String Voltage

Key Takeaways

• String voltage = N × module voltage.
• Cold Voc bound: NEC 690.7 + inverter MPPT max.
• Hot Vmp bound: inverter MPPT min.
• Always temperature-correct using ASHRAE extreme min/max.
• Document on SLD with calculation.

Why the Temperature Correction Isn’t Optional

A module’s nameplate Voc and Vmp are measured at Standard Test Conditions (25°C), but a string almost never sits at 25°C in the field. On the coldest morning of the year, crystalline silicon Voc rises as temperature drops, so the string’s actual open-circuit voltage at dawn on a sub-zero day can run meaningfully higher than the nameplate-based sum would suggest.

That corrected cold-day Voc is what gets checked against both the NEC 690.7 ceiling (1000 V commercial, 600 V residential, 1500 V utility-scale) and the inverter’s maximum DC input rating, and the lower of those two numbers governs.

On the opposite end, hot-day Vmp drops as module temperature climbs, and if the string falls below the inverter’s MPPT minimum voltage the inverter simply can’t track the array’s maximum power point, which shows up as clipped or unstable energy production rather than a compliance failure.

Because both bounds depend on site-specific temperature extremes rather than a single national number, the correction has to use the local ASHRAE design temperatures (or equivalent site data), a string that’s compliant in a coastal, mild-winter climate can be over-voltage in a high-desert or northern-latitude site with the same module and same string length.

This is why string voltage calculations are one of the first checks in a permit review and one of the first things a solar SLD needs to document explicitly.

Related Reading

String voltage sizing sits right next to MPPT window compliance in the design workflow, once a designer confirms cold-day Voc is within limits, the next question is whether the inverter’s MPPT range covers the hot-day Vmp floor, which is the exact topic covered in string voltage vs. MPPT voltage in inverter design. For a deeper look at how the MPPT algorithm itself tracks the array’s power point once voltage is within range, the MPPT glossary entry breaks down the tracking logic.

On the documentation side, string voltage calculations feed directly into the one-line diagram that accompanies a permit submission, the solar SLD software guide covers how NEC 2023-compliant SLDs auto-generate these voltage tables, and the same string-level Voc/Vmp checks are a standard part of the PVsyst tracker yield study methodology when the array uses trackers rather than fixed-tilt racking.

The current side of the same check is covered under string current, and the full method is in string sizing using Voc.

Disclosure: Qbits Energy is a Heaven Designs group company.