Solar String Sizing: Max Panels in Series per NEC 690.7
Reading time: ~9 min read
📋 Key Takeaways
- Cold raises voltage; cold is what kills inverters. Module Voc rises as temperature falls — roughly 0.25–0.35% per °C below the 25°C rating point. NEC 690.7 requires the string's maximum voltage to be calculated with the lowest expected ambient temperature, and that corrected value must stay under the inverter's absolute maximum input.
- Two limits, one string. The series count must be low enough that cold-corrected Voc stays under the inverter max (NEC 690.7) and high enough that hot-weather operating voltage stays above the MPPT minimum — otherwise the inverter clips or never wakes up on summer afternoons.
- A 600V residential inverter on a 41V-module design tops out around 11–13 panels per string depending on climate; a 1,000V commercial unit takes 20+; 1,500V utility strings run into the high 20s.
- String fusing per NEC 690.9 becomes relevant once you parallel three or more strings — check the module's maximum series fuse rating on its datasheet.
- Use the module's actual Voc and temperature coefficient from the datasheet, not rule-of-thumb values — a 0.05%/°C difference in coefficient changes the max string count in cold climates.
String sizing is the one solar calculation where a wrong answer destroys hardware. Overstack a string and the first cold, clear morning pushes open-circuit voltage past the inverter's input rating — a failure that happens at dawn, under no load, with warranty implications. Understack it and the array sags below the MPPT window every hot afternoon, silently clipping production for 25 years. The calculator below applies the NEC 690.7 cold-temperature correction to your module's Voc and checks both ends of the inverter's voltage window, returning the maximum and minimum modules per series string.
Once the string count is set, the wire and protection come next: our solar wire size calculator covers PV-wire ampacity and the NEC 690.8 current multipliers, and the solar panels collection has datasheets with Voc, Vmp, and temperature coefficients for current modules.
Solar String Sizing Calculator
Magnitude from the datasheet (listed as a negative, e.g. −0.29%/°C — enter 0.29).
ASHRAE extreme annual minimum for the site — many AHJs publish this.
Cold-corrected module Voc: —
Maximum modules per string (NEC 690.7): —
Minimum modules per string (MPPT floor): —
String Voc at record low (max string): —
Sizing logic: cold-corrected Voc = module Voc × (1 + coefficient × (25 − Tmin)) per NEC 690.7(A) methodology; max series count = inverter max input ÷ cold Voc, rounded down; min series count = MPPT minimum ÷ hot-weather Vmp (estimated at cell temperature 45°C above a 35°C ambient using the same coefficient), rounded up. Some AHJs require the NEC Table 690.7(A) correction factors instead of the coefficient method — the table below lists them. Informational — inverter manufacturer limits and the AHJ govern.
NEC Table 690.7(A): Voltage Correction Factors
Where the AHJ requires the table method rather than the manufacturer coefficient, multiply STC Voc by the factor for the lowest expected ambient temperature:
| Ambient temp (°C) | Correction factor | Ambient temp (°C) | Correction factor |
|---|---|---|---|
| 24 to 20 | 1.02 | −1 to −5 | 1.14 |
| 19 to 15 | 1.04 | −6 to −10 | 1.16 |
| 14 to 10 | 1.06 | −11 to −15 | 1.18 |
| 9 to 5 | 1.08 | −16 to −20 | 1.20 |
| 4 to 0 | 1.10 | −21 to −25 | 1.21 |
| — | — | −26 to −30 | 1.23 |
Note the table and coefficient methods give different answers — the table is deliberately conservative and assumes a generic silicon response. Most modern AHJs accept the manufacturer coefficient per 690.7(A)(2) or its informational note; a few still mandate the table. Ask before the design goes to permitting.
Worked Examples: Common Modules on Common Inverters
Using the coefficient method with a −10°C design minimum (correction ≈ 1.10–1.12 for typical coefficients):
| Module | Voc (STC) | Coeff (%/°C) | Cold Voc @ −10°C | 600 V inverter max string | 1,000 V inverter max string |
|---|---|---|---|---|---|
| 410 W residential, 108 half-cell | 37.1 V | 0.27 | 40.6 V | 14 | 24 |
| 450 W residential/commercial | 41.0 V | 0.29 | 45.2 V | 13 | 22 |
| 500 W large-format | 44.0 V | 0.30 | 48.6 V | 12 | 20 |
| 590 W commercial, 144 half-cell | 48.6 V | 0.26 | 53.0 V | 11 | 18 |
Two design habits pay off. First, don't string right at the ceiling — leaving 3–5% voltage headroom absorbs datasheet tolerance, measurement error, and the one morning colder than the "record" low. Second, keep parallel string lengths identical on a shared MPPT; mismatched strings force the tracker to a compromise voltage and cost real production.
The Other End: MPPT Floor and Summer Heat
Everyone checks the cold ceiling; fewer check the hot floor. Cell temperature runs 25–35°C above ambient on a sunny afternoon, and Vmp falls with heat using the same coefficient family. A string sized at the minimum for a 125V MPPT floor can sag below it on a 38°C rooftop day, dropping the inverter out of tracking precisely when irradiance peaks. The calculator's minimum estimate uses an 80°C cell temperature as a conservative proxy — if your inverter lists a wide MPPT range (e.g., 80–550V), mid-window string counts are the safe default. Also remember the floor applies to operating voltage, not Voc: a string that "starts" at dawn can still drop out at noon.
String Fusing Under NEC 690.9
Once strings are paralleled, NEC 690.9(A) asks whether each string needs its own overcurrent device. The common outcomes: one string alone needs no string fuse; two parallel strings are generally exempt because neither can drive more fault current into the other than the module series-fuse rating; three or more parallel strings typically require a fuse per string, sized per 690.8's 156% logic but never above the module's marked maximum series fuse value (usually 15–20A on the datasheet). Inline MC4 fuses or combiner-box fuse holders both satisfy it — our solar wire size calculator covers the 690.8 ampacity math behind that 156%.
Frequently Asked Questions
How many solar panels can I connect in series?
It depends on the module's Voc, your site's coldest temperature, and the inverter's maximum input voltage — there is no universal number. Divide the inverter's max input by the cold-corrected Voc and round down. On a 600V residential inverter with a typical 41V module and a −10°C design minimum, that's about 13 panels; on a 1,000V commercial unit it's about 22. Always verify with the NEC 690.7 correction for your exact module and climate.
What happens if string voltage exceeds the inverter's maximum input?
Permanent damage, typically at the worst possible time: the overvoltage occurs on cold, sunny mornings at open circuit — before the inverter even starts drawing current. Input-stage components are not rated beyond the marked maximum, and the failure mode (blown input capacitors or DC stage) is usually not covered under warranty because exceeding max input voltage is an installation error, not a product defect. This is why NEC 690.7 exists.
Which temperature do I use for the NEC 690.7 correction?
The lowest expected ambient temperature for the site — in practice the ASHRAE Extreme Annual Mean Minimum Design Dry Bulb temperature for the nearest weather station, which many AHJs publish in their permitting guides. Conservative designers use the all-time record low. Do not use an average winter low; the correction exists for the single coldest morning of the system's 25-year life, because one overvoltage event is enough.
Do I need a fuse on every string?
Usually only when three or more strings are paralleled on one MPPT or combiner circuit. NEC 690.9(A) exempts configurations where the worst-case backfeed current can't exceed the module's marked series-fuse rating — which covers single strings and most two-string parallels. With three or more parallel strings, install one fuse per string at or below the module's maximum series fuse rating (commonly 15–20A), sized with the NEC 690.8 156% current logic.
Is series or parallel wiring better for a rooftop array?
Series first, parallel only when you must. Series strings raise voltage, which keeps current low — meaning smaller PV wire, less voltage drop, and better inverter efficiency. Parallel strings raise current and trigger the NEC 690.9 fusing question. The exceptions: heavy shade across part of the roof (split shaded and unshaded modules onto different MPPTs rather than one long string), and module counts that can't reach the MPPT floor, where two short parallel strings beat one long one.
Designing an Array?
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