Solar PV System Sizing and Design per NEC 690
Solar photovoltaic (PV) system sizing begins with matching the system's energy production to the facility's energy consumption. The fundamental equation is: System kW = (Daily kWh consumption) / (Peak Sun Hours × System Efficiency). Peak sun hours (PSH) vary by location — ranging from 3-4 hours in the Northeast US to 5-7 hours in the Southwest. System efficiency (typically 75-85%) accounts for inverter losses (3-5%), wiring losses (1-3%), soiling (2-5%), temperature derating (5-15%), module mismatch (1-3%), and shading losses (0-20%). NREL's PVWatts calculator provides location-specific solar resource data for accurate sizing.
NEC Article 690 governs PV system installations with increasingly detailed requirements. Key provisions: conductors must be sized for 125% of the maximum circuit current (Isc × 1.25 for source circuits per NEC 690.8(A)), creating an effective 156% sizing factor (1.25 × 1.25) for continuous duty. String voltage must not exceed inverter maximum input voltage — calculated using the lowest expected temperature and Voc temperature coefficient (modules produce higher voltage in cold weather). Equipment grounding per 690.43 — all metallic parts must be bonded. Ground-fault protection per 690.41 for grounded systems.
String sizing determines how many modules connect in series and is temperature-dependent. Maximum modules per string = Inverter Vdc_max / Module Voc_at_lowest_temp. The temperature correction: Voc_corrected = Voc_STC × [1 + (T_min - 25°C) × Voc_tempco/100]. Example: 400W module, Voc = 49.5V, tempco = -0.30%/°C, at -10°C: Voc_corrected = 49.5 × [1 + (-35 × -0.003)] = 49.5 × 1.105 = 54.7V. For a 600V inverter: max string = 600/54.7 = 10 modules. Minimum modules per string = Inverter MPPT_min / Module Vmp_at_highest_temp (Vmp drops in heat).
NEC 690.12 Rapid Shutdown has undergone significant evolution. NEC 2017: conductors outside the array boundary must de-energize to ≤30V within 30 seconds. NEC 2020/2023: conductors within the array boundary must also de-energize to ≤80V within 30 seconds and ≤1V within 3 minutes — requiring module-level shutdown capabilities. Solutions include microinverters (inherently compliant — no DC on the roof), DC optimizers with safe shutdown mode, and module-level rapid shutdown devices. This requirement significantly impacts system design and component selection for rooftop installations.
PV system grounding and bonding protects against electrical shock, lightning, and ground faults. Grounded systems use either positive or negative ground (negative ground is standard for most crystalline silicon modules). NEC 690.41 requires ground-fault protection for grounded PV systems on buildings. Transformerless (non-isolated) inverters require ungrounded PV arrays — NEC 690.35 mandates specific overcurrent and ground-fault protection for ungrounded systems. Equipment grounding per NEC 690.43: all metallic frames, racking, and enclosures must be bonded. WEEB (Washer, Electrical Equipment Bond) clips or equivalent module bonding hardware simplifies frame grounding.
Battery-coupled PV systems require additional sizing for the battery bank, charge controller, and hybrid inverter. The battery bank must store enough energy for autonomy days at the desired depth of discharge: Battery kWh = Daily kWh × Autonomy Days / (DoD × Inverter Efficiency). Grid-tied with battery backup typically provides 4-8 hours of essential load coverage (8-20 kWh for residential). Off-grid systems require 3-5 days of autonomy with oversized PV arrays to account for consecutive cloudy days. The charge controller must handle the total PV array short-circuit current with NEC 690.8 factors applied, and must match the PV string voltage range to the battery voltage.