How Many REC Alpha Pure-R Modules Per String on a 480 V Inverter?
A REC Alpha Pure-R has a nameplate Voc of 44.8 V at STC. Cold weather pushes that up, and NEC 690.7 makes the cold-corrected number — not the nameplate — the one that must stay under your inverter's 480 V maximum. Here is the whole matrix.
Maximum modules in series, by design minimum temperature
Voc corrected per 690.7's coefficient method: Voc × [1 + (Tmin − 25) × (β ÷ 100)], with β = -0.27 %/°C. "Headroom" is what you have left under the 480 V limit at that string length — the margin a plan reviewer looks for.
| Design min temp | °C | Corrected Voc / module | Max modules in series | String Voc at that length | Headroom |
|---|---|---|---|---|---|
| -30 °F | -34.4 °C | 51.99 V | 9 | 467.9 V | 12.1 V |
| -20 °F | -28.9 °C | 51.32 V | 9 | 461.9 V | 18.1 V |
| -10 °F | -23.3 °C | 50.65 V | 9 | 455.8 V | 24.2 V |
| 0 °F | -17.8 °C | 49.97 V | 9 | 449.8 V | 30.2 V |
| 10 °F | -12.2 °C | 49.3 V | 9 | 443.7 V | 36.3 V |
| 20 °F | -6.7 °C | 48.63 V | 9 | 437.7 V | 42.3 V |
| 30 °F | -1.1 °C | 47.96 V | 10 | 479.6 V | 0.4 V |
| 40 °F | 4.4 °C | 47.29 V | 10 | 472.9 V | 7.1 V |
Note how the answer moves: 9 modules at −30 °F versus 10 at 40 °F. That is the entire reason 690.7 exists, and it is why a string length copied from a job in another climate is a genuine hazard rather than a technicality.
The mistake that costs you a string
690.7 wants the ASHRAE Extreme Annual Mean Minimum Design Dry Bulb temperature for the site's nearest weather station. That is not the record low, and the two are far apart — a location can have a record low near −17 °F and a design minimum around 10 °F. In the table above that gap is frequently a whole module per string.
Both errors are real. Use the record low and you build shorter strings than you need, adding parallel runs and cost. Use a warm guess and you exceed 480 V on the coldest morning of the year, which is where inverters fail and inspectors fail you.
Does the temperature coefficient change the answer?
Module datasheets vary between roughly −0.24 and −0.30 %/°C. At a 0 °F design minimum:
| Voc coefficient | Corrected Voc | Max in series | String Voc |
|---|---|---|---|
| -0.24 %/°C | 49.4 V | 9 | 444.6 V |
| -0.27 %/°C | 49.97 V | 9 | 449.8 V |
| -0.3 %/°C | 50.55 V | 9 | 454.9 V |
Pull β from the actual datasheet for the module you are installing. In this case the spread does not change the string length, so the sizing is robust to that variation.
Related
- The 690.7 string-sizing reference
- 690.8 conductor & OCPD sizing
- Cold-temperature Voc correction, explained
Computed with OneLine Studio's NEC engine — the same deterministic code tables that size a real permit set, not an estimate. Conductor ampacity from NEC 310.16, correction factors from 310.15(B)(1), fill adjustment from 310.15(C)(1). Always confirm the enforced NEC edition and any local amendments with your AHJ.
FAQ
How many REC Alpha Pure-R modules can I put in one string on a 480 V inverter?
It depends on your site's design minimum temperature. At 0 °F the limit is 9 modules; at −20 °F it is 9. Use the ASHRAE Extreme Annual Mean Minimum for your nearest weather station, not the record low.
What Voc does the REC Alpha Pure-R have?
44.8 V at STC (25 °C). NEC 690.7 requires you to correct that upward for your coldest expected temperature before comparing it to the inverter's maximum input voltage.
Can I just use −40 °F to be safe?
You can, and it is always code-compliant, but it is not free. A blanket −40 °F forces shorter strings than the site needs, which means more parallel strings, more homeruns and more cost — a real penalty in a warm climate where the actual design minimum might be 30 °F.
Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.
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