PV wire vs USE-2: which conductor goes in a solar array

PV wire and USE-2 look alike on the reel: single conductor, black, cross-linked insulation, rated 90 °C wet. For the exposed conductors inside a solar array, PV wire is the safe pick: every recent NEC edition permits it there, it comes rated up to 2 kV, and the 2014 edition accepted nothing else on ungrounded arrays. USE-2 is an underground service and feeder conductor. The NEC lets it into the array only under specific conditions, and never above its 600 V rating.

What each one is

PV wire (also sold as PV cable) is listed to UL 4703. Its insulation is usually thicker than USE-2 of the same size, most of all at the higher voltage ratings. It is sunlight resistant by listing and comes in 600 V, 1 kV and 2 kV ratings. Many PV wires are also marked RHH or RHW-2, which matters when the run continues into a building.

USE-2 is listed to UL 854 and covered by NEC Article 338. It is rated 600 V. It is not automatically sunlight resistant: it has to be marked that way to be used in the sun (310.10). Most USE-2 sold is triple rated USE-2/RHH/RHW-2.

Side by side

What the NEC says, by edition

NEC Article 690 covers PV systems, and 690.31 sets the wiring methods.

In the 2014 edition, 690.35 covered ungrounded PV arrays, and it required exposed single conductors in those arrays to be PV wire or PV cable. USE-2 was not an option. Most arrays built on transformerless inverters fall in this group. Starting with the 2017 edition, the NEC calls them functionally grounded, and 690.35 was removed.

In the 2020 and 2023 editions, 690.31(C)(1) permits exposed single-conductor cable within the array to be either PV wire or cable, or single conductor marked sunlight resistant and Type USE-2 and Type RHW-2. The 2017 wording differs. Read the edition your jurisdiction enforces, and check the inverter and module installation manuals, which often name PV wire outright.

Voltage decides many jobs before the code does. 690.7 limits DC circuits on or in one- and two-family dwellings to 600 V, but other buildings and ground-mount arrays often run higher. Above 600 V, a 600 V USE-2 is out and a 1 kV or 2 kV PV wire is the answer.

Where each fits

PV wire fits every exposed run inside the array: module leads, jumpers between rows, and home runs to the combiner or inverter. The thicker wall and sunlight rating are there for years of UV and for wind rubbing the wire against racking.

USE-2 fits the underground part of a ground-mount system, direct buried or in conduit, and conductors in raceway from the array to the inverter. Where the circuit stays at or under 600 V and the edition allows it, sunlight-resistant USE-2/RHW-2 can serve in the array too.

DC circuits that run inside a building have their own rules in 690.31: metal raceway, metal enclosures or Type MC cable in most cases, from the point of penetration. The subsection letter moved between 2017 and 2020, so look it up in your edition.

Common mistakes

• Using 600 V USE-2 on a string that can exceed 600 V on a cold morning. Calculate maximum voltage with the temperature correction in 690.7.
• Running USE-2 that is not marked sunlight resistant across a roof.
• Pulling PV wire alone into a raceway inside a building when it has no RHW-2 or RHH marking.
• Mixing connectors of different types on the same pair. 690.33 and the connector listings govern what may mate.
• Letting conductors hang loose or lie on the roof. 690.31 sets support intervals for exposed cables.

Shop PV and USE-2 conductors

• 10 AWG PV wire, 2 kV and 12 AWG PV wire, 2 kV
• 10 AWG PV wire, 600 V and 8 AWG PV wire, 2 kV
• 4/0 aluminum PV wire for long home runs
• 10 AWG USE-2/RHH/RHW-2 copper and 6 AWG

Verify with your local code. PV wiring rules changed in every recent NEC edition, and module and inverter makers add their own. Check the edition your jurisdiction has adopted and ask your inspector before you string the array.