NABCEP PVDS cheat sheet

Power formula: P = V × I; for DC array sizing use module Vmp × Imp at STC (1000 W/m², 25 °C cell temperature, AM 1.5).

Ampacity rule (NEC 690.8): conductor ampacity ≥ 1.25 × continuous current; for PV source and output circuits the continuous current is already Isc × 1.25, so the combined factor is 1.25 × 1.25 = 1.5625 × Isc.

Inverter output current for a grid-tied inverter is rated AC output power ÷ nominal AC voltage; size the AC output conductor at ≥ 1.25 × that current (NEC 690.9).

OCPD rating rule: the overcurrent device must be ≥ 1.25 × Isc and ≤ the module's maximum series fuse rating; if 1.25 × Isc exceeds the max fuse rating, strings must be paralleled only with string fusing or a listed combiner.

Voltage temperature correction: Voc at lowest expected ambient = Voc(STC) × [1 + βVoc × (Tmin − 25 °C)], where βVoc is negative (typically −0.25 to −0.35 %/°C). Use record low ambient for the coldest design condition.

Maximum system voltage check: corrected cold-temperature Voc of the series string must not exceed the inverter max DC input voltage or the module/equipment voltage rating (commonly 600 V residential, 1000 V or 1500 V commercial).

Voltage drop: VD = 2 × L × I × R / 1000 for single-phase DC or AC two-wire circuits (L in feet, R in ohms/1000 ft); keep total DC + AC drop typically ≤ 2–3 % for performance, though code sets no mandatory limit.

Grounding: 690.41 requires DC-side equipment grounding or functional grounding per system type; 690.45 sizes equipment grounding conductors per 250.122, and 690.47 governs grounding electrode connections for PV.

Rapid shutdown (690.12): conductors within 1 ft of the array and 30 V within 30 seconds after shutdown initiation must be limited to ≤ 30 V; module-level electronics or a listed shutdown device accomplish this.

Roof load check: total dead load = module weight (typ. 10–15 kg/module, ~2–3 lb/ft²) + racking + ballast; verify against the structural report's allowable dead load and add snow load per ASCE 7.

Wind uplift: design pressure p = q × (GCp − GCpi) using ASCE 7 velocity pressure q; edge and corner zones have higher GCp magnitudes, so increase attachment density there.

Attachment spacing rule of thumb: module clamps must be placed within the module manufacturer's listed clamp zone (typically 1/4 span from frame ends) - clamping outside that zone voids the module's mechanical warranty.

Rail span and cantilever: follow rack manufacturer's span tables; keep cantilever overhang ≤ 1/3 of the adjacent span unless the table states otherwise.

Standoff/penetration sealing: use EPDM or silicone flashing with a curb or pitch pan on low-slope roofs; all penetrations must be above the water line of the roof course.

Thermal expansion: aluminum rail expands ~23 µm/m·°C; allow slip joints or expansion gaps on runs over ~24 m (80 ft) to prevent buckling.

Setback and access: IRC/IBC require a 36 in (0.91 m) clear path at ridge and eave for firefighters on residential roofs; keep modules outside these zones.

Array-to-array row spacing on flat roofs: spacing ≈ module height × (sin tilt / tan solar altitude) to avoid inter-row shading at the winter solstice design hour.

Solar window: measure the horizon profile at the array's azimuth; obstructions above ~15–20° elevation in the winter solar path indicate shading loss requiring modeling.

Shading tool: a Solmetric SunEye or smartphone sun-path app produces a shade report giving annual solar access percentage; below ~70 % solar access, reconsider array location.

Roof condition: check age, remaining life, and covering type; if the roof has < 10 years remaining life, re-roofing before PV installation is the standard recommendation.

Structural: obtain span, rafter/joist size and spacing, and sheathing thickness; a licensed structural engineer signs off if the added load exceeds code prescriptive limits.

Electrical service assessment: record main panel bus rating, main breaker size, available breaker spaces, and service voltage; the 120 % rule (705.12(B)) limits backfed PV breaker to busbar × 1.2 minus main breaker.

Irradiance data: use NREL NSRDB/PVWatts or a site pyranometer; typical GHI for a good US site is 4.5–6.0 kWh/m²/day; apply tilt and azimuth correction factors.

Site survey checklist items: GPS coordinates, roof pitch and azimuth, obstruction dimensions, distance to main panel, trench/conduit path, and utility meter location.

Soiling and snow: identify local soiling sources (dust, pollen, agricultural) and design snow-shedding tilt; snow cover can reduce winter production 20–100 % depending on tilt.

Permit plan set typically includes: site plan, roof plan, electrical single-line diagram (SLD), three-line with grounding, structural calc, module/ inverter datasheets, and string sizing table.

SLD must show: module model and count, string configuration, combiner/recombiner, DC disconnect, inverter, AC disconnect, main service panel, OCPD ratings, conductor sizes, and grounding electrode conductor.

Labeling requirements (NEC 690.53 and 690.54): DC PV system labels at the disconnect stating maximum system voltage and maximum current; AC labels at the point of interconnection stating rated AC output current.

Interconnection application: utilities require a completed application, SLD, and sometimes a signed net-metering agreement before energizing; approval timelines vary from days to months.

Arc-flash and shock hazard: PV modules produce voltage whenever illuminated; cover modules with opaque material during service and treat the DC side as always energized.

Lockout/tagout: a listed DC-rated disconnect must be within sight of the inverter; label the disconnect 'Photovoltaic System' and mark the maximum voltage.

Commissioning documentation: record open-circuit voltage, short-circuit current, insulation resistance (megohmmeter), and ground continuity before energizing; retain for the project record.

Fall protection: OSHA 1926.501 requires fall protection above 6 ft (1.8 m) in construction; roof anchors, guardrails, or personal fall arrest systems must be in the safety plan.

Permit plan set typically includes: site plan, roof plan, electrical single-line diagram (SLD), three-line with grounding, structural calc, module/inverter datasheets, and string sizing table.

SLD must show: module model and count, string configuration, combiner/recombiner, DC disconnect, inverter, AC disconnect, main service panel, OCPD ratings, conductor sizes, and grounding electrode conductor.

Labeling requirements (NEC 690.53 and 690.54): DC PV system labels at the disconnect stating maximum system voltage and maximum current; AC labels at the point of interconnection stating rated AC output current.

Interconnection application: utilities require a completed application, SLD, and sometimes a signed net-metering agreement before energizing; approval timelines vary from days to months.

Arc-flash and shock hazard: PV modules produce voltage whenever illuminated; cover modules with opaque material during service and treat the DC side as always energized.

Lockout/tagout: a listed DC-rated disconnect must be within sight of the inverter; label the disconnect 'Photovoltaic System' and mark the maximum voltage.

Commissioning documentation: record open-circuit voltage, short-circuit current, insulation resistance (megohmmeter), and ground continuity before energizing; retain for the project record.

Fall protection: OSHA 1926.501 requires fall protection above 6 ft (1.8 m) in construction; roof anchors, guardrails, or personal fall arrest systems must be in the safety plan.