PV Wire vs USE-2 vs THHN: Which Solar Cable Goes Where
Every solar build has several kinds of wire in it, and they aren't interchangeable. The cable on the back of a panel lives in direct sun and rain for decades. The wire inside a conduit run to the house is protected. The cable between battery and inverter carries huge currents in a short, flexible run. Picking the right type for each run matters as much as picking the right gauge.
Three questions that decide the wire type
Before gauge, ask three things about each run:
- Is it exposed to sunlight? UV breaks down many insulations. Exposed runs need insulation rated sunlight-resistant.
- Is it wet, or could it be? Outdoors, inside conduit that collects condensation, or underground all count as wet locations.
- What's the maximum voltage? PV strings can reach several hundred volts DC, and the insulation must be rated above the cold-corrected open-circuit voltage.
Then ask about flexibility and how it will be terminated, which matters most for battery cables.
The cable types side by side
| Type | Typical use in a solar build | Sunlight resistant | Notes |
|---|---|---|---|
| PV wire / PV cable | Panel leads, array wiring, home runs to a combiner or controller | Yes | Designed for exposed PV use; commonly available with higher DC voltage ratings; thick, tough insulation |
| USE-2 | Underground or exposed service-type runs | Yes | Often dual-listed with RHW-2; check local code for use on ungrounded PV arrays |
| THHN / THWN-2 | Inside conduit, between combiner, disconnect and controller | Not as typically sold | Common, inexpensive building wire; needs a raceway outdoors |
| Welding cable | Battery-to-inverter, bus bars | Varies | Very flexible fine strands; often not listed as building wire; voltage rating may be low |
| Listed battery / flexible cable | Battery-to-inverter and battery interconnects | Varies | Flexible like welding cable, with a listing appropriate for the use |
PV wire
PV wire is the cable on the back of nearly every panel. It's designed for exactly the hard conditions of an exposed array: long-term UV exposure, wide temperature swings, rain and abrasion against racking. It's commonly sold in single-conductor rolls, in red and black, with insulation rated for high DC voltage. For runs from panels to a combiner box or into a conduit entry, PV wire is the straightforward choice. Use proper MC4-style connectors for panel connections; our MC4 connector guide explains matching.
USE-2
USE-2 (underground service entrance) cable is also sunlight-resistant and wet-rated, and it's sometimes cheaper than PV wire. It shows up in older installations and in ground-mount runs. Code in many US jurisdictions requires PV wire specifically for exposed conductors in ungrounded PV arrays, which covers many modern installations, so USE-2 isn't always a drop-in replacement. Check with your local inspector before substituting.
THHN/THWN-2
This is the standard building wire found inside conduit throughout houses and commercial buildings. It's thin, slippery (easy to pull), inexpensive and widely available. Once the array wiring reaches a junction box or combiner, many installers transition to THWN-2 in conduit for the run to the controller or inverter. The "W" and "-2" indicate wet-location and higher temperature ratings. It's not designed for open-air runs in direct sunlight, so keep it in a raceway outdoors.
Welding cable and flexible battery cable
Fine-stranded cable is easy to bend in tight battery boxes and holds up to vibration. That's why it's popular between batteries, bus bars and inverters. Two cautions. First, many welding cables aren't listed as building wire, which can matter for inspected installations. Second, fine strands need lugs and terminals designed for them. Set-screw terminals can cut fine strands, and some lugs are specified only for standard stranding. Our battery-to-inverter cable guide covers sizing.
Copper, aluminum and "CCA"
Use copper for solar DC wiring in small and mid-size builds. Aluminum conductors are used in large installations with appropriately rated terminals, but they require larger sizes and careful termination. Avoid copper-clad aluminum (CCA) cable for battery and PV circuits. It's sold cheaply as "power cable" in car audio and generic listings, but it has higher resistance than pure copper and behaves differently at terminals. Listings that say "OFC" or "pure copper" are what you want, and it's worth scraping a strand end to check the color all the way through.
Tinned vs bare copper
Tinned copper strands have a thin coating that resists corrosion, especially in humid or coastal locations. It's common in marine cable and often a good pick for battery cables in sheds and outbuildings. Bare copper is fine in dry indoor locations and inside conduit.
Gauge and voltage drop still apply
Choosing the type is the first decision; gauge is the second. Gauge is set by ampacity (how much current the wire can carry safely in its conditions) and voltage drop (how much voltage is lost over the run). Long runs at low voltage need much heavier wire. Our voltage drop guide for long runs shows the method. Ampacity depends on insulation temperature rating, ambient temperature, the number of conductors bundled together and whether they're in conduit, so use the tables your local code references rather than a generic chart.
Reading the print on the jacket
Listed wire carries its identity printed along the insulation every so often: the type designations (for example "PV WIRE" or "USE-2 / RHW-2"), the gauge, the voltage rating, the temperature rating, and often "SUNLIGHT RESISTANT" and the listing mark. If a listing online shows a cable with no visible jacket printing, or the printing in the photos doesn't match the title, that's a reason to look elsewhere. When the cable arrives, read the jacket before you cut it. Inspectors do the same thing, and a run of the wrong type is expensive to pull out once it's in conduit.
Also note the temperature rating. Wire on or near a sunny roof runs much hotter than the air around it, and higher-temperature insulation keeps more of its ampacity in those conditions.
Routing and protection tips
- Clip array wiring to racking with UV-resistant stainless clips or UV-rated ties so it doesn't rub or sag onto the roof.
- Use a proper cable entry gland or conduit fitting where wires enter a box or a building.
- Leave drip loops so water runs off before reaching an entry point.
- Protect cables from sharp edges with grommets or edge guards.
- Label positive and negative at both ends, especially for long runs in identical black cable.
- Bury only cable rated for direct burial, or run it in conduit at the depth your local code requires.
When it connects to the house. Wire types for the AC side of an inverter, transfer equipment, a subpanel or any grid-tie connection are governed by electrical code and usually by permit. Have a licensed electrician design and install those runs.
FAQ
Can I use THHN wire for solar panels?
Not as exposed array wiring in sunlight. THHN/THWN-2 is fine inside conduit, which is how it's commonly used between a combiner or junction box and the controller.
What's the difference between PV wire and USE-2?
Both are sunlight-resistant and wet-rated, but PV wire is designed specifically for PV arrays and is commonly available with higher voltage ratings. Local code may require PV wire for exposed conductors on ungrounded arrays.
Is welding cable okay for battery cables?
It's widely used because it's flexible, but check its voltage rating and listing, and use lugs and terminals designed for fine-stranded cable.
Is CCA cable okay for solar?
It's best avoided. Copper-clad aluminum has higher resistance than pure copper and can cause heating at terminals. Use pure copper for PV and battery wiring.
The bottom line
For most DIY builds the pattern is simple: PV wire for anything exposed on the array, THWN-2 in conduit once the run reaches a protected path, and flexible, pure-copper battery cable with proper lugs at the battery. Confirm each run's sun, water and voltage conditions, size the gauge for both ampacity and voltage drop, and bring in a licensed electrician for the AC and grid side.