Check your string configuration against inverter voltage and current 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
rated Voc and temperature
coefficient.
2
Enter your site's minimum design temperature per the adopted NEC edition or AHJ requirement.
3
Enter proposed
string count.
4
Enter the inverter's maximum DC input voltage and MPPT voltage window.
5
Review the pass/fail result and adjust string count as needed.
What the Calculator Checks
The calculator checks temperature-corrected string voltage against the inverter's maximum DC input voltage, and checks string and combined input current against the inverter's applicable DC input ratings.
Formula: Maximum string length = Inverter max DC voltage ÷ Temperature-corrected module Voc
Example Calculation
-- Inverter max DC input: 600V
-- Module Voc: 41V
-- Temperature-corrected Voc at site minimum design temp: 44.6V
600 ÷ 44.6 = 13.45 Maximum string length: 13 modules
Where This Appears on Plans
String sizing shows up on the electrical plan set as part of the DC circuit design, and it's one of the first things a plan reviewer checks against the inverter datasheet. Getting it wrong is a common cause of permit correction requests, since it's easy to use an average low temperature instead of the design minimum.
Related NEC Calculators
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?
This calculator evaluates temperature-corrected string voltage against the inverter's DC input and MPPT range. Combined current from parallel strings should be checked separately against the inverter's maximum input current and per-MPPT current limits, since paralleling strings adds current without changing string 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. This calculator checks both directions using the module and inverter values you enter.