What Size PV Wire for a Trina Vertex S+ String at 149 °F With 7 Conductors in Conduit?
Short answer: 10 AWG copper. A Trina Vertex S+ carries 13.9 A Isc, which 690.8(A) turns into a continuous design current of 17.38 A — and at a 149 °F ambient with 7 conductors bundled in the raceway, the derating is severe enough that the intuitive answer is often one gauge too small.
Every gauge, against all three checks
NEC 690.8(B) is not one test. A PV conductor has to satisfy the derated ampacity, the 90 °C-base comparison, and the 110.14(C) terminal rating — and different gauges fail different ones.
| Conductor | 90 °C base ampacity | Temp correction | Fill adjustment | After derating | Terminal (75 °C) | Result |
|---|---|---|---|---|---|---|
| 14 AWG | 25 A | 0.65 | 0.7 | 11.4 A | 20 A | no |
| 12 AWG | 30 A | 0.65 | 0.7 | 13.7 A | 25 A | no |
| 10 AWG | 40 A | 0.65 | 0.7 | 18.2 A | 35 A | passes |
| 8 AWG | 55 A | 0.65 | 0.7 | 25 A | 50 A | passes |
| 6 AWG | 75 A | 0.65 | 0.7 | 34.1 A | 65 A | passes |
| 4 AWG | 95 A | 0.65 | 0.7 | 43.2 A | 85 A | passes |
Where the numbers come from
- Design current — 690.8(A)(1): 13.9 A × 1.25 = 17.38 A.
- OCPD — 690.8(B)(1): 17.38 × 1.25 rounded up per 240.6(A) = 25 A.
- Temperature correction — 149 °F (65 °C) on a 90 °C conductor gives a factor of 0.65.
- Fill adjustment — 7 current-carrying conductors gives 0.7 per 310.15(C)(1).
- Combined — 40 A × 0.65 × 0.7 = 18.2 A, against 17.38 A required. Margin: 0.8 A.
That margin is thin — 0.8 A. If this run picks up another circuit, or the roof runs hotter than 149 °F, it stops passing. Size up.
What actually raises the ambient
A conduit on a hot roof is not at air temperature. Rooftop raceways in direct sun run well above ambient, and the correction factor is applied at the conductor, not at the weather station. A 149 °F design ambient is a realistic rooftop figure in much of the US in July, and it is the number the plan reviewer will expect to see justified.
Related
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
What size wire do I need for a Trina Vertex S+ string at 149 °F?
With 7 current-carrying conductors in the raceway, 10 AWG copper is the smallest gauge that satisfies all three NEC 690.8 checks. Design current is 17.38 A and the conductor delivers 18.2 A after temperature and fill derating.
What OCPD goes with it?
25 A — that is 13.9 A Isc × 1.25 × 1.25 per 690.8(B)(1), rounded up to the next standard size in 240.6(A).
Why does the conductor count matter so much?
NEC 310.15(C)(1) reduces allowable ampacity as current-carrying conductors are bundled, because they cannot shed heat independently. At 7 conductors the adjustment is 0.7 — it multiplies with the 0.65 temperature correction, so the two together can remove more than half the conductor's rated capacity.
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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