Solar PV circuit current and wire sizing
PV source circuit current is figured at 125% of the parallel modules' rated short-circuit current, and conductors need at least 125% of that current before correction and adjustment factors. That first check works out to about 1.56 times the short-circuit current.
What the code requires
For a PV source circuit, the maximum current is the sum of the rated short-circuit currents (Isc) of the modules in parallel, multiplied by 125%. For DC-to-DC converter circuits it is the sum of the converters' continuous output current ratings. For an inverter output circuit it is the inverter's continuous output current rating.
Systems with inverter capacity of 100 kW or more may instead use an industry-standard calculation stamped by a licensed professional electrical engineer, based on the highest 3-hour average current. The result may not be less than 70% of the 125% method.
Conductors must have an ampacity of at least 125% of the maximum current, before adjustment and correction factors are applied. Applying 125% twice works out to about 1.56 times Isc. The current with temperature correction and adjustment factors applied is a separate check, and the conductor must meet the larger of the two.
On the job
- For bifacial modules, which produce power from more than one surface, the Isc to use is the one on the module label.
- Revisions for the 2026 NEC proposed removing the 100-kW floor on the engineered method, but California's current code is based on the 2023 NEC, where the floor applies.
Exceptions that matter
- Where a circuit's assembly, together with its overcurrent devices, is listed for continuous operation at 100% of its rating, it may be used at 100% instead of the 125% conductor rule.
What changed in 2023
The 2023 edition removed the old PV output circuit current item, renamed 690.8(D) Multiple PV String Circuits, and added a note that bifacial modules are labeled with the applicable Isc.
Related sections
- NEC 690.7
- NEC 690.9
- NEC 690.31
- NEC 705.28
- NEC 310.15: Derating wire for heat and crowded conduits
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