Originally Posted by goboatingnow
rate the cables for the solar panel to the MPPT controller on short circuit panel amps ( assuming all panels are in parallel it will therefore be "no of panel x short circuit amps")
Rate the mppt cable to the battery on the max MPPT controller output amps.
Nothing is ever simple on CF Dave
From the panels to the controller I would use Imp not Ioc. The maximium power current can occasionally be exceeded, but this is very rare and the excess will only be very slight. The short circuit current will never be reached. This is for a single
panel, or panels wired in series. If wired in parallel you need to to add the Imp of each panel.
I am not sure what you mean by "max MPPT controller output amps". If you mean the maximium rating of the controller, this is not relevant. The simplest way is to covert the total solar panel wattage to current. The battery will be charging
so the voltage is unlikely be below 13v, but you can use 12.5v or even 12.0v if you want to cover all possibilities. Solar panels can very occasionally exceed their specified watts but once again this is very rare and the excess will only be slight.
So for this example:
Panel to controller 5.97x 2 = 11.94A
Controller to batteries 435 x 2 / 13 = 66.9A
If we work out the voltage drop with various wires:
Panel to controller
8AWG = 0.90 v
6AWG = 0.57 v
4AWG = 0.36 v
Controller to batteries
4AWG = 0.40 v
3AWG = 0.32 v
2AWG = 0.25 v
So what voltage drop do we accept? All the above results are calculated on the maximium panel output so the voltage drop will normally be less. There is no perfect answer. Some common solutions are a total voltage drop of 10%, others like 5%, others 3%. The voltage drop from the controller to the batteries is more critical as this will effect the voltage set points.
I would suggest 4 AWG between the panel and the controller and 2 AWG between the battery and the controller would be a reasonable compromise. One caution is to actually measure the wire distance once it passes around bulkheads etc. The actual distance is often greater than the estimation.
There, I made it complicated