Maximum PV Source-Circuit Current Comparison
The case study illustrates that using the new NEC 2017 maximum circuit-current calculation method results in a 15.4% improvement in PV source-circuit values compared to the traditional calculation method. Since the 690.8(A)(1)(2) calculation is based on the highest 3-hour current average and irradiance is the primary driver of PV source-circuit currents, maximum current calculations based on simulation results will vary from system to system.
The data in Table 2 illustrate how the percentage improvement associated with the new PV source-circuit current calculation method varies according to location, weather file and array orientation. Some trends are clearly evident in these results.
Location. On average, the percentage improvement is biggest at cold-weather sites. The average benefit (across racking type and weather files) in both Boston and Minneapolis is over 20%, while the average benefit in Phoenix is just 14%.
Orientation. On average, the percentage improvement is biggest in systems with lower tilt angles. The average benefit (across all sites) in dual-tilt (east-west) mounting systems is 21%, whereas the average improvement is just over 15% at a 25° tilt.
Weather data. The percentage difference associated with weather data source is relatively small. On average, satellite-based Prospector data resulted in a 1% improvement compared to ground measurement–based TMY2 or TMY3 data.
In the case of location and orientation, there is a strong inverse correlation between kWh/kWp and percentage benefit. This makes sense because lower total irradiance will typically lead to lower peak irradiance values. Since the rated current is calculated based on the peak irradiance, lower-irradiance systems will get the most benefit from the new calculations.
While this analysis does not include different modules, the results should be relatively agnostic to module selection. The temperature coefficient of current is the only module-specific factor in the calculations. This factor has a small impact on the results in comparison to irradiance.
—Paul Grana / Folsom Labs / San Francisco / folsomlabs.com