Calculate the correct number of panels per string using NEC 690.7 temperature correction — both Method 1 (exact αVoc) and Method 2 (table factors) — and verify your strings fit within your inverter’s MPPT voltage window.
Incorrect string sizing is one of the most common causes of solar permit rejections and inverter failures. This calculator applies NEC 690.7 temperature correction and MPPT window verification to give you code-compliant string configurations — fast.
Supports both Method 1 (exact temperature coefficient αVoc calculation) and Method 2 (NEC Table 690.7(A) correction factors) so your design documentation satisfies any AHJ.
Calculates both the maximum panels per string (cold Voc limit with 0.95 safety factor) and minimum panels per string (hot Vmp check against inverter MPPT minimum) to keep you in-window all year.
Determines how many strings can run in parallel before NEC 690.8(A) requires overcurrent protection — preventing costly re-design after permit submission.
Before finalizing your panel layout and inverter selection, run string sizing to confirm your chosen configuration is code-compliant and within MPPT range. Catching errors here saves expensive re-designs after equipment is ordered.
Many AHJs require explicit string sizing calculations with temperature correction shown in the permit package. Use this tool's NEC 690.7 method labels to produce documentation that passes the first review.
Replacing failed panels with a different model? Run the string sizing check to confirm the new panel's Voc characteristics still keep the existing string within safe voltage limits — especially critical if cell temperature coefficients differ.
Input your panel's Voc (open-circuit voltage at STC), Vmp (max power voltage at STC), temperature coefficient αVoc (for Method 1), and αVmp. All values are on the panel datasheet under "Electrical Characteristics."
Input the inverter's maximum DC input voltage (e.g., 600 V, 1000 V, or 1500 V) and the MPPT voltage range (minimum and maximum MPPT operating voltages). These are in the inverter datasheet under "DC Input."
Enter the record low ambient temperature for the installation site. Use ASHRAE 99% design temperatures — not the historical record minimum. Also set the expected maximum operating temperature for the hot Vmp check (default 75°C cell temp).
Method 1 uses the panel's exact αVoc coefficient for the most precise correction. Method 2 uses NEC Table 690.7(A) correction factors based on temperature range — required when αVoc is not listed on the datasheet. Method 1 typically yields a slightly higher panel count.
The calculator displays the corrected cold Voc, maximum panels per string (cold limit with 0.95 safety factor), minimum panels per string (hot MPPT minimum), and the maximum parallel strings before fuses are required per NEC 690.8(A).
The panel's Voc adjusted for your site's record low temperature. This is the highest voltage a string will produce — the value that must not exceed the inverter's maximum DC input voltage. Factoring in the 0.95 safety margin gives the true design ceiling.
The largest number of panels you can wire in series without exceeding the inverter's maximum DC voltage (with the 0.95 NEC safety factor applied). Going above this count risks inverter damage or tripping on overvoltage protection — and will fail a permit inspection.
The fewest panels you can wire in series and still keep the string operating above the inverter's MPPT minimum voltage during hot summer conditions. Fewer panels means the inverter may lose tracking on hot days.
The number of strings that can run in parallel without requiring string-level overcurrent protection per NEC 690.8(A). If your design exceeds this count, each string must be individually fused — a critical detail for both safety and permit compliance.
All calculations follow NEC 2020 Article 690, specifically Section 690.7 (maximum voltage) and Section 690.8 (overcurrent protection). The two voltage correction methods and the MPPT window check are applied sequentially to produce a complete string design.
Worked example: Jinko 400W panels (Voc = 41.5V, Vmp = 34.5V, αVoc = −0.28%/°C) on a SMA Sunny Boy 7.7 (Vmax = 600V, MPPT range 200–480V). Record low temp: −10°C. Cold Voc: 41.5 × (1 + (−0.0028 × (−10 − 25))) = 41.5 × 1.098 = 45.6V per panel. Max string: floor((600 × 0.95) / 45.6) = floor(570 / 45.6) = 12 panels. Min string: ceil(200 / (34.5 × (1 + (−0.0028 × 50)))) = ceil(200 / 29.7) = 7 panels. Valid range: 7–12 panels per string.
Calculations sourced from SurgePV’s String Sizing Calculator — surgepv.com/tools/string-sizing-calculator/
| Ambient Temp (°C) | Ambient Temp (°F) | Correction Factor |
|---|---|---|
| 24 to 20°C | 76 to 68°F | 1.02 |
| 19 to 15°C | 66 to 59°F | 1.04 |
| 14 to 10°C | 57 to 50°F | 1.06 |
| 9 to 5°C | 48 to 41°F | 1.08 |
| 4 to 0°C | 39 to 32°F | 1.10 |
| −1 to −5°C | 30 to 23°F | 1.12 |
| −6 to −10°C | 21 to 14°F | 1.14 |
| −11 to −15°C | 12 to 5°F | 1.16 |
| −16 to −20°C | 3 to −4°F | 1.18 |
| −21 to −25°C | −5 to −13°F | 1.20 |
Source: NEC 2020 Table 690.7(A). Factors apply to crystalline silicon panels at STC (25°C). For Method 1, use panel's αVoc coefficient directly instead of this table.
Never use the historical record extreme low — it is too conservative and will reduce your string count unnecessarily. The ASHRAE 99% heating design temperature (exceeded only 1% of hours annually) is the NEC-recognized standard for string voltage calculations.
NEC 690.7 requires that calculated string voltage not exceed the inverter's rated max voltage limit with the safety factor applied. Omitting this is one of the most common reasons string sizing calculations are rejected on permit review. Always apply it.
These are two different temperature coefficients. αVoc is used in Method 1 to find the cold Voc ceiling. αVmp is used separately to calculate how much Vmp drops on a hot day — verifying the string stays above the inverter's MPPT minimum. Using the wrong coefficient produces a code violation or under-producing system.
Most designers focus on the cold Voc maximum, but a system that drops below MPPT minimum on a hot summer day will lose production during peak hours. Always run the hot Vmp check and ensure your minimum panel count keeps the string in-window even at 75°C cell temperature.
Solar panels are semiconductor devices, and their voltage output is temperature-dependent. As temperature decreases, the Voc of a crystalline silicon panel increases — typically by about 0.3% to 0.45% per degree Celsius below STC (25°C). On a very cold day, a string of panels can produce significantly higher voltage than at STC, which is why NEC 690.7 requires temperature correction to prevent the cold-weather voltage spike from exceeding the inverter's maximum input voltage.
Method 1 (NEC 690.7(A)(1)) uses the panel manufacturer's exact temperature coefficient for Voc (aVoc) to calculate the precise cold-corrected voltage. Method 2 (NEC 690.7(A)(2)) uses conservative correction factors from NEC Table 690.7(A) organized by temperature ranges. Method 2 is required when aVoc is not listed on the panel datasheet and typically results in a slightly lower maximum panel count because the table factors are conservatively rounded. Both methods are accepted by all AHJs.
NEC 690.7 references the ASHRAE Handbook of Fundamentals for design temperatures. Use the "99% heating design temperature" — the temperature that the location's outdoor air falls below only 1% of all hours in a year. It is not the historical absolute record minimum. You can find ASHRAE 99% design temperatures through the ASHRAE website, Climate Consultant software, or many utility and state energy office databases.
Exceeding the inverter's maximum DC input voltage can cause the inverter to shut down on overvoltage protection, void the inverter warranty, and in severe cases damage the inverter's input electronics or DC disconnect components. In the field, this often shows up as the system going offline on cold mornings and coming back online once panels warm up — frequently misdiagnosed as a communication fault. It is also a code violation that can result in failed inspections and insurance claim denials.
Target a string voltage that sits comfortably in the middle to upper portion of the inverter's MPPT voltage range at STC conditions — typically 70% to 90% of the MPPT maximum. This ensures the string stays in-window during hot summer afternoons (when voltage drops) and operates near peak efficiency during spring and fall. Avoid being too close to either the MPPT minimum or maximum at STC, as seasonal temperature swings will push the voltage toward the edges.
Per NEC 690.8(A), string-level overcurrent protection (fuses or circuit breakers) is required when the number of parallel strings exceeds the threshold where fault current from other strings could damage the wiring of any single string. The calculator determines this threshold by dividing the inverter's maximum DC input current by the panel's short-circuit current (Isc). Most residential systems with two or three strings do not require fusing; larger commercial systems nearly always do.
Mixing panel models in a series string is strongly discouraged and may violate the inverter warranty. Panels with different Vmp values will force the lower-voltage panel to operate outside its optimal point, reducing total string output. Panels with different Isc values will limit the string current to the lowest Isc panel. For string sizing purposes, always calculate using only one panel model's specifications.
No. NEC 690.7 string voltage calculations apply to systems where multiple panels are connected in series to a string inverter or DC optimizer system. Microinverters convert DC to AC at each individual panel, so there is no series string voltage to calculate. Each microinverter's maximum input voltage simply needs to exceed the single panel's cold-corrected Voc — a much simpler check covered by the microinverter's panel compatibility list. This calculator is designed for string inverter and DC optimizer systems.
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