11.901

At 4 AWG, the voltage drop meets my minimum, and if you go to the NEC table, you can use a 70-95 Amp rated breaker. You have to look at the Outback manual to see if they have a maximum wire AWG (what fits the holes) and a maximum output breaker--Lets say 60 amps maximum output current:

• 60 amps * 1.25 NEC deratring = 75 Amps maximum (guess) breaker) ~80 amps (next standard size)

From the above (using my guesses), you would have a choice of ~50 to 80 amps for your breaker choice (depending on if you choose 6 or 4 AWG wiring).

For the panels--The fuses/breaker are there to protect the panel and the panel's jbox+attached wiring. Usually, panels have a series fuse rating (larger panels have a larger series fuse ratings vs smaller panels). If the panel does not have a series fuse rating, I would suggest a 1.25*1.25=1.56 derating over the Imp rating minimum (to prevent false trips). Say you have an Imp=5 amps:

• 5 amp Imp * 1.56 derating = 7.8 amp ~ 8-10 amp recommended fuse/breaker for that panel string (and wiring rated for 8-10 amps minimum).

You do not need a fuse/breaker on the combined input string of the Outback... Just that the wiring the carries all of the combined current be ~1.56x higher than Imp for the array. It would be assumed that the array will never be able to output more than 1.56x Imp (really 1.25 peak solar output current x 1.25 NEC wiring/breaker derating)--So no single input master breaker is needed. Although, many people like this so they can easily shut down the array for charge controller/wiring maintenance (a single breaker/switch easier to get at versus having to pull fuses/turn off breaker at the array).

-Bill

Normally, I would suggest that you put identical panels in series (2x) and run a Vmp~35 volt array. That would let you reduce the wire AWG a lot.

But, you have 3x of one panel--So, unless you "dumped" one 135 Watt panel (or got 1x more 135 watt panel to match--2x in series x 2x parallel strings)--That won't work.

Also--If the panels have different shading (shadows from lines, different sides of a deck house, etc.)--The pairs of panels will not work for you either--Blocking/shading part of one panel will kill the output from the entire 2x series string.

How long of cable run from the array to the Outback controller?

-Bill