Using Voc and Isc output for site specific NEC calcs

Is anyone using the Voc and Isc hourly outputs to calculate the maximum Isc and Voc for use in the NEC voltage and current calculations in chapter 690 for PV systems over 100kW? I see in the help file description that these values are for reference only so does that mean they are not accurate enough to depend on?

Thanks

Hello,

You might be interested in the System Sizing macro that generates a report of operating voltages among other things. We could modify the script to do some of the calculations you mention, but it would be helpful to know how you would use that information.

To run the macro, click Macros below the Simulate button, and then click System Sizing in the list of macros.

Best regards,
Paul.

The NEC allows the use of calculated values for equipment sizing for PV systems 100kW and over. So instead of using the module data sheet values of Voc and Isc we can use calculated values, which are usually less and can allow the use of longer strings or smaller conductors. That is why I am asking about the Voc and Isc values that SAM calculates. I'm not looking for the operating values but what the Voc and Isc values would be during the year. 
There is an article in Solar Professional magazine (July/Aug 2018) where the author uses SAM and post-processing to calculate these values but the author chose to not use the values provided directly by SAM because the SAM values did not seem accurate based on his analysis. 

Back to my basic question then, are the Voc and Isc values calculated by SAM accurate? What does it mean "for reference only" for these values?

Thanks,

Marvin
 

Hi Marvin,

Thank you for sharing that excellent article: solarprofessional.com/articles/design-installation/simulating-nec-voltage-and-current-values?v=disable_pagination&nopaging=1

SAM reports the array's open-circuit voltage (Voc) in each time step based on the cell temperature in that time step. The cell temperature, in turn, is calculated from the incident irradiance and ambient temperature in the time step. The author's post-processing equations calculate a worst-case value of Voc, not a more accurate value. This is similar to the macro I mentioned above, where you can enter temperature values for worst-case calculations and compare those to the operating values SAM calculates.

Best regards,
Paul.

Thanks Paul. I thought the same for the Voc calculation, adjust for ambient temperature instead of cell temperature will give a higher value. What's your opinion on Isc?

Hi Marvin,

SAM calculates values of Voc and Isc in each time step based on a simulation of the system output given ambient conditions in the weather file. For both Voc and Isc, the worst-case value is likely to be higher than the maximum value SAM calculates. The SolarPro article discusses an approach to calculating worst-case values for both. I don't really have an opinion about that -- the article does a nice job of putting the worst-case estimates in the context of simulation results.

Best regards,
Paul.