Solar PV Wire vs USE-2: Conductor Selection for Source Circuits

Definition

PV Wire and USE-2 are both common conductor types for solar PV source circuits. PV Wire is single-conductor, sunlight-resistant, suitable for exposed outdoor installation. USE-2 is direct-burial-rated but not approved for prolonged sun exposure above ground.

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.

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

4 commonly searched questions about PV Wire vs USE-2.

PV Wire vs USE-2?

PV Wire: sunlight-resistant single-conductor, exposed outdoor. USE-2: direct-burial-rated, underground. Both 90°C rated. NEC 690.31 lists permitted uses.

Can USE-2 cable be run exposed on a rooftop instead of PV Wire?

Not for long-term exposed runs. USE-2 is rated for direct burial and wet locations, but its insulation is not certified for prolonged, uncovered sunlight exposure the way PV Wire is. Where a source-circuit conductor will sit exposed on a rooftop or racking run, PV Wire (UL 4703) is the correct listing to specify; USE-2 belongs underground or inside conduit.

Why are PV Wire and USE-2 both rated at 90°C?

The 90°C insulation rating is what lets designers use the conductor's higher ampacity when running temperature and conduit-fill correction factors under NEC 690, rather than derating to a lower-temperature column. Both conductor types share this rating, but the rating only describes thermal performance, not where the cable is permitted to be installed.

Which one should a solar EPC default to for DC source-circuit home runs?

Most EPCs standardize on PV Wire for exposed source-circuit and home-run conductors between modules, combiner boxes, and inverters, since it is purpose-built for sunlight exposure and is the conductor NEC 690.31 most often assumes. USE-2 is reserved for segments that transition underground, such as trench runs between a combiner box and an inverter pad.