NABCEP PVDS flashcards

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

STC definition

Standard Test Conditions: 1000 W/m² irradiance, 25 °C cell temperature, and AM 1.5 spectrum. Module nameplate ratings are given at STC.

1.25 × 1.25 factor

PV source and output circuits use Isc × 1.25 for continuous current, then conductor ampacity must be ≥ 1.25 × that value, giving 1.5625 × Isc total.

Cold-temperature Voc correction

Voc at the lowest expected ambient equals Voc(STC) × [1 + βVoc × (Tmin − 25 °C)], where βVoc is negative. Use record low ambient to check maximum system voltage.

Maximum series fuse rating

The largest overcurrent device the module manufacturer lists for string protection. The string OCPD must be at least 1.25 × Isc and no greater than this rating.

Rapid shutdown threshold

NEC 690.12 requires conductors within 1 ft of the array to be limited to 30 V within 30 seconds of shutdown initiation, achieved with module-level electronics.

Inverter AC output current

Rated AC power divided by nominal AC voltage. The AC output conductor and OCPD must be sized at ≥ 1.25 × this current per NEC 690.9.

Voltage drop formula

VD = 2 × L × I × R / 1000 for two-wire circuits, with L in feet and R in ohms per 1000 ft. Keep total system drop typically at or below 2–3 %.

Equipment grounding conductor sizing

PV equipment grounding conductors are sized per NEC 250.122 based on the rating of the overcurrent device protecting the circuit.

120 % busbar rule

NEC 705.12(B): the sum of the main breaker and the backfed PV breaker must not exceed 120 % of the panel busbar rating.

String sizing window

Minimum string length must keep Vmp above the inverter's MPPT minimum at the hottest expected cell temperature; maximum length must keep cold-corrected Voc below the inverter max DC voltage.

Wind uplift design pressure

p = q × (GCp − GCpi) per ASCE 7. Corner and edge roof zones have larger GCp magnitudes, so attachment density must increase there.

Module clamp zone

The region along the module frame where the manufacturer permits mounting clamps, typically within one-quarter of the span from each end. Clamping outside it voids the warranty.

Rail cantilever limit

The unsupported overhang of rail beyond the last attachment should not exceed one-third of the adjacent span unless the manufacturer's span table allows more.

Thermal expansion of aluminum

Aluminum expands about 23 µm per meter per °C. Long rail runs over roughly 24 m need slip joints or expansion gaps to avoid buckling.

Roof dead load check

Sum module, racking, and ballast weight and compare to the structural report's allowable dead load, adding snow load per ASCE 7 for the site.

Firefighter access setback

IRC and IBC require a clear 36 in (0.91 m) path at the ridge and eave on residential roofs; modules must stay outside these zones.

Inter-row shading spacing

On flat roofs, row spacing ≈ module height × (sin tilt / tan solar altitude) evaluated at the winter solstice design hour to prevent inter-row shading.

Roof penetration sealing

Penetrations must sit above the water line of the roof course and use EPDM or silicone flashing, or a curb/pitch pan on low-slope roofs.

Solar window assessment

Measure the horizon profile at the array azimuth; obstructions above roughly 15–20° elevation in the winter path indicate shading loss needing modeling.

Solar access percentage

The share of annual sun-path hours unobstructed at a point, reported by shade tools. Below about 70 % access, relocate or redesign the array.

Roof remaining life rule

If the roof covering has less than about 10 years of remaining life, re-roofing before installing PV is the standard recommendation to avoid costly removal later.

Service panel survey data

Record busbar rating, main breaker size, available breaker spaces, and service voltage. These drive the 120 % rule and interconnection approval.

Typical GHI range

A good US site receives about 4.5–6.0 kWh/m²/day of global horizontal irradiance. Apply tilt and azimuth correction factors to estimate plane-of-array energy.

Structural survey inputs

Collect rafter or joist size, spacing, span, and sheathing thickness. A licensed structural engineer reviews when added load exceeds prescriptive code limits.

Soiling and snow losses

Local dust, pollen, or agricultural sources raise soiling loss; snow cover can cut winter production 20–100 % depending on tilt and shedding behavior.

Site survey record items

GPS coordinates, roof pitch and azimuth, obstruction dimensions, distance to main panel, conduit or trench path, and utility meter location.

Permit plan set contents

Site plan, roof plan, electrical single-line diagram, grounding details, structural calculations, equipment datasheets, and a string sizing table.

Single-line diagram contents

Shows module model and count, string configuration, combiner, DC disconnect, inverter, AC disconnect, service panel, OCPD ratings, conductor sizes, and grounding electrode conductor.

DC PV system label

NEC 690.53 requires a label at the DC disconnect stating maximum system voltage and maximum circuit current, marked 'Photovoltaic System'.

AC interconnection label

NEC 690.54 requires a label at the point of interconnection stating the rated AC output current of the PV system.

Commissioning test records

Record open-circuit voltage, short-circuit current, insulation resistance by megohmmeter, and ground continuity before energizing, and retain the results in the project file.

Fall protection threshold

OSHA 1926.501 requires fall protection for work at heights above 6 ft (1.8 m) in construction, via anchors, guardrails, or a personal fall arrest system.