FilterGuy
Solar Engineering Consultant - EG4 and Consumers
OK, Here are the key specs for the panelsMy 4 panels are CanadianSolar CS3U-380MS
Pmax: 380 not 385W, Vmp: 40.0V, Imp: 9.5A, Voc: 47.8, Isc:10.1A
Also I go down to -40C in winter averaging -22C so how does that affect the 174Voc @ -10C?
With two in series the string voltage @ STC is 2*47.8=95.6V. That leaves 54.4V or 54.4/95.6 = 57% headroom. Anything above 25% headroom is plenty for even the most extreme cold conditions. 3 in series would come to 143.4V and that is too high for cold conditions.
If you want to learn more about adjusting Voc for temperature, this resource may help.
Adjusting Solar Panel Voc for temperature
When designing you solar panel system, it is important to adjust you solar panel Voc for temperature in order to ensure you do not over-voltage the PV inputs of your solar charge controller. This paper shows how to calculate the Temperature...
diysolarforum.com
Current on PV line:
Even the short circuit current of these panels is 'only' 10.01A so you should never see much above 30A with 3 strings in parallel. However, for safety reasons the NEC requires a multiplier of 1.55. Even with that multiplier the max would only be 46.6A. I am still baffled on how that would ever trip a 100A breaker (even in very hot conditions).
6AWG is more than enough from a safety point of view with 3 strings in parallel and since the normal operating current is going to be closer to 28A, it is good from a voltage drop point of view for runs as long as 100 ft from the controller to the panels. (Longer will still work but the voltage drop will start inching up above 3%)The from the Pv to the combiner box are #6 AWG
For 4 strings in parallel, the max current with the NEC safety margin would be 62A. The 6AWG can easily handle that from a safety point of view.
Operating current will be around 38A so runs as long as 80 ft would still be less than a 3% voltage drop. (Since the controller will probably cap the current at a lower value, you can probably still go 100ft without any significant worries about voltage drop)
The big remaining question is can the Classic handle over-paneling with a 4th string. I am pretty sure it can.... but would like to get confirmation from Midnite. If over-paneling is allowed, you should be able to just add the 4th string and not worry about taking them in and out of the circuit.