Weather conditions can exceed STC rating of 1000 Watts per meter squared. For example, on a clear day with snow reflection, or high altitude locations. This is the “irradiance factor”. We have to take this into account regardless of the location of the system for the periods of time where we can exceed 1,000 W/m^2 – when modules make more current than what’s on the module labels, in order to determine wire size.
Therefore the maximum PV circuit current is calculated by:
ISC (the short circuit current from label of modules) X 1.25 (irradiance factor).