NABCEP PVIS cheat sheet
NABCEP PVIS · Keep the key ideas within reach.
PV Installation Professional (PVIP): Site Assessment, Design, and Electrical Calculations
Standard test conditions (STC) are 1000 W/m² irradiance, 25 °C cell temperature, and AM 1.5 spectrum; nameplate ratings use STC, while PTC/NMOT ratings reflect ~20 °C ambient, 1 m/s wind, and open-rack mounting and are typically 10–15% lower.
Module temperature derating: cell temperature ≈ ambient + (NOCT − 20) × irradiance/800; power loss ≈ γ × (Tcell − 25), with γ ≈ −0.30%/°C to −0.45%/°C for crystalline silicon.
String sizing: maximum string Voc at record-low cell temperature must not exceed inverter max DC input; minimum string Vmp at record-high cell temperature must stay above inverter MPPT lower limit. Cold-temperature Voc uses Voc × [1 + βVoc × (Tmin − 25)].
NEC 690.7 requires the maximum PV system voltage to be calculated using the lowest expected ambient temperature correction factors in Table 690.7(A) for crystalline silicon; 1.25× Voc is not a substitute for the table method.
Conductor ampacity: continuous PV output current × 1.25 (NEC 690.8(A)) for conductors and overcurrent devices, and inverter output current × 1.25 for inverter output circuits (NEC 690.8(A)(3)).
Voltage drop on DC circuits should generally be ≤ 2–3% and total system drop ≤ 5%; Vdrop = 2 × L × I × R per conductor length for two-wire circuits, using resistance at operating temperature.
Array-to-inverter DC/AC ratio (clipping ratio) is typically 1.1–1.3 for fixed-tilt; higher ratios increase clipping losses but improve low-irradiance harvest. Estimate clipping loss from the irradiance duration curve, not from nameplate alone.
Rapid shutdown (NEC 690.12) requires conductors within 1 ft of the array boundary to be limited to ≤ 30 V within 30 seconds of shutdown initiation; module-level electronics or string-level disconnecting means satisfy this.
Roof clearance and access: NEC 690.12 boundary, fire setbacks per IBC/IFC (typically 3 ft ridges and 18–36 in. egress paths), and structural checks for dead load (typically 2–4 lb/ft² for flush-mounted arrays).