PV String Sizing at Record Low Temperature: Voc Correction, Maximum Modules per String for 600 V, 1000 V and 1500 V, and NEC 690.7
Sep 10, 2026
Summary: A PV module's open-circuit voltage rises as it gets colder, so string voltage is checked at the record low temperature for the site, not at 25 C. NEC 690.7(A) permits three methods: the module's temperature coefficient of Voc from the datasheet, Table 690.7(A) correction factors for crystalline modules, or an engineered calculation. Maximum modules per string equals the system voltage limit divided by the cold-corrected Voc, rounded down. Example: a 450 W module with Voc 41.5 V and a coefficient of −0.27 percent per C, at a record low of −20 C, has a corrected Voc of 41.5 × (1 + 0.0027 × 45) = 46.5 V; on a 600 V residential limit that is 12 modules, on 1000 V it is 21, on 1500 V it is 32. Every connector, fuse, disconnect, combiner and conductor in the string must be rated for that system voltage: 1000 V UL 6703 connectors on a 1500 V string are a violation and a fire risk.
Why cold matters
Voc falls about 0.25 to 0.35 percent for every degree C above 25 C and rises by the same for every degree below. A module rated 41.5 V at 25 C reads about 47 V at −20 C before sunrise on a clear January morning, when the string is at open circuit because the inverter has not started. That is the moment the inverter's DC input, the connectors and the wire insulation see the highest voltage of the year. Exceeding the inverter's maximum DC voltage voids its warranty and can destroy the input stage; exceeding the connector or fuse rating is a listing violation under 690.7 and 690.33.
The three NEC methods
| Method | NEC | How | Use when |
|---|---|---|---|
| 1. Temperature coefficient | 690.7(A)(1) | Voc corrected = Voc × [1 + (β/100) × (Tmin − 25)], where β is the Voc temperature coefficient in percent per C (negative), Tmin the record low in C | Datasheet coefficient available; the standard method |
| 2. Table 690.7(A) | 690.7(A)(2) | Multiply Voc by the factor for the temperature band: 1.02 at 24 to 20 C down to 1.25 at −36 to −40 C | Crystalline or multicrystalline modules with no coefficient on the datasheet |
| 3. Engineered | 690.7(A)(3) | PV system design software or a licensed engineer using site data | Utility-scale and where the AHJ accepts stamped calculations |
Table 690.7(A) correction factors
| Record low, C | Factor | Record low, C | Factor |
|---|---|---|---|
| 24 to 20 | 1.02 | −1 to −5 | 1.12 |
| 19 to 15 | 1.04 | −6 to −10 | 1.14 |
| 14 to 10 | 1.06 | −11 to −15 | 1.16 |
| 9 to 5 | 1.08 | −16 to −20 | 1.18 |
| 4 to 0 | 1.10 | −21 to −25 | 1.20 |
| −26 to −30 | 1.21 | ||
| −31 to −35 | 1.23 | ||
| −36 to −40 | 1.25 |
Worked examples, coefficient method
| Site | Record low | Module Voc (25 C) | β, %/C | Corrected Voc | Max modules at 600 V | at 1000 V | at 1500 V |
|---|---|---|---|---|---|---|---|
| Ventura, CA | −3 C (27 F) | 41.5 V | −0.27 | 44.6 V | 13 | 22 | 33 |
| Denver, CO | −31 C (−24 F) | 41.5 V | −0.27 | 47.8 V | 12 | 20 | 31 |
| Minneapolis, MN | −41 C (−41 F) | 41.5 V | −0.27 | 48.9 V | 12 | 20 | 30 |
| Phoenix, AZ | −9 C (16 F) | 49.2 V (72-cell) | −0.28 | 53.9 V | 11 | 18 | 27 |
Round down, always. Then check the other end: at the record high cell temperature (ambient high plus 25 to 30 C for a roof mount), the string's Vmp must stay above the inverter's MPPT minimum, or the inverter drops out on hot afternoons. Most residential designs land between the two limits with two or three modules of margin.
Where to get the record low
NEC 690.7(A) informational note points to the ASHRAE Extreme Annual Mean Minimum Design Dry Bulb Temperature for the location. Solar design tools (Aurora, HelioScope, SolarEdge Designer) carry it built in; the Solar ABCs Expedited Permit Process reference tables list it by city, and NOAA daily records give the absolute low. AHJs vary on which they accept; ASHRAE extreme mean minimum is the code reference and is usually a few degrees warmer than the all-time record.
Everything in the string must match the voltage
| Component | 600 V string | 1000 V string | 1500 V string |
|---|---|---|---|
| Module | Any | 1000 V or 1500 V rated | 1500 V rated |
| MC4 connectors | UL 6703, 1000 V | UL 6703, 1000 V | UL 6703, 1500 V |
| PV wire | UL 4703, 600 V | UL 4703, 1000 V | UL 4703, 2000 V |
| String fuses | 600 V gPV | 1000 V gPV | 1500 V gPV |
| Combiner, disconnect | 600 V DC | 1000 V DC | 1500 V DC |
| Inverter DC input | 600 V max | 1000 V max | 1500 V max |
Residential and dwelling-unit rooftops are limited to 600 V by 690.7(D) unless the array meets the conditions for 1000 V; commercial rooftops run 1000 V; ground mounts and utility arrays run 1500 V. The connector rating is the one most often missed on upgrades.
Quick answers
How do I calculate maximum modules per string? Divide the system voltage limit by the module Voc corrected to the site's record low temperature, and round down.
What temperature do I use? The ASHRAE extreme annual mean minimum design dry-bulb temperature for the site, per the NEC 690.7 note, or the all-time record low if the AHJ requires it.
What is the Voc correction at −20 C? Using Table 690.7(A), multiply by 1.18; using a typical coefficient of −0.27 percent per C, multiply by 1.12. The coefficient method is more precise and usually allows one more module.
Can I use 1000 V connectors on a 1500 V string? No. Every connector, fuse, wire and disconnect must be rated at or above the system voltage.
Conversions Tech stocks UL 6703 MC4 and Evo2 connectors in 1000 and 1500 V, UL 4703 2 kV PV wire and custom jumpers, and 1500 V gPV fuses, combiners and DC disconnects. Send the module datasheet and site to CS@ConversionsTech.com and engineering will run the string check. Related: 1500 V strings and MC4 Evo2, MC4 crimp procedure.
