Calculate the minimum and maximum modules per string from temperature-corrected module voltage. Then check string voltage and combined input current against your inverter's limits before you submit for permitting.
This tool is for preliminary design checks only and does not replace a stamped plan set or AHJ review. Confirm all values against current code, the adopted NEC edition, and manufacturer specifications before submitting for permitting. For the full walkthrough of Voc/Vmp temperature correction, cold and hot string limits, and common sizing errors, see our Solar String Sizing Guide.
How to Use the NEC 690.7 Solar String Sizing Calculator
1
Enter the module's Voc, Vmp, and Isc and their temperature coefficients from the current datasheet.
2
Enter the site's minimum and maximum design temperatures per the adopted NEC edition or AHJ.
3
Enter the proposed number of modules per string and parallel strings per MPPT input.
4
Enter the inverter's maximum DC input voltage, MPPT voltage window, and maximum input current per MPPT.
5
Review the pass/fail results and adjust modules per string or string count as needed.
What Does the Solar String Calculator Check?
The calculator runs three string-sizing checks. Cold-weather string Voc must stay at or below the inverter's maximum DC input voltage. Hot-weather string Vmp must stay at or above the inverter's minimum MPPT operating voltage. It also checks whether combined current from parallel strings stays within the applicable inverter input-current limit.
Formula: Maximum string length = Inverter max DC voltage ÷ Temperature-corrected module Voc
How to Calculate Maximum Modules per String
Maximum string length is limited by the module's open-circuit voltage at the project's lowest expected ambient temperature. NEC 690.7 requires maximum PV circuit voltage to account for cold-temperature voltage increase.
Cold-adjusted Voc = Voc × [1 + (|Voc temperature coefficient| × (25°C − Tmin))]
Maximum modules per string = Inverter maximum DC input voltage ÷ Cold-adjusted Voc
Always round down to the nearest whole module. Use the module's Voc temperature coefficient as a decimal in the formula. For example, −0.29%/°C becomes 0.0029.
How to Calculate Minimum Modules per String
Minimum string length depends on whether the string can maintain enough operating voltage for the inverter's MPPT range when module voltage falls at high operating temperatures.
Hot-adjusted Vmp = Vmp × [1 − (|Vmp temperature coefficient| × (Tmodule max − 25°C))]
Minimum modules per string = Inverter minimum MPPT voltage ÷ Hot-adjusted Vmp
Always round up to the next whole module. Use the inverter's minimum MPPT operating voltage, not its startup voltage. Startup voltage is a separate inverter specification and may be higher.
Example Solar String Sizing Calculation
Assume:
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Module: Voc 41 V at −0.29%/°C; Vmp 34.5 V at −0.35%/°C
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Inverter: 600 V maximum DC input; 250 V minimum MPPT operating voltage
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Site: −5°C minimum design ambient temperature; 70°C assumed maximum module temperature
Maximum modules per string
Cold-adjusted Voc:
41 V × [1 + (0.0029 × 30)] = 44.6 V
Maximum modules:
600 V ÷ 44.6 V = 13.45
Round down, so the string can contain no more than 13 modules.
Minimum modules per string
Hot-adjusted Vmp:
34.5 V × [1 − (0.0035 × 45)] = 29.1 V
Minimum modules:
250 V ÷ 29.1 V = 8.6
Round up, so the string needs at least 9 modules.
For these assumed design temperatures and equipment specifications, the acceptable range is 9 to 13 modules per string. Actual project sizing should use the module and inverter manufacturers' current specifications and project-specific temperature conditions. Module operating temperature depends on factors such as ambient temperature, irradiance, airflow, mounting configuration, and roof conditions.
Related NEC Calculators
Where Is String Sizing Shown on a
Permit Plan Set?
String configuration is documented in the electrical plan set, usually on the single-line or three-line diagram. It appears alongside module and inverter specifications, conductor sizing, and overcurrent protection. Plan reviewers often check it against the inverter datasheet first.
Using an average winter low instead of the design minimum temperature is a common cause of permit correction requests.
GreenLancer prepares complete solar permit plan sets for residential and commercial projects in all 50 states, with string configuration documented on the electrical diagrams. When a jurisdiction requires it, PE-stamped solar engineering can be added to the same project.
FAQs on NEC 690.7 Solar String Sizing
What temperature should I use for solar string sizing?
Use the minimum design temperature required by the adopted NEC edition, your AHJ, or the project's design criteria, not an average winter low. Confirm the temperature source shown on the plan set before calculating maximum cold-weather Voc.
How many modules can I put in one string?
It depends on your module's open-circuit voltage, the temperature correction for your site's design minimum, and your inverter's maximum DC input voltage. The calculator applies all three.
Does this replace a stamped plan set?
No. This is a preliminary design-check tool. Submitted plan sets still require PE stamping in states that require it, and AHJ review regardless of state.
How many modules fit a 600V inverter with a 41V module Voc?
Using a temperature-corrected Voc of roughly 44.6V (based on a -5°C site minimum and a typical -0.29%/°C coefficient), a 600V inverter supports up to 13 modules per string. Actual results depend on your project's module coefficient and design minimum temperature, so run your own numbers through the calculator above rather than reusing this example.
What happens if my string length fails the voltage check?
If calculated cold-weather Voc exceeds the inverter's maximum DC input voltage, reduce the number of modules in the string. If hot-weather Vmp falls below the inverter's minimum MPPT voltage, add modules where the design allows, or select an inverter with a lower minimum MPPT threshold. Our solar string sizing guide covers current-related fixes and other common sizing errors in more depth.
Does this calculator check current and parallel strings, or only string voltage?
It checks both. The calculator compares temperature-corrected string voltage with the inverter's DC input and MPPT range, and compares combined current from parallel strings with the inverter's per-MPPT input current rating. Adding modules in series raises voltage but not current, while adding parallel strings raises current but not voltage.
What's the difference between maximum and minimum string size?
Maximum string size is set by cold-adjusted Voc and the inverter's maximum DC input voltage. Minimum string size is set by hot-adjusted Vmp and the inverter's minimum MPPT operating voltage, which can differ from its startup voltage. The calculator checks both limits using the values you enter.