Cold-temperature Voc correction for PV strings

It seems backwards: a solar array produces its highest voltage on the coldest morning, not under a blazing summer sun. But that quirk of physics is exactly why NEC 690.7 exists, and why it — not heat — sets the ceiling on how many modules you can put in series. Get the correction wrong and you either exceed the inverter's maximum voltage or leave production on the table.

Why cold raises Voc

A photovoltaic cell is a semiconductor. As temperature drops, the band-gap behavior shifts and the open-circuit voltage (Voc) rises. Current barely moves with temperature, but voltage swings meaningfully. Datasheet Voc is quoted at Standard Test Conditions (25 C); on a -10 C morning the real Voc is noticeably higher.

Because inverters have a hard maximum DC input voltage (and modules a maximum system voltage, typically 600 V or 1000 V residential), the design must survive the coldest moment the site will ever see with the sun just hitting a cold array.

The 690.7 method

690.7 gives you two ways to correct:

• Temperature-coefficient method (preferred). Multiply rated Voc by the module's temperature coefficient of Voc (a negative %/C or mV/C value) across the difference between 25 C and the lowest expected ambient. This is the accurate route and uses the actual module.
• Table correction factors. When the coefficient isn't available, the NEC provides a table of correction factors for crystalline silicon keyed to the low temperature — multiply rated Voc by the factor.

The corrected Voc per module, times the number of modules in series, must stay at or below the module's maximum system voltage and the inverter's maximum input voltage.

Which temperature?

Use the lowest expected ambient temperature at the site — most designers use the ASHRAE extreme annual minimum design dry-bulb temperature for the location. A mountain site in Colorado and a coastal site in Florida produce very different string limits from the same modules.

Why it caps string length

Corrected Voc is the maximum-modules side of string sizing. The other side — minimum modules — comes from hot-weather Vmp needing to stay inside the inverter's MPPT window. Cold sets the top of the string; heat sets the bottom. Our PV string sizing reference shows both bounds in one worked example, and the broader how to size a residential solar system walks the whole flow.

Common mistakes

• Using 25 C or an average low instead of the site's extreme minimum — this quietly overloads the inverter on the coldest days.
• Ignoring the coefficient sign or mixing %/C with mV/C.
• Forgetting the module's own max system voltage, which can be the binding limit, not the inverter.
• Rounding the string up "to be safe" — one extra module can quietly push corrected Voc past the ceiling on the coldest morning.

A worked feel for the numbers: a module with a -0.28%/C Voc coefficient and a 40 V rated Voc gains roughly 0.11 V per degree below 25 C. Drop to -15 C — a 40-degree swing — and that's about 4.4 V per module more than the datasheet. Across a 12-module string that's an extra 53 V, enough to turn a comfortable design into a violation. That is why the cold-morning check, not the summer worst case, decides the maximum string length.

Site temperature data and the enforced NEC edition vary; confirm the low-temperature input and any local amendments with your AHJ, and have a licensed electrician or PE review, sign and stamp the design before filing or energizing.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.