From HeatSpring.com PV Boot Camp
Can someone go into further detail on the enhanced irradiance conditions that would cause a PV source circuit current to exceed the short circuit current? Is there some sort of magnification of irradiance under certain conditions? If so, has anyone been able to determine these conditions are happening with modern data acquisition systems?
Usually irradiance is less than 1000W per square meter, but in certain places you can have irradiance over 1000W per square meter on a regular basis, such as in the high desert. Irradiance outside of the earth's atmosphere is 1366 watts per square meter, so you do not need any reflection to get over 1000 watts per square meter.
You can also get reflections off of clouds, which we call the edge of cloud effect that gives us higher irradiance. I check the Enphase monitoring of some systems I installed and on a very regular basis, partly cloudy days have bursts of more power than days without clouds. You can see it spike down then up then down, etc. with partly cloudy days. I have measured 1300W per square meter with an irradiance meter on a partly cloudy day.
Hopefully we will never have a short circuit current, but Isc is about 7% more than Imp. That would make 125% of Isc about 132% of Imp and that would take about 1320 watts per square meter.
In the 2017 NEC, there are going to be some new ways to make these calculations for current, so we will have some more options.