NABCEP - Certified North American Board of Certified Energy Practitioners Specialist PV System Design Principles Questions and Answers — 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?
- 0.95
- 1.00
- 1.25 (Correct answer)
- 1.75
Correct answer: 1.25
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?
- Peak summer cell temperature
- The module's open-circuit voltage (Voc) temperature coefficient (Correct answer)
- The module's short-circuit current (Isc) at STC
- Average annual ambient temperature
Correct answer: The module's open-circuit voltage (Voc) temperature coefficient
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?
- Using a larger gauge conductor for the string wiring
- Installing the array at a steeper tilt angle
- Increasing the DC-to-AC load ratio
- Employing module-level power electronics (MLPE) (Correct answer)
Correct answer: Employing module-level power electronics (MLPE)
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?
- 10.0 A
- 12.5 A
- 15.0 A
- 15.6 A (Correct answer)
Correct answer: 15.6 A
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?
- True south at a tilt angle equal to the site's latitude (Correct answer)
- Magnetic west at a 45-degree tilt angle
- True north at a tilt angle 15 degrees less than latitude
- Due east at a low tilt angle (10 degrees)
Correct answer: True south at a tilt angle equal to the site's latitude
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]
Question 6: When designing the mechanical mounting and racking plan for a rooftop PV system, which of the following environmental forces is a primary consideration for ensuring the structural integrity of the array and roof?
- Seismic load
- Dead load
- Snow load
- Wind uplift load (Correct answer)
Correct answer: Wind uplift load
While dead load (the system's weight), snow load, and seismic loads are all important considerations, wind uplift is often the most critical environmental force acting on a rooftop PV array. [23, 24] The airfoil shape created by modules mounted on a roof can generate significant negative pressure (lift), especially at the corners and edges of the roof. [23] The racking system and its attachments must be engineered to withstand these uplift forces as specified by standards like ASCE 7 to prevent the array from being detached from the roof during high-wind events. [24, 25, 29]
NABCEP - Certified North American Board of Certified Energy Practitioners Specialist PV System Design Principles Questions and Answers
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?