Does voltage correction apply to every climate, or only genuinely cold regions?

NEC 690.7 requires the calculation to be performed for every installation, since even relatively mild climates experience some minimum temperature below the standard 25°C test condition, producing at least some voltage increase — the magnitude of the required correction and its practical effect on maximum string length varies by how cold the specific site's documented minimum design temperature actually is, but the calculation itself is a universal requirement.

Where does the required documented lowest expected temperature value come from for a specific site?

This is typically derived from recognized meteorological reference data sources reflecting the specific site's historical extreme low temperature record, often referenced through ASHRAE climate design data or similar recognized sources — not simply an average winter temperature, since the code requirement specifically addresses the coldest realistic operating condition, not typical conditions.

Can this voltage rise ever be beneficial rather than just a design constraint to work around?

The voltage increase itself does not provide additional useful energy output — it is purely a voltage-level shift the electrical design has to safely accommodate, not an efficiency or performance benefit. It is exclusively a design and safety consideration, addressed by correctly limiting maximum string length rather than something to be exploited for any positive effect.

Engineering·6 min read·August 14, 2026

🥶 NEC 690.7: Why Cold Weather Increases Solar Array Voltage

Why photovoltaic panel voltage rises as temperature falls, why this makes cold-climate voltage correction a required NEC 690.7 calculation, and what happens when it is overlooked.

By EngineersUniverse Editorial Team

Engineering Editorial & Technical Review Team

Published August 14, 2026

A Counterintuitive Temperature Relationship

It might seem intuitive that colder conditions would reduce solar panel performance across the board, similar to how cold weather reduces battery capacity in many other contexts. For photovoltaic panel voltage specifically, the relationship runs the opposite direction: panel open-circuit voltage (Voc) actually increases as panel temperature decreases, a well-established semiconductor physics behavior that becomes directly relevant to system design and safety.

Why Voltage Rises as Temperature Falls

Photovoltaic cells are semiconductor devices, and their voltage output has an inherent, predictable temperature coefficient — commonly a negative correlation with temperature, meaning voltage rises as cell temperature drops below the standard 25°C rating condition. This is a fundamental characteristic of how photovoltaic cells generate voltage, not a manufacturing defect or unusual behavior specific to any particular panel brand — it applies broadly across standard crystalline silicon photovoltaic technology.

Why This Matters More at the String Level Than the Single-Panel Level

An individual panel's voltage increase from cold weather is a real but modest effect on its own — the consequential design concern arises because panels in a typical residential or commercial array are connected in series strings, and voltage adds directly across a series string, meaning a string of many panels multiplies this per-panel temperature effect across the entire string. A string voltage that is safely within equipment ratings at the standard 25°C test condition can rise to a genuinely different, higher voltage during actual cold-weather operation, and this cumulative string-level voltage increase is what NEC 690.7 specifically requires designers to calculate and verify.

What NEC 690.7 Actually Requires

NEC 690.7 requires correcting a photovoltaic string's calculated open-circuit voltage for the lowest expected ambient temperature at the specific installation site — using the panel manufacturer's published temperature coefficient for voltage, combined with the site's documented coldest expected ambient temperature (typically derived from a recognized source of local historical extreme temperature data), the code requires calculating the actual maximum string voltage the system could realistically experience under its coldest expected operating condition, not just its standard-test-condition voltage.

Why Exceeding Equipment Voltage Ratings Is a Real Safety and Compliance Failure

Every component in a photovoltaic string circuit — panels themselves, string wiring, combiner boxes, disconnects, and the inverter's DC input — carries a maximum voltage rating that must not be exceeded during any realistic operating condition. If a string is sized without properly accounting for cold-weather voltage rise, the string's actual voltage during a genuinely cold day could exceed one or more of these component ratings, creating a real safety hazard (insulation breakdown, arc flash risk, equipment damage) and a direct code compliance failure that a plan reviewer or inspector should catch, though undersized strings that pass through unnoticed on paper do sometimes reach the field.

Why This Is a Bigger Design Concern in Cold Climates

The magnitude of the voltage correction directly scales with how cold the site's documented lowest expected ambient temperature actually is — a site with a design low temperature well below freezing requires a correspondingly larger voltage correction, and therefore a more conservative (shorter) maximum string length to stay within equipment ratings, compared to a site with a much milder documented minimum design temperature. This is exactly why the same panel and inverter combination might support a longer series string in a warm climate than the maximum safe string length for an otherwise identical installation in a cold climate — the temperature correction genuinely changes the safe design outcome, not just a calculation formality.

Why This Calculation Follows, Not Precedes, Preliminary Array Sizing

Preliminary array sizing (as this site's Solar PV Array Sizing Calculator addresses) determines the overall system size and panel count needed to meet a usage offset target — a genuinely separate design question from how that panel count is actually wired into individual series strings, which is where the NEC 690.7 voltage correction becomes directly relevant. A complete design process moves from preliminary sizing to detailed electrical stringing design, where cold-weather voltage correction (along with the companion NEC 690.8 current and overcurrent protection sizing) determines the actual safe, code-compliant string configuration for the specific site and equipment selected.

Topics covered

NEC 690.7 voltage correctionsolar panel cold weather voltagePV open circuit voltage temperaturesolar string voltage sizing