Bifacial PV modules introduce new complexities into determining maximum circuit current for circuits as the National Electrical Code (NEC) doesn’t provide a clear method specific to bifacial modules and manufacturer documentation provides numerous short-circuit current values. This article examines what the 2026 NEC says, why it says it, and dives into relevant product safety standards to provide a clear understanding of how to determine maximum circuit current for bifacial PV.
The NEC
Section 690.8(A)(1)(a) offers three options for determining photovoltaic source circuit maximum current:
- The sum of the highest short-circuit current rating of the PV modules connected in parallel times 125%.
- The sum of the short-circuit current ratings of the PV modules connected in parallel calculated in accordance with manufacturer instructions included in the listing or labeling.
- Calculated using an industry-standard maximum current calculation method, provided by a licensed professional electrical engineer as a part of a documented and stamped PV system design, with further stipulations:
- The calculated value must be based upon the highest 3-hour current average resulting from the simulated local irradiance on the PV array, accounting for orientation and elevation.
- The calculated value cannot be less than 70% of what is calculated in #1 above.
How do bifacial modules fit within these options? There were in-depth proposals and discussions in the 2026 cycle about adding a definition for bifacial PV modules, along with specific requirements for calculating current, but the committee decided it was not necessary for bifacial modules to be addressed separately as the product safety standards for PV modules now include testing and marking requirements that serve as the basis for bifacial calculations.
To make sure that the additional current produced by bifacial modules is taken into account when calculating maximum current, the term “highest” was added to 690.8(A)(1)(a)(1). The code-making panel stated that the revisions “were made to clarify the reference to “short-circuit” current since there are some PV modules, such as bifacial modules, that may have higher max current values depending on application. For this simple method, the highest value is required.”
We need to identify the “highest” value to use either Option #1 or Option #3, but what is that value for a bifacial module? The NEC doesn’t specify, requiring us to look at the relevant product safety standards.
PV Module Standards
The relevant international standard is IEC 61730-1 Photovoltaic (PV) module safety qualification – Part 1: Requirements for construction. UL 61730-1 contains the same requirements with the exception of a few necessary modifications for compliance with existing US regulations.
The 3rd edition of IEC 61730-1 was released in 2023, but publication of the updated UL edition has been delayed until mid-2026. It will continue to mirror IEC 61730-1, so we can look there for the most up-to-date guidance. IEC 61730-1:2023 includes four different possible irradiance values for bifacial modules:
1. Standard test conditions (STC): 1000W/m2 on the module front
2. Bifacial Nameplate Irradiance (BNPI): 1000W/m2 on the module front and 135W/m2 on the module rear. BNPI is used for nameplate verification and provides an approximation of typical power gain due to bifaciality.
3. Applied Bifacial Stress Irradiance (aBSI): 1000W/m2 on the module front combined with the great of 300 W/m2 or the manufacturer’s “claimed” irradiance on the module rear.
4. Bifacial Stress Irradiance (BSI): 1000W/m2 on the module front and 300W/m2 on the module rear.
A bifacial PV module must be marked with Isc values for the first three listed above (STC, BNPI, and aBSI). The value for aBSI is almost always going to use the 300 W/m2 irradiance value. Manufacturers can claim a higher value for aBSI, but this can be complicated as the module will have to be tested and marked accordingly.
Technical note: aBSI irradiance does not translate directly into a 30% increase in current gain as it depends upon the bifaciality coefficient for Isc (φIsc), which typically ranges from 70-90%. This reduces the backside gain accordingly:
IscaBSI = IscSTC x (1+0.3 x φIsc)
Overcurrent Protection Sizing
The 2nd of UL 61730 currently requires that the rating of overcurrent protection is not less than 1.56 times IscaBSI for bifacial modules, which is anticipated to remain unchanged in the next edition. This means that the code-calculated minimum OCPD size should be equal to or below the maximum series fuse rating of a module, alleviating a potential conflict between application of the NEC and manufacturer’s instructions.
Conclusion
Examples of bifacial PV module data sheets and nameplates that do not align with the requirements in PV module standards abound. We anticipate this issue will gradually resolve itself as manufacturers align their products and documentation with the 3rd edition of UL 61730 / IEC 61730.