NEC 690.7(A) PV solar string voltage sizing. Enter module data, site temperatures, and inverter specs — output includes corrected open-circuit voltage, maximum modules per string, MPPT window compliance, and NEC 690.8 conductor/OCPD sizing. Applies to both monofacial and bifacial modules.
ⓘ NEC 690 governs PV-only systems. For DC arc flash hazards from PV strings, use the Arc Flash calculator. For string fault contribution, use the Short Circuit Current calculator.
Module Parameters
Open-circuit voltage at Standard Test Conditions (1000 W/m², 25°C, AM1.5)
Maximum power-point voltage at STC
Short-circuit current at STC — basis for NEC 690.8(A) conductor sizing
Current at maximum power point at STC
Typically −0.27 to −0.30 %/°C for crystalline Si; −0.35 to −0.40 %/°C for thin-film
Typically −0.35 to −0.40 %/°C for crystalline Si
Site Conditions
ASHRAE 99.6% heating design dry-bulb temperature or field-measured minimum
ASHRAE 0.4% hot design dry-bulb temperature — drives MPPT window upper limit
Inverter
Do not exceed — limits maximum string length per NEC 690.7(A)(1)
V_mp at maximum site temperature must stay ≥ MPPT_low to track maximum power
V_mp at minimum site temperature must stay ≤ MPPT_high to stay within inverter range
Total string current (I_sc × 1.5625 × n_parallel) must not exceed this value
Bifacial Gain (Optional)
NEC 690.7(A)(3): if bifacial gain can be documented, apply additional irradiance multiplier. Default 1.10× (10% gain). Requires engineering justification on project documentation.
Combiner / Parallel Strings
Each string = one source circuit per NEC 690.8
PASS — NEC 690.7(A)
Recommended configuration
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String Voltage Summary
V_oc @ T_min
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V/module
Max Modules (V)
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modules
Rec. Config
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modules
MPPT window compliance
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PV String Sizing Results
NEC 690.8 — Conductor & OCPD
Check
Value
Result
A — next standard conductor size Use next standard conductor size above the calculated ampacity. Fuse rating is per-string — combiner must have one fuse per input.
Export calculation report
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NEC 690.7(A) is US-code. IEC projects use IEC 61730 voltage limits with temperature correction per the module manufacturer's datasheet.
FAQ
NEC 690.7(A)(1) requires that the open-circuit voltage of a PV string, corrected for the minimum expected ambient temperature, must not exceed the maximum DC voltage rating of any connected equipment (inverter, combiner, conductor). The corrected voltage is computed using the module temperature coefficient β_Voc and the site-specific minimum temperature (ASHRAE 99.6% design dry-bulb or field data).
NEC 690.7(A)(3) allows an additional irradiance gain factor for bifacial modules when the gain can be documented — typically 1.05–1.15× depending on ground albedo and mounting height. FrameAI applies the gain factor as a voltage multiplier (in addition to the temperature correction) when "Enable bifacial gain" is checked. The gain must be substantiated in the project documentation.
The MPPT (Maximum Power Point Tracking) window defines the voltage range in which the inverter can operate to harvest maximum power from the array. At high temperatures, module voltage drops — V_mp must stay above the MPPT_low so the inverter can still track. At low temperatures, voltage rises — V_mp must stay below MPPT_high to avoid over-voltage damage. This creates a valid module-count range for each string.
NEC 690.8(A) requires conductors to be sized at 125% of the PV source-circuit current (I_sc). NEC 690.8(B) requires overcurrent devices (fuses, breakers) to be rated at 125% of I_sc as well. The combined factor is 1.25 × 1.25 = 1.5625, so string ampacity = I_sc × 1.5625. Combiner fuses use I_sc × 1.56.
For every 1°C below 25°C, crystalline Si modules gain ~0.27% in V_oc. At −20°C, this adds ~12% voltage. At −40°C (Arctic or high-altitude sites), the gain reaches ~18% — enough to push a 1000V inverter string over its limit. Always use the 99.6% heating design dry-bulb (ASHRAE) or field-measured coldest temperature, not the average winter temperature.
This tool sizes individual PV strings per NEC 690.7(A). The Short Circuit Current calculator evaluates the contribution of PV strings to overall fault levels in the inverter AC output circuit and downstream equipment. Use both: string sizing for NEC compliance and DC side design, fault current for protective device coordination and equipment duty.