Last Updated: 2026-09-30
This article is part of the Print Pro AZ NEC 2026 solar labeling series. Start with the pillar guide: NEC 2026 Solar Labeling Changes Installers Must Know.
Last Updated: April 21, 2026
NEC 2026 Article 690.8(A)(1)(a)(1) now requires installers to use the highest short-circuit current (Isc) value from a bifacial module's datasheet when sizing conductors and overcurrent devices. Bifacial modules carry three Isc ratings, STC, BNPI, and aBSI. Using any value below the highest on your permit plans or field-applied labels is a code violation that fails inspection.
Your bifacial module datasheet lists three different Isc values. You pick one for the permit set. The AHJ inspector pulls out the datasheet at the job site, compares your label values to the listed ratings, and your system gets red-tagged.
That's the scenario catching installers off guard under NEC 2026. The code update to Article 690.8 is one line long, but it changes which Isc number drives your conductor sizing, overcurrent protection, and field-applied labels for every bifacial PV system you permit.
This guide covers exactly what changed in NEC 2026 Article 690.8, which Isc value the code now requires, how overcurrent devices must be sized and marked, and what your labels need to show to pass the first time.
If you're installing bifacial panels on any project permitted under NEC 2026, or any state that has adopted it, this applies to your next job.
What Changed in NEC 2026 Article 690.8?
The 2026 National Electrical Code Article 690.8(A)(1)(a)(1) added one word to the existing language: "highest."
The updated text reads: "The sum of the highest short-circuit current rating of the PV modules connected in parallel multiplied by 125 percent."
Before 2026, the code didn't specify which Isc value to use when a module's datasheet listed more than one. For standard monofacial panels, that wasn't a problem, there's only one STC Isc value. But bifacial modules have always carried multiple rated current values, and different engineers were making different calls.
The 2026 NEC ends that ambiguity. You must use the highest Isc listed on the module's datasheet or installation manual, no exceptions.
Actionable takeaway: Pull your bifacial module's datasheet before starting any permit set and identify which Isc value is the highest. That's the number that drives your conductor sizing, OCPD selection, and every field-applied label that shows system current.
Why Bifacial Modules Have Three Different Isc Values
Bifacial panels capture sunlight on both sides of the module. The rear side generates additional current when light reflects off the ground, roof surface, or nearby structures. That rear-side gain is real, and it varies based on installation angle, surface albedo, and the module's bifacial factor.
Because rear-side gain is measurable and variable, the product safety standard for PV modules, UL 61730, 2nd edition, now requires bifacial modules to be listed and marked with three distinct Isc values:
| Rating | Irradiance Condition | Typical Rank |
|---|---|---|
| STC (Standard Test Conditions) | 1,000 W/m² front only | Lowest |
| BNPI (Bifacial Nameplate Irradiance) | 1,000 W/m² front + 135 W/m² rear | Middle |
| aBSI (Applied Bifacial Stress Irradiance) | 1,000 W/m² front + 300 W/m² rear | Highest |
The aBSI value, 1,000 W/m² front irradiance combined with 300 W/m² rear irradiance, represents the maximum stress condition a bifacial module can realistically see in the field. This is the condition UL 61730 uses for overcurrent protection qualification testing.
Critical rule: NEC 2026 requires using the highest Isc from the datasheet. In most bifacial installations, that means aBSI.
These three values appear on the module's nameplate label and in the installation manual. Before 2026, there was no code directive on which one to use for circuit calculations. That created inconsistency. Now there isn't one.
Which Isc Value Do You Use for Labels and Conductor Sizing?
Under NEC 2026, the answer is clear: use the highest Isc from the datasheet, in most cases, the aBSI value.
Here's how that number flows through your system:
- Maximum PV source circuit current, Per NEC 690.8(A)(1)(a)(1): sum of parallel module Isc ratings (using the highest value) × 125%
- Conductor ampacity, Per NEC 690.8(B): conductors must be sized to carry the maximum circuit current calculated above
- Overcurrent device rating, Must protect conductors sized to that aBSI-derived current, within the module's maximum series fuse rating
- Field-applied labels, Any label showing system current or overcurrent ratings must reflect values derived from the correct, highest Isc
Here's a scenario Print Pro AZ sees regularly. A solar installer in Phoenix brings in permit plans for a bifacial ground-mount system. They used the STC Isc, the lowest of the three values, for conductor and OCPD sizing. The plan reviewer compares it to the module datasheet and kicks it back for revision. The installer recalculates using aBSI, the conductors need to be upsized one AWG, and the labels in the existing order are now wrong. Two weeks of delays, a reorder, and a second permit submission.
The fix is to use aBSI from the start. If your module's installation manual lists a different method for determining Isc for your specific application, you can use that instead, but only if it produces a value higher than or equal to aBSI (see the section below on manufacturer instructions).
Get the right labels for your bifacial system from the start: solar electric label packs from Print Pro AZ are built to NEC specs and sized for standard solar combiner, disconnect, and junction box applications.
A New Option: Using Manufacturer Instructions for Isc Calculation
NEC 2026 added a second calculation method in Article 690.8(A)(1)(a)(2). This subsection allows the maximum circuit current to be calculated using the module manufacturer's installation instructions rather than defaulting to the datasheet's aBSI value.
This matters for bifacial installations where the actual rear-side irradiance is well-documented. If a manufacturer's installation manual includes a specific method for calculating Isc based on the installation type, say, a tracked ground mount with high-albedo gravel, versus a low-slope roof with dark membrane, you can use that method under the 2026 code.
What you need to use 690.8(A)(1)(a)(2):
- The calculation method must be in the module manufacturer's published installation instructions (not just the datasheet)
- The resulting Isc must represent the highest value produced by that method for your specific installation
- Keep the relevant pages from the installation manual in your permit package, AHJs can and do ask for them
Per NEC 690.8(A)(1)(a)(2), the manufacturer's method is an alternative to the standard aBSI approach, not a way to justify lower values. If the manufacturer's method gives you a number lower than aBSI, you cannot use it to downsize conductors or OCPDs. The code still requires the highest.
How Do You Size Overcurrent Devices for Bifacial Solar Under NEC 2026?
Overcurrent device sizing for bifacial modules follows the same NEC 690.8 formula, but with aBSI as the input for maximum circuit current.
The sizing sequence:
- Pull the highest Isc from the module datasheet (aBSI for most modules)
- Calculate maximum PV source circuit current: (parallel module Isc values × 125%)
- Size conductors to carry that current per NEC 690.8(B)
- Select an OCPD rated at or above conductor ampacity, at or below the module's maximum series fuse rating (MSFR)
For bifacial modules specifically, UL 61730 2nd edition requires that the MSFR printed on the module label is not less than 1.56 × IscaBSI. That built-in margin ensures the code-calculated OCPD minimum falls within the module's rated protection range.
Check your module nameplate: The MSFR must be ≥ 1.56 × IscaBSI. If it's below that threshold, the module is not listed for use under NEC 2026 Article 690.8.
OCPD marking requirement: Per NEC 2026 Article 690.9(D), every overcurrent protection device installed in a PV system DC circuit must be permanently marked "Photovoltaic" or "PV." This applies to bifacial source circuit OCPDs the same as any other solar circuit. A standard residential breaker or fuse with no PV rating does not meet 690.9(D), and an AHJ who catches it will fail the inspection.
AHJs vary in how strictly they enforce the PV marking requirement, confirm with your local authority before assuming unlabeled components are acceptable. For any inspection where the AHJ is going by the book, 690.9(D) is a line item.
Review common NEC violations for solar installations in EC&M's guide: Avoiding Common NEC Violations on Solar PV and BESS Projects, OCPD marking issues appear consistently.
What Field-Applied Labels Must Show for Bifacial Systems
Field-applied labels on bifacial PV systems under NEC 2026 need to reflect the updated current values. Here's what changes compared to a monofacial install:
Labels affected by the bifacial Isc change:
- PV source circuit labels, Current values must match aBSI-based conductor sizing; if you upsized conductors due to bifacial current, your labels need to match
- Disconnect labels, Per NEC 690.13(B), each PV system disconnecting means must be permanently marked "PV SYSTEM DISCONNECT" (the 2026 code simplified this wording; the previous Electrical Shock Hazard label from 690.13(B) has been removed)
- DC voltage labels, NEC 690.7(D) now allows rounding up the maximum PV dc circuit voltage to the next standard value, reducing the number of unique voltage labels needed per system
- OCPD labels, Any OCPD in the DC circuit must display PV or Photovoltaic marking per 690.9(D)
Material durability: For outdoor rooftop or ground-mount applications, labels need to hold up under UV exposure, temperature cycling, and moisture. Vinyl-coated or aluminum-backed labels rated for outdoor use are the standard. Paper labels or standard inkjet prints degrade quickly and fail AHJ visual inspection.
Print Pro AZ's NEC solar label bundles are made for outdoor installation, ship fast, and include the label types required for NEC 2026 systems, disconnect, source circuit, rapid shutdown, and more.
Have a commercial bifacial project with a specific label specification? Send us your plan sets and we'll match the label to your permit drawings.
Related reading: Rapid Shutdown Labels: NEC 2017 vs 2020 vs 2023 Requirements
Frequently Asked Questions
What Isc value does NEC 2026 require for bifacial solar module calculations?
NEC 2026 Article 690.8(A)(1)(a)(1) requires using the highest short-circuit current (Isc) value listed on the module's datasheet or in the manufacturer's installation manual. For most bifacial modules, that's the aBSI value, rated at 1,000 W/m² front irradiance plus 300 W/m² rear irradiance. Using any lower value (STC or BNPI) does not meet the 2026 code.
Do bifacial solar systems need different labels than monofacial systems?
The label types required by NEC Article 690 are the same for both. The difference is in the current values printed on the labels. Bifacial systems calculated using aBSI Isc will show higher current ratings than the same system calculated using STC Isc. Your field-applied labels must match the values used in your approved permit plans, if you correct the Isc after plan check, reorder the labels.
What does "Photovoltaic" marking on an OCPD mean?
Per NEC 2026 Article 690.9(D), any overcurrent protection device installed in a PV system DC circuit must be marked "Photovoltaic" or "PV." This indicates the device has been tested and rated for the DC arc interruption characteristics specific to solar systems. Standard AC-rated breakers and fuses typically do not carry this marking and do not satisfy 690.9(D).
What happens if I use the wrong Isc value on a bifacial permit set?
Using a lower Isc than required by NEC 690.8 means conductors and OCPDs may be undersized for the actual circuit current. AHJ plan reviewers and field inspectors who compare your permit documentation to the module datasheet will catch the discrepancy. The result is a rejection or failed inspection, a correction notice, and often a requirement to upsize components before re-inspection can be scheduled.
Can manufacturer instructions replace the datasheet Isc for NEC 2026 sizing?
Yes, under NEC 2026 Article 690.8(A)(1)(a)(2), if the module manufacturer provides a specific calculation method in the installation manual for determining Isc for your installation type, that method is an approved alternative. The resulting value must still be the highest produced by that method, this option cannot be used to justify a lower Isc than aBSI when aBSI is the higher value.
Conclusion
NEC 2026 Article 690.8 changed one word, "highest", but that word changes which Isc value drives conductor sizing, OCPD ratings, and field-applied label values on every bifacial system you permit.
The three rules: use the highest Isc (aBSI for most modules), ensure every DC circuit OCPD carries "Photovoltaic" or "PV" marking per 690.9(D), and make sure your field labels match the aBSI-based values in your approved plans. Catch the right Isc value at the design stage, not at re-inspection.
Shop NEC 2026-compliant solar label packs →
Questions? Call Brent: (602) 649-5305
Brent Hanke | Print Pro AZ | (602) 649-5305 | b.hanke@printproaz.com Brent Hanke is the founder of Print Pro AZ, supplying NEC-compliant labels to contractors across the country.
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