NABCEP - Certified North American Board of Certified Energy Practitioners Specialist PV System Design Principles Questions and Answers

Question 1

A PV system designer is selecting a string inverter for a 12 kW STC DC array located in a moderate climate. To optimize annual energy harvest by balancing performance gains against minimal power clipping, which of the following DC-to-AC load ratios is most appropriate for this design?

Accepted Answer

1.25

Answer

A DC-to-AC ratio of 1.25 is a common and effective industry practice for optimizing PV system performance. [2, 10, 12] This means the DC nameplate rating of the solar array is 25% larger than the AC power rating of the inverter. This 'oversizing' allows the inverter to operate at or near its peak efficiency for more hours of the day, capturing more energy during lower light conditions (morning, evening, cloudy days) and compensating for real-world losses like temperature derating and soiling. [2] A ratio of 1.00 is a direct match and often suboptimal, while 0.95 represents an oversized inverter. A ratio of 1.75 is typically too aggressive and would lead to excessive energy loss from clipping. [12]

Question 2

According to NEC® 690.7, what is the most critical factor to use when calculating the maximum system voltage of a PV string to ensure it does not exceed equipment voltage ratings?

Accepted Answer

The module's open-circuit voltage (Voc) temperature coefficient

Answer

NEC 690.7 requires the maximum system voltage to be calculated for the lowest expected ambient temperature at the site. [1, 4] PV module voltage increases as temperature decreases. Therefore, the module's open-circuit voltage (Voc) temperature coefficient, provided by the manufacturer, is essential for accurately determining the highest voltage the string will ever produce in cold conditions. [4, 11] This calculation ensures the voltage does not surpass the maximum DC input voltage rating of the inverter and other components. [1]

Question 3

Which of the following design strategies is most effective at mitigating energy production losses caused by partial shading of a single module within a larger series string?

Accepted Answer

Employing module-level power electronics (MLPE)

Answer

Module-level power electronics (MLPE), such as microinverters or DC optimizers, are the most effective solution for mitigating shading losses. [17, 19] In a standard string, the current is limited by the lowest-performing (shaded) module, which can significantly reduce the output of the entire string. [8, 19] MLPEs manage the power output of each individual module, so the poor performance of a shaded module does not impact the production of the unshaded modules in the array. [17]

Question 4

A designer is sizing the conductors for a PV source circuit. The module Isc is 10A. Per NEC® 690.8, the conductors must be sized to carry at least what minimum ampacity before any adjustments for conditions of use?

Accepted Answer

15.6 A

Answer

According to NEC 690.8(B), PV circuit conductors must be sized to handle a minimum of 125% of the maximum current. [3] The maximum current, per NEC 690.8(A), is calculated as 125% of the module's short-circuit current (Isc) to account for periods of enhanced irradiance. [1, 18] Therefore, the total calculation is Isc × 1.25 × 1.25, which equals Isc × 1.56. In this case, 10A × 1.56 = 15.6A. [1, 22]

Question 5

For a fixed-tilt residential PV system located in the Northern Hemisphere, which orientation is generally recommended to maximize total annual energy production?

Accepted Answer

True south at a tilt angle equal to the site's latitude

Answer

To maximize annual energy production for a fixed-tilt system in the Northern Hemisphere, the array should be oriented toward true south (180° azimuth). [13, 14, 15, 16] The generally accepted rule of thumb for the optimal tilt angle for annual production is to set it equal to the site's latitude. [13, 15] This combination provides the best average exposure to the sun's path throughout the year. [15]

(NABCEP) Certified North American Board of Certified Energy Practitioners Specialist Practice Test

(NABCEP) Certified North American Board of Certified Energy Practitioners Specialist Practice Test

NABCEP - Certified North American Board of Certified Energy Practitioners Specialist PV System Design Principles Questions and Answers