When to use: Sizing PV source/output circuit conductors and overcurrent protection. Per NEC 690.8(A)(1) the maximum circuit current is Isc × 1.25 (irradiance factor). Per 690.8(B)(1) conductors and OCPD are sized at 125% of that (continuous duty) — an effective 1.25 × 1.25 = 1.5625× factor on Isc.
This calculator sizes PV source and output circuit conductors and overcurrent protective devices (OCPD) per NEC 690.8 and 690.9, applying the required 1.25 × 1.25 current multiplier chain to the array short-circuit current. Engineers use it during PV system design to specify wire gauge, string fuses, and combiner breaker ratings.
NEC 690.8(A)(1) establishes the maximum PV source circuit current as 1.25 × Isc, where Isc is the module short-circuit current at STC. For a parallel array, the combined Isc is the sum of all string Isc values. This 1.25 factor accounts for irradiance levels that can exceed the STC reference of 1000 W/m² under certain atmospheric conditions (e.g., cloud-edge effects), producing currents above nameplate Isc.
NEC 690.8(B)(1) then requires PV circuit conductors and OCPD to have an ampacity at least 125% of the maximum circuit current calculated above — a continuous-duty multiplier. Combined, conductors must handle at least Isc × 1.25 × 1.25 = 1.5625 × Isc. OCPD is selected as the next standard rating at or above this calculated ampacity per NEC 240.6(A).
Temperature derating (NEC 310.15(B)(1)) and conduit fill derating (NEC 310.15(B)(3)(a)) further increase the required conductor table ampacity. The required table (unducted) ampacity = (Isc × 1.5625) / (temperature_factor × fill_factor). A conductor selected from NEC Table 310.12 or 310.16 must have a table ampacity at or above this value before derating.
NEC Article 690 (Solar Photovoltaic Systems) is the comprehensive code for PV electrical systems. Section 690.8 addresses maximum circuit current and conductor sizing, while 690.9 covers overcurrent protection. These sections reference the general overcurrent protection rules of NEC Article 240 for standard OCPD ratings.
NEC 690.9(A) requires overcurrent protection for PV source circuits when the source can back-feed through a faulted string into parallel strings. NEC 690.9(B) provides exemptions when the number of parallel strings is small enough that the remaining strings cannot exceed the module's backfeed current rating — string fuses may be omitted in those cases.
UL 1741 (Inverters, Converters, Controllers and Interconnection System Equipment) governs inverter certification, while UL 4703 covers PV wire and USE-2 cable. NEC 690.31(C) requires PV wire or USE-2 cable for exposed outdoor wiring in PV systems. Conduit wiring in arrays must use conductors with insulation rated for wet locations.
Temperature derating is particularly significant for PV systems because roof-mounted arrays experience conduit temperatures of 70–90°C in summer — far above the 30°C ambient assumed in NEC ampacity tables. Per NEC 310.15(B)(1), conductors in conduit on a rooftop must apply a rooftop adder (typically +17°C for conduit within 7/8" of the roof surface) before looking up the temperature correction factor from Table 310.15(B)(1)(a).
Fusing strategy for parallel strings: when N or more parallel strings can back-feed a faulted string with more current than the module's max series fuse rating, individual string fuses are required per NEC 690.9. Each string fuse is sized per the conductor ampacity, typically the next standard size above Isc × 1.5625. DC-rated string fuses (Class gPV or listed PV fuses) must be used — standard AC fuses cannot interrupt DC arc current safely.
Single-string systems (one string, no parallel combining) are exempt from OCPD requirements if the conductor ampacity equals or exceeds the maximum circuit current (NEC 690.9(A) exception). This simplification reduces hardware cost for small residential systems.
Enter the module short-circuit current (Isc) from the module datasheet at STC. Enter the number of parallel strings. The calculator multiplies string Isc by number of strings for the combined array Isc.
Enter the temperature correction factor (from NEC Table 310.15(B)(1)(a) at your design conduit temperature, typically 0.82–0.91 for 60–75°C conduit) and conduit fill factor (1.0 for 3 or fewer conductors, 0.80 for 4–6 conductors per NEC 310.15(B)(3)(a)).
Review the Calculation Steps for the full NEC multiplier chain. The OCPD Rating shows the next standard fuse or breaker size per NEC 240.6. Use the minimum table ampacity to select conductor size from NEC Table 310.12 (PV wire, 90°C column) or 310.16 (THWN-2, 90°C column, with derating applied separately).
The first 1.25 (NEC 690.8(A)(1)) accounts for irradiance exceeding 1000 W/m² — cloud-edge effects can boost irradiance to 1200–1300 W/m², increasing Isc proportionally. The second 1.25 (NEC 690.8(B)(1)) is the standard continuous-load multiplier applied to any load that operates for 3 hours or more, since PV systems generate throughout the day. Together they ensure the conductors and OCPD are never operating near their thermal limit during normal conditions.
String fuses are required per NEC 690.9(A) when the maximum number of parallel source circuits (strings) multiplied by the module short-circuit current exceeds the module's maximum series fuse rating. For example, if a module is rated for a 15A max fuse and 3 parallel strings produce 3 × 9.5A = 28.5A, more than twice the fuse rating, individual string fuses are required on each string to protect the module wiring.
DC PV circuits require fuses specifically listed and rated for DC service. Standard AC Class J, CC, or RK fuses cannot safely interrupt DC arc current because DC has no natural zero-crossing. PV-specific fuses include IEC gPV Class fuses and UL listed "PV fuses" (or string combiners with listed PV fuse holders). SIBA, Mersen, Bussmann, and Littelfuse offer UL/IEC-listed PV fuses in standard ampere ratings.
NEC 690.31(C) requires PV Wire (XLPE insulation, 90°C wet, rated per UL 4703) or USE-2 (underground service entrance cable, 90°C wet) for exposed single-conductor wiring in PV systems. These cables are sunlight resistant, moisture resistant, and rated for the higher voltages (1000V or 1500V) required in PV circuits. THWN-2 in conduit is an alternative for conduit-protected runs.
NEC 310.15(B)(3)(a) requires ampacity derating when 4 or more current-carrying conductors share a conduit: 80% for 4–6 conductors, 70% for 7–9 conductors. For a combiner with 6 strings feeding a single output conduit, the required table ampacity increases by 1/0.80 = 1.25× compared to 3 conductors or fewer. Large combiners routing multiple string pairs through one conduit may require significant wire upsizing to maintain adequate ampacity after derating.
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