Keyur Rakholiya PV Wire vs USE-2 compares two conductor types used in solar DC circuits. PV Wire, listed to UL 4703, is a sunlight-resistant single conductor built for exposed module and array wiring. USE-2, listed to UL 854, is designed for underground and wet locations. NEC 690.31 sets where each may be used in PV source circuits.
Comparison
| Type | Sunlight | Burial | UL Listing |
|---|---|---|---|
| PV Wire | Yes | Yes (in conduit) | UL 4703 |
| USE-2 | No (limited) | Yes | UL 854 |
| THHN/THWN-2 | No | No (conduit only) | UL 83 |
Key Takeaways
- PV Wire: outdoor exposed; sunlight-resistant.
- USE-2: direct-burial; limited sun exposure.
- Both 90°C rated insulation.
- Choose based on installation environment.
- NEC 690.31 governs permitted uses.
Why the Distinction Matters
The two conductor types get confused because they overlap on paper, both are single-conductor, both carry a 90°C insulation rating, and both show up in the same source-circuit runs on a rooftop or ground-mount array. The difference that actually matters to a designer is exposure.
PV Wire’s UL 4703 listing certifies the insulation against continuous sunlight and weathering, which is exactly the condition a module home run sits in for the life of the system.
USE-2’s UL 854 listing is built around wet and buried environments; it isn’t tested for the same long-term UV exposure, so specifying it for an exposed run trades a code-compliant, sunlight-rated conductor for one that was never meant to see the sun.
Worked Example
Consider a rooftop array where source-circuit conductors run exposed along the racking from the module strings to a combiner box on the roof, then drop through a wall penetration and continue underground in conduit to the inverter. The exposed racking segment calls for PV Wire, since it will sit in direct sunlight indefinitely.
Once the conductor enters conduit below grade, USE-2 (or another wet-location, direct-burial-rated conductor) becomes the appropriate choice for that buried segment. Mixing them the other way around, USE-2 exposed on the roof, PV Wire buried without conduit protection, is the kind of substitution an AHJ plan reviewer will flag during permit design.
Related Reading
Conductor selection only closes out once it is tied back to wire sizing and conductor ampacity calculations, since the same NEC 690 temperature and derating rules that govern PV Wire and USE-2 also set the minimum conductor gauge for a given circuit.
Where several source-circuit conductors share a raceway, conduit fill limits come into play alongside the insulation choice. Designers documenting these conductor call-outs on a permit set often do so directly on the single-line diagram, our guide to solar SLD software and NEC 2023 auto-generation walks through how that documentation gets built out for AHJ submittal.
Frequently Asked Questions
3 commonly searched questions about PV Wire vs USE-2.
Can USE-2 be used instead of PV Wire on a rooftop?
Why are PV Wire and USE-2 both rated 90°C?
Which conductor should a solar EPC use for DC home runs?
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