I have a Classic 150 hooked to 10 100 watt panels They are arranged with 2 banks of 5 in series parallel together. when I first start the controller it says around 100 volts coming in, but once it begins to put our power to the batteries it drops to around 70 volts. Is this to be expected?
Also the terminal size for 90 amp / 12 volt output seems a little small to me..
You will have some drop when the circuit is under load. The I2R losses in the cabling will depend on the size of cable coming from your array and the current. The size of the array will also be a factor. The size of the terminals have been discussed before and #4 AWG will fit in the terminals and is sufficient for any load the classic can output.
What you are seeing is the difference between the open circuit voltage of the panel with NO load and the max power point voltage with a load. So to get the maximum current out of a PV module you have to load it down, usually this is around 75-80% of the open circuit voltage