Isc (Short-Circuit Current)

Quick Facts

What is Isc?

Isc is the current flowing through a PV module’s terminals when shorted (V = 0). Reported at STC on the datasheet.

Relationship to Imp

Imp (current at maximum power) ≈ 0.92 × Isc typically.

Why short-circuit current is safe to measure

PV cells are current-limited devices. Shorting a module doesn’t damage it; current is capped by the photogenerated current at the prevailing irradiance.

NEC 690.8 Sizing

These calculations aren’t just a design-office exercise, the resulting conductor and OCPD ratings are what the permit reviewer checks against the electrical CEIG drawings submitted with the application, so getting Isc math right early avoids a rejected drawing set later.

Step 1: Maximum circuit current

I_max = 1.25 × Isc

Step 2: Continuous-duty adjustment

I_conductor = 1.25 × I_max = 1.5625 × Isc

Step 3: Conductor selection

Choose conductor with ampacity ≥ I_conductor (with temperature derating).

Step 4: OCPD sizing

OCPD = 1.25 × Isc, rounded up to standard rating

Worked Example

Module Isc = 13.5 A. Wire run at 50°C ambient, in conduit, 90°C insulation.

I_max = 1.25 × 13.5 = 16.875 A
I_conductor = 1.25 × 16.875 = 21.09 A

10 AWG PV Wire ampacity at 90°C: 40 A
Derate for 50°C ambient (NEC Table 310.15(B)(2)(a)): × 0.82 → 32.8 A
Derate for conduit fill: assume × 0.80 → 26.2 A

26.2 A > 21.09 A required → 10 AWG is suitable.

OCPD = ceil(1.25 × 13.5) = 17 A → next standard size 20 A

Bifacial Adjustment

Bifacial modules produce 5-15% additional current from rear side. The “bifacial Isc” or “Isc with bifacial gain” is published on bifacial module datasheets.

For NEC sizing, use the bifacial Isc value (manufacturer’s specified Isc at relevant albedo) plus standard 1.25 multiplier. See Bifacial Gain for how manufacturers quantify and publish this rear-side uplift.

Common Mistakes

1. Using Imp instead of Isc for conductor sizing.
2. Forgetting the second 1.25 continuous-duty factor.
3. Ignoring bifacial Imp uplift.
4. Not derating conductor ampacity for ambient and conduit fill.
5. Using STC Isc without accounting for cloud-edge enhancement (rare but possible).

Best Practices

• Document Isc on the SLD per inverter and per string.
• Apply NEC 690.8(A)(1) and (B) corrections directly on the SLD.
• For bifacial designs, use bifacial-adjusted Isc.
• Verify per-MPPT current limit includes bifacial uplift.
• At the combiner box, size busbars and terminals for the sum of parallel string Isc values, not a single string’s.

Standards & Certifications

• IEC 60904, STC measurement.
• NEC 690.8, Conductor and OCPD sizing.

Key Takeaways

• Isc is the short-circuit current of a PV module at STC.
• NEC 690.8 conductor sizing uses 1.25 × Isc × 1.25 = 1.56 × Isc.
• Bifacial modules have higher effective Isc; account for the uplift in design.
• OCPD = next standard rating ≥ 1.25 × Isc.
• Per-MPPT current limit must accommodate parallel string Isc with bifacial gain.

Related Reading

Isc rarely stands alone on a drawing set, it feeds directly into the auto-generated conductor and OCPD callouts that tools covered in our Solar SLD Software roundup produce for NEC 2023 permit packages.

Bifacial Isc uplift also has to be carried through into energy modeling, not just electrical sizing; the PV Yield Simulation Software guide walks through how bankable tools handle rear-side gain assumptions that ultimately trace back to the same bifacial Isc figure used here.

Once conductors and OCPDs are sized off Isc, the last step on the DC side is making sure the array is properly bonded, QbitsEnergy’s inverter grounding guide is a good next stop for that part of the installation.

Disclosure: Qbits Energy is a Heaven Designs group company.