Solar PV All Codes
A near-universal safety multiplier shows up in PV cable sizing under PEC, AS/NZS 5033, and the NEC. Here is why it exists and how it is applied.
If you've sized DC cable for a solar array under more than one electrical code, you've probably noticed the same number keep showing up: 1.25. It appears in PEC solar cable sizing, in AS/NZS 5033, and in the NEC's Article 690 — not as a coincidence, but because it's solving the same physical problem everywhere.
A solar panel's short-circuit current (Isc) on its datasheet is measured under Standard Test Conditions — 1000 W/m² irradiance, 25°C cell temperature. Real-world conditions can briefly exceed that irradiance level (bright, clear, cold days with reflective cloud edges are a classic cause), pushing actual short-circuit current above the datasheet figure by a meaningful margin.
Minimum cable ampacity ≥ 1.25 × Iₛₛ
This factor accounts for that real-world overshoot before any further derating (temperature, grouping) is even applied. In some jurisdictions and calculation methods, a second 1.25 factor is stacked on top for standard continuous-load overcurrent protection margin — meaning the effective multiplier some installers use is closer to 1.56, not 1.25. Always check which version of the rule your governing code and calculator apply.
| Step | What happens |
|---|---|
| 1 | Take the module's rated Isc from the datasheet |
| 2 | Multiply by 1.25 (or the applicable stacked factor) to get minimum required ampacity |
| 3 | Check that requirement against temperature-derated cable ratings — a hot rooftop needs the same kind of correction as any other high-temperature installation |
| 4 | Check voltage drop separately against the DC-specific limit (see the voltage drop comparison article) |
Whether you're working under PEC, AS/NZS 5033, or NEC Article 690, the underlying reason for the multiplier doesn't change — only the exact numeric factor and how it's presented in each code's text. Size to whichever code actually governs your installation.
Run this against your own module's Isc in the PV/Solar DC Cable Sizing calculators across all three code tabs.