PV wire and USE-2 look identical on a reel: black or red single-conductor copper, cross-linked polyethylene insulation, sunlight-resistant, direct-burial rated. Installers have used both in solar arrays for two decades, and plenty of spec sheets list one where the drawings say the other. They are not the same product. UL evaluates them under different standards, they carry different voltage ratings, and the jacket print tells you which one you are holding. This guide covers what separates them, how to read the print legend and the spec sheet, and which one to put on the purchase order.
The Short Version
| PV Wire | USE-2 | |
|---|---|---|
| UL standard | UL 4703, Photovoltaic Wire | UL 854, Service-Entrance Cables |
| Voltage ratings available | 600 V, 1000 V, 2000 V | 600 V (some constructions dual-rated 600/1000 V) |
| Temperature rating | 90°C wet, 105°C dry (higher dry ratings on some constructions) | 90°C wet or dry |
| Insulation wall | Heavier for a given size; often a two-layer or thicker single-layer XLPE | Standard RHW-2 wall thickness |
| Low-temperature test | Cold bend and cold impact at −40°C | Not required by UL 854 |
| Flame test | Required (VW-1 or equivalent per UL 4703) | Not required for USE-2 alone; RHH/RHW-2 dual-rated product passes the UL 44 flame test |
| Sunlight resistance | Required, extended exposure | Required |
| Direct burial | Yes | Yes |
| Original purpose | Exposed module interconnect wiring in PV arrays | Underground service entrance to a building |
| NEC article | 690 (PV systems) | 338 (service-entrance cable) and 690 |
Why UL Treats Them Differently
USE-2 came first. It is an underground service-entrance conductor, designed to run from the utility's transformer to a building's service equipment, buried or in conduit, in a wet environment. UL 854 tests it for exactly that: moisture, sunlight, and heat. It does not have to pass a flame test, which is why the NEC does not allow bare USE-2 to be run inside a building beyond the point where it terminates in service equipment. Most USE-2 sold today is triple-rated RHH/RHW-2/USE-2, meaning the same conductor also passed the UL 44 tests for thermoset building wire, including the flame test, and can be used indoors as RHW-2.
PV wire was created in the 2000s because module interconnect wiring lives a harder life than a service entrance. It hangs in free air under the array where ultraviolet exposure is continuous, it sees the full daily thermal cycle on a rooftop, it flexes in wind, it is installed in winter, and increasingly it runs at 1000 or 1500 volts DC. UL 4703 adds cold bend and cold impact at −40°C, a flame test, extended sunlight-resistance exposure, and higher voltage ratings, and it requires a heavier insulation wall for a given conductor size.
The practical result: PV wire meets or exceeds every USE-2 requirement, but USE-2 does not meet the PV wire requirements. A single conductor can carry both listings, and many do. The jacket print is the only way to know.
Reading the Print Legend
Everything you need is printed on the insulation at regular intervals. Here is a representative 2 kV PV wire legend broken into its parts.
Three fields deserve a second look on every reel:
- Voltage. A 1500 V DC utility-scale system needs 2000 V PV wire. Residential and small commercial string systems run at or under 600 V DC and either product's rating covers them. A 1000 V system needs a 1000 V or 2000 V rating; standard 600 V USE-2 does not qualify.
- Type. "USE-2" alone is service-entrance cable. "PV WIRE" is photovoltaic wire. Both together means it is listed to both standards and is acceptable wherever either is called for.
- Dry temperature. The 105°C dry rating on PV wire is what lets it survive in a hot combiner box or against a dark rooftop; USE-2 is 90°C regardless.
Where Each One Is Permitted
NEC 690.31(C)(1) permits single-conductor cable in exposed outdoor locations within a PV array in two forms: cable listed and identified as PV wire, and Type USE-2. Before 2017, arrays on transformerless (ungrounded) inverters were required to use PV wire; the 2017 edition replaced that scheme with the "functionally grounded" concept and dropped the requirement, so under current editions both types are permitted for exposed module-to-module wiring provided the voltage rating covers the circuit. Confirm the edition your jurisdiction has adopted. In practice, three things still push projects toward PV wire:
- System voltage above 600 V DC. Nearly all commercial and utility-scale work, and a growing share of residential string inverters, run at 1000 or 1500 V. Only PV wire is commonly available at 2000 V.
- Module leads and connectors. Module manufacturers ship PV wire pigtails, and the MC4-style connectors are listed with PV wire of a specific size and insulation diameter. Mixing in USE-2 with a different outside diameter can void the connector listing.
- Specifications. Most engineering specifications for exposed array wiring simply say "PV wire, UL 4703," and a USE-2 substitution is a deviation that has to be approved.
USE-2 (as RHH/RHW-2/USE-2) remains the right product for the parts of the system it was designed for: DC feeders in conduit from the combiner to the inverter, AC output conductors, and the underground run to the point of interconnection. It costs less per foot than 2 kV PV wire in the same size, and inside a raceway the extra sunlight and cold-weather margin of PV wire buys nothing.
Reading the Spec Sheet
The print legend confirms what is on the reel; the manufacturer's specification sheet is what you compare to the engineer's requirements before you order. Check these items in this order.
| Spec sheet field | What to confirm |
|---|---|
| Listing | UL 4703 for PV wire; UL 854 for USE-2; UL 44 for RHH/RHW-2. A dual- or triple-listed product shows all of them. CSA marking if the project is in Canada. |
| Voltage rating | 600, 1000, or 2000 V. Match or exceed the maximum system DC voltage on the one-line diagram. |
| Conductor | Copper or aluminum; bare or tinned; stranding class. Class B (7 or 19 strand) is standard for fixed wiring. Finer Class C, G, or K stranding is specified where the wire is flexed or where a particular connector requires it. Tinned copper is worth specifying in coastal and high-humidity sites. |
| Insulation | Material (XLPE is standard; some PV wire uses EPR or a two-layer XLPE) and nominal thickness. Compare the finished outside diameter to the connector manufacturer's accepted range. |
| Temperature | Wet rating, dry rating, and minimum installation temperature. Rooftop work in the northern half of the country benefits from the −40°C cold-bend rating. |
| Environmental | Sunlight resistant, direct burial, and (for PV wire) the flame rating. Oil resistance if the spec mentions it. |
| Colors | Black and red are standard for DC polarity identification. White or gray for a grounded conductor and green for equipment grounding are available by order. |
| Put-up | Standard reel lengths (500 ft, 1000 ft, 2500 ft) and whether cut lengths are available. Long collection-circuit pulls may need a specific continuous length. |
Copper or Aluminum?
PV wire is available in aluminum as well as copper, and on large collection circuits the weight and cost difference is significant. Aluminum PV wire uses an 8000-series alloy conductor with the same 2 kV XLPE insulation and requires aluminum-rated terminations. We stock aluminum 2 kV PV wire from 6 AWG to 1000 kcmil alongside the copper. The cost comparison is worth doing carefully, and it is the subject of an upcoming guide.
What We Stock
- Copper PV wire, 2000 V, Class B in 14 through 6 AWG, the standard for string wiring.
- Copper PV wire, 2000 V, Class C and Class G in 10 through 6 AWG where a finer stranding is specified.
- Aluminum PV wire, 2000 V in 6 AWG through 1000 kcmil for collection circuits.
- PV-TC tracker cable for single-axis tracker drive and communication runs.
- Copper RHH/RHW-2/USE-2 and aluminum RHH/RHW-2/USE-2 for the feeder and interconnection side, 14 AWG through 1000 kcmil.
Browse the complete Solar & PV category for current sizes and colors.
Request a PV Wire QuoteTell us the system voltage, conductor size, copper or aluminum, colors, and footage. We will confirm the listing on the reel matches your specification before it ships.
Related Reading
- Copper vs Aluminum Building Wire covers the termination and ampacity rules that apply equally to aluminum PV wire.
- THHN vs XHHW-2 explains why cross-linked polyethylene is the insulation of choice for wet and outdoor service.
- EV Charger Wiring Requirements for the AC side of a solar-plus-charging installation.