Voltage drop has been something I have struggled with, and still I know I don't understand it completely- but I will, eventually. The thing is there is so much more to it than just this basic equation…
When calculating voltage drop in AC conductors, it is standard practice to use the nominal voltage on the AC side. For example, if you were calculating the voltage drop in a conductor in a 240V residential system, you would use the 240V. However, when it comes to PV there are more variables to consider and even knowing the nominal voltage or array voltage wouldn't really be completely all inclusive to determine the drop in voltage. A PV system designer may decide to use the short circuit current [Isc] as a conservative measure in the voltage drop calculation to consider the potential voltage drop under extreme conditions imposed by high currents from fluctuations in irradiance. Also, the designer may chose to consider temperature fluctuations and the possible drop in system voltage due to high ambient heat, using the adjusted high- temperature system voltage on the DC side.
For example, when using a voltage drop formula to determine the PV output circuit conductor size in a grid-tied PV System, you would use the array voltage [module] and the maximum power current [Imp] of the module to determine the voltage drop potential under standard operation conditions.
In the case of #'s 47 and 49 of the 2009 NABCEP PVI SG, we are calculating a stand alone system. It is stated in these questions what we must consider "that Im is flowing" [that] but not that the voltage is that of the modules- therefor we use the nominal voltage of the batteries.
When calculating the voltage drop in question #50 of the 2009 PVI Study Guide, we are asked to calculate the voltage drop "under maximum power conditions at STC". This makes the question different in stating that we are designing for a stand alone system under maximum power conditions which would only be achieved with a MPPT tracker- in which case we could consider the voltage of the array.
The first thing I would do if I was taking an exam and I saw this would be to say – “wait just one gosh darned second…”. I would discern that because of the similarity in these questions and this little differentiation in wording, that something was being asked of me that was ‘specific’ and that there was something fishy. Something is different in these 2 questions, but you may not realize what was really being asked in the previous questions  until you get to the last one .
I hope this clarifies!
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