Sometimes, the riskiest parts of a solar installation are the ones that look almost identical. Put a spool of PV Wire next to a spool of USE-2 cable, and you’d be hard-pressed to spot the difference just by looking at them. In fact, they have subtle differences in insulating chemical properties, flame retardancy, and critical voltage threshold.
Trying to decide between PV Wire and USE-2 for your installation? Here’s what you need to know to make the right call.
For a broader look at the entire ecosystem of solar wiring before we dive deep, start with our guide: All About Solar Cable.
What Standards Define PV Wire and USE-2?
Before we argue about which is better, we need to define what they are according to UL (Underwriters Laboratories).
USE-2 stands for Underground Service Entrance – 90°C Wet/Dry.
- Standard: UL 854.
- History: This is a legacy cable. It was originally designed to bring power from the utility transformer to a house’s meter. Because it was rated for direct burial and harsh outdoor conditions, the solar industry adopted it early on as the default “solar cable.”
- Key Trait: It is tough, but it is “general purpose” tough.
PV Wire stands for Photovoltaic Wire.
- Standard: UL 4703.
- History: This standard was born out of necessity. As the 2008 National Electrical Code (NEC) introduced stricter requirements for solar (specifically Article 690), the industry needed a cable engineered specifically for the unique stresses of a rooftop array—higher voltages, extreme UV, and vertical flame resistance.
- Key Trait: It is a specialized, high-performance asset designed strictly for solar applications.
If you are exploring the wider range of options beyond just these two, check our guide on what are the types of solar cables to see how they fit into the market.
PV Wire vs USE-2: Design and Performance
While they both conduct electricity, the physical construction differs in ways that matter to an engineer.
- Insulation Thickness
This is the most visible difference. PV Wire has thicker insulation.
- USE-2: Typically has about 45-mil insulation (for 10 AWG).
- PV Wire: Can have up to 60-mil or even 75-mil insulation for the same size.
Thicker insulation means better resistance to mechanical abuse. On a roof, cables rub against shingles, sharp aluminum rails, and concrete tiles. That extra “meat” on the PV Wire is your insurance policy against ground faults.
Here is a “rookie mistake” I see constantly. Because PV Wire has a larger Outer Diameter (OD) due to that thick insulation, you fit fewer wires in a pipe. If you design a system based on standard THHN or USE-2 conduit fill charts but install PV Wire, you may violate NEC Chapter 9 fill limits. Always recalculate your conduit fill when switching to PV Wire to avoid jamming the pipe or failing inspection.
- Voltage Ratings
- USE-2: Almost always rated for 600V.
- PV Wire: Rated for 600V, 1000V, or 2000V.
Modern residential systems and almost all commercial systems now operate at or near 1000V DC. Utility farms are pushing 1500V. USE-2 is functionally obsolete for these high-voltage systems. If your string voltage calculation exceeds 600V, you must use PV Wire.
- Flame Resistance(The Vertical Burn Test)
This is where safety inspectors get strict.
- USE-2: Usually passes a horizontal burn test, but it is not required to pass the strict vertical flame tests (VW-1).
- PV Wire: Is required to pass the VW-1 (Vertical Wire) flame test if it is installed in a tray or conduit.
Fire moves differently vertically. If you are running cables up the side of a building or inside a wall cavity, PV Wire offers superior fire propagation resistance.
PV Wire vs USE-2: Grounded vs Ungrounded System
Here is the technical nuance that catches most installers off guard. It comes down to your inverter.
Grounded Inverters: Older systems had the negative conductor bonded to ground. In these setups (and under 600V), USE-2 was acceptable.
Transformerless (Ungrounded) Inverters: Today, most modern string inverters (like those from SMA, Fronius, SolarEdge) are Transformerless / Ungrounded. In an ungrounded system, both the positive and negative wires are “hot” relative to the ground. They float. The code states that for ungrounded PV power systems, the wiring used must be listed as PV Wire. If you are using a modern transformerless inverter, you cannot use USE-2. You are legally required to use UL 4703 PV Wire.
For a look at how European standards have similarly evolved, you might find our comparison of PV1-F vs H1Z2Z2-K interesting.
PV Wire vs USE-2: Temperature and Environmental Endurance
USE-2 is made for the stable, cool environment of a buried trench—90°C in wet or dry conditions, and generally tested for 300 hours of weathering.
PV Wire is designed for the extreme reality of a solar array. While its wet rating remains at 90°C, high-quality PV Wire is often rated for significantly higher dry temperatures—typically 105°C, 125°C, or even 150°C. PV Wire undergoes a far more brutal UV aging test (720 hours of carbon-arc or xenon-arc exposure).
For insight into why standard building wire (like THHN) fails in these conditions, see our deep dive: PV Wire vs. THHN Wire.
PV Wire vs USE-2: Flexibility and Installation
If you’re the one installing the cable, flexibility matters.
- USE-2:Thinner insulation makes it quite flexible, but the coarser stranding can make it prone to kinking.
- PV Wire: Uses finer copper stranding (often Class K or similar). Even though the insulation is thicker, the stranded core makes it incredibly pliable.
Pro Tips: Because PV Wire has a larger Outer Diameter (OD) due to the thick insulation, you must ensure your MC4 connectors are sized correctly. Trying to shove a thick PV Wire into a gland seal designed for thin USE-2 will result in a failed seal. Water will get in. Always check the OD range of your connectors.
If you are dealing with calculating the right gauge for these connectors, review our guide on solar cable size to ensure you aren’t oversizing the copper beyond what the connector can handle.
Practical Scenarios: Which Cable Makes Sense for Your Project?
Use USE-2 If:
- You’re working on an older, grounded (under 600V) legacy system.
- You’re running an underground lateral line from a combiner box to a meter (outside the PV source circuit).
- You have a specific, code-compliant scenario where PV Wire is unnecessary and you need to save on cost.
Use PV Wire If:
- You have a transformerless (ungrounded) inverter.
- Your system voltage is above 600V.
- Your cable will be exposed to direct sunlight on a roof.
- You need VW-1 flame compliance for tray or vertical installations.
Conclusion
The move from USE-2 to PV Wire marks the solar industry’s shift from adapting general-purpose materials to using solutions engineered for high-voltage, high-exposure, modern PV systems. USE-2 still has its place in certain underground or legacy applications, but PV Wire is the clear choice for new rooftop arrays. It provides the voltage headroom, mechanical protection, and code compliance that today’s systems demand.
Sourcing Note: There is a difference between generic “solar wire” and quality PV Wire. When sourcing solar photovoltaic PV wire, prioritize manufacturers who provide full traceability and UL test reports. For a list of vetted suppliers, refer to: Leading 7 Solar Cable Manufacturers Worldwide.
