NABCEP PVIS flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

STC vs NMOT

STC: 1000 W/m², 25 °C cell, AM 1.5. NMOT/PTC: ~800 W/m², 20 °C ambient, 1 m/s wind, open rack. NMOT ratings are typically 10–15% lower than STC.

Temperature coefficient of Voc

Negative value (about −0.25 to −0.35%/°C for c-Si) used to calculate cold-temperature maximum string voltage. Cold increases Voc, so size strings for record-low temperature.

Cold-temperature string sizing

Vmax_string = N × Voc × [1 + βVoc × (Tmin − 25)]. N must keep this value below inverter max DC input voltage and any 600/1000/1500 V system limit.

Hot-temperature string sizing

Vmp_string = N × Vmp × [1 + βVmp × (Thot − 25)]. The result must remain above the inverter MPPT lower voltage limit to avoid derating or dropout.

NEC 690.8 continuous current

PV source and output circuits are continuous; multiply by 1.25 for conductor ampacity and overcurrent device sizing. Inverter output circuits also use 1.25× rated output current.

Rapid shutdown 690.12

Conductors within 1 ft of the array boundary must be limited to 30 V or less within 30 seconds of shutdown initiation. Module-level or string-level electronics typically provide compliance.

DC/AC ratio (clipping)

Array STC kW divided by inverter AC kW. Fixed-tilt systems commonly use 1.1–1.3; higher ratios clip more at peak but harvest more at low irradiance and diffuse conditions.

Voltage drop formula

Vdrop = 2 × L × I × R for two-wire DC circuits, where L is one-way length and R is conductor resistance at operating temperature. Keep DC drop ≤ 2–3%.

String fuse rating

Fuse must be ≥ 1.25 × Isc (per 690.8) and ≤ the module maximum series fuse rating. Parallel strings of three or more require string overcurrent protection.

Irradiance vs irradiance units

Irradiance is instantaneous power density in W/m²; irradiation (insolation) is energy per area in kWh/m²/day. Design uses irradiance for power and irradiation for energy yield.

Air mass (AM)

Ratio of path length through atmosphere to vertical path. AM 1.5 is the reference spectrum for STC; higher AM (low sun) shifts spectrum and reduces output.

Shading analysis tools

Solmetric SunEye, Solar Pathfinder, and 3D modeling (Helioscope, PVsyst, Aurora) quantify horizon shading. Use hourly simulation for inter-row and object shading losses.

Row spacing calculation

Minimum row spacing = collector height × cos(azimuth tilt) / tan(solar altitude at winter solstice). Use 9 a.m.–3 p.m. window to avoid inter-row shading.

Grounding electrode system

NEC 250 requires bonding PV module frames, racking, and inverter enclosures to the equipment grounding conductor; array mounting structures need a grounding electrode conductor sized per 250.66.

GFCI vs GFDI

GFCI protects personnel from ground faults on AC circuits. GFDI (ground-fault detection and interruption) is required on grounded PV arrays by NEC 690.5 to detect DC ground faults.

Arc-fault circuit protection

NEC 690.11 requires DC arc-fault circuit protection on PV systems operating at 80 V or more. Listed AFCI devices detect series arcs and interrupt the circuit.

Module IV curve

Plot of current vs voltage. Key points: Isc (short circuit), Vmp (max power voltage), Imp, Voc. Fill factor = (Vmp × Imp)/(Voc × Isc); higher FF means better cell quality.

Bypass diode function

Allows current to bypass a shaded or failed cell substring, preventing hot spots and reducing mismatch loss. Typically one diode per 20–24 cells in a module.

Blocking diode function

Prevents reverse current from flowing into a lower-voltage string or battery. Rarely needed with modern string inverters that have integrated reverse-current protection.

Inverter efficiency (CEC)

California Energy Commission weighted efficiency accounts for part-load performance. Typical string inverters: 96–98%; microinverters: 95–97%. Use CEC value for energy yield estimates.

Performance ratio (PR)

Ratio of actual AC energy to theoretical DC energy at STC irradiance. PR of 0.75–0.85 is typical; losses include temperature, soiling, mismatch, wiring, and inverter.

Soiling loss

Energy lost to dirt, dust, bird droppings, and snow on modules. Typical 2–5% annually in moderate climates; higher in arid or agricultural areas. Cleaning schedule based on measured loss.

Module mismatch

Power loss when series-connected modules have different current or voltage due to shading, soiling, or manufacturing tolerance. String sizing and sorting by flash-test data reduce mismatch.

Battery depth of discharge

Percentage of capacity withdrawn from a battery. Lead-acid typically limited to 50% DoD for cycle life; lithium-ion can often reach 80–100% DoD with BMS protection.

Charge controller MPPT vs PWM

MPPT controllers track the array's max power point and convert voltage, giving 10–30% more harvest in cold or low-irradiance conditions. PWM is simpler and less efficient.

Article 690 vs 705

NEC 690 covers PV source and output circuits. Article 705 covers interconnections of power production sources with primary or secondary distribution systems, including load-side taps.

Supply-side connection

PV system connected on the line side of the main service disconnect. Requires a dedicated disconnect and often utility approval; no 120% busbar rule applies.

120% busbar rule

NEC 705.12(B): sum of main breaker plus backfed PV breaker must not exceed 120% of busbar rating, unless the PV breaker is at the opposite end of the busbar.

Line-side tap

Connection made between the utility meter and the main service disconnect. Must comply with NEC 230.82 and utility interconnection requirements; typically requires a visible disconnect.

Inverter anti-islanding

Required by UL 1741: inverter must detect loss of grid and stop exporting within 2 seconds. Prevents backfeeding during outages and protects line workers.

Module-level power electronics

Microinverters or DC optimizers mounted at each module. Enable per-module MPPT, rapid shutdown compliance, and module-level monitoring, at higher cost and component count.

Structural dead load

Flush-mounted PV adds roughly 2–4 lb/ft² to a roof; ballasted flat-roof systems add 3–7 lb/ft². Verify against structural engineer's allowable load and local snow/wind loads.