Solar String Sizing Calculator
Size a PV string per NEC 690.7. Get the maximum and minimum modules per string from module Voc/Vmp, temperature coefficients, your record-low and high design temperatures, and the inverter voltage window. Free, no sign-up.
What to calculate next
Tools commonly used alongside this calculation
Wire Size & Ampacity Calculator
Find the minimum copper or aluminum wire size for a load using NEC Table 310.16, with 310.15 ambient and conductor-count derating and the 110.14(C) terminal limit. Free, no sign-up.
Voltage Drop Calculator
Calculate voltage drop % and the minimum copper or aluminum wire size to stay within the NEC 3% / 5% recommendation, single- or three-phase. Free, no sign-up.
Conduit Fill Calculator
Size conduit per NEC Chapter 9. Add THHN, XHHW, or other conductors and get the minimum trade size and fill % for EMT, IMC, RMC, PVC, ENT, FMC, and LFMC. Free, no sign-up.
Explanation
A PV string is a group of solar modules wired in series so their voltages add up. Wire too many and the cold-weather voltage overshoots the inverter’s maximum DC input rating; wire too few and the hot-weather voltage drops below the inverter’s MPPT window and the array stops harvesting power. NEC 690.7(A) fixes the upper limit by requiring the maximum system voltage to be calculated at the lowest expected temperature. This calculator returns both ends — the maximum and minimum modules per string — from the module datasheet and the inverter voltage window.
How string sizing works
A module’s voltage rises as it gets colder and falls as it gets hotter — the temperature coefficient on the datasheet (a negative %/°C) says by how much. The maximum string is set by the coldest temperature, where the open-circuit voltage (Voc) peaks: divide the inverter’s maximum input voltage by the corrected Voc and round down. The minimum string is set by the hottest cell temperature, where the max-power voltage (Vmp) bottoms out: divide the inverter’s minimum MPPT voltage by the corrected Vmp and round up.
Vmp_hot = Vmp × [1 + (T_max + T_add − 25) × Tk_Vmp / 100]
| Term | Meaning |
|---|---|
| Voc / Vmp | Module open-circuit and max-power voltage (datasheet, STC) |
| Tk_Voc / Tk_Vmp | Voltage temperature coefficient, %/°C (negative) |
| T_min / T_max | Lowest and highest design ambient temperature (°C) |
| T_add | Cell-temperature rise above ambient for the hot case |
The standard test condition (STC) temperature is 25 °C, which is why every correction is measured from 25. Because a colder day pushes more voltage, the maximum count is always the binding NEC 690.7 limit — round it down so the array can never exceed the inverter rating even on the coldest morning of the year.
Design temperatures
Use site-specific design temperatures, not record extremes from the news. For the cold case, most designers take the ASHRAE Extreme Annual Mean Minimum dry-bulb temperature for the location (or the lowest recorded temperature for a more conservative result). For the hot case, the ASHRAE 2% design high is the common choice, plus a cell-temperature adder for how the modules are mounted — air moves freely under a ground rack but is trapped under a roof-hugging array.
| Mounting | Cell-temp adder T_add |
|---|---|
| Roof mounted, parallel to surface | +35 °C |
| Roof mounted, rack / tilted | +30 °C |
| Ground or pole mounted | +25 °C |
Once the string length is set, size the PV source and output conductors with the wire size & ampacity calculator, and check the loss on a long home run with the voltage drop calculator.
Notes and limits
NEC 690.7(A) lets you correct voltage two ways: with the module’s temperature coefficient (used here, the more precise method) or with the correction factors in Table 690.7(A) when no coefficient is available. Temperature coefficients are negative — enter them with the minus sign. The maximum string is the code limit; the minimum string is a functional limit so the array stays inside the inverter’s MPPT window in summer. When the string conductors share a raceway, count them in the conduit fill calculator. Always confirm the final design against the inverter datasheet, the current NEC, and the authority having jurisdiction.