Solar conductor types — PV Wire, THWN-2, USE-2

Solar systems use several conductor types, each suited to a different part of the run — exposed on the roof, inside conduit, or between the array and the inverter. Picking the right one is about environment and rating, not just ampacity. Here is where PV Wire, THWN-2, and USE-2 each belong.

Why the environment drives the choice

Conductors in a PV system see sunlight, heat, and moisture that ordinary building wire is not built for. The key properties that separate solar conductors are:

• Sunlight (UV) resistance for exposed runs.
• Wet-location rating for outdoor and underground use.
• Temperature rating — most solar conductors are 90°C rated.
• Voltage rating — 600 V, 1000 V, or 2000 V insulation.

PV Wire

PV Wire (also called photovoltaic wire) is a single-conductor cable built for the harshest part of the system: the exposed DC circuits between modules and from the array to the first junction box or transition to conduit.

• Sunlight- and moisture-resistant, suitable for free-air use without conduit.
• Commonly available with 1000 V or 2000 V insulation ratings.
• Thick, durable insulation that resists abrasion on racking.
• Required for ungrounded PV arrays and widely used with module connectors.

This is the wire you see zip-tied to racking between panels.

THWN-2

THWN-2 is a standard building wire used once conductors enter a raceway (conduit). It is not sunlight-resistant on its own, so it lives inside conduit.

• 90°C rated in wet and dry locations (the "-2" denotes 90°C wet).
• Used for DC runs inside conduit and for AC output circuits to the panel.
• Economical and widely stocked.

When a transition fitting takes the exposed PV Wire into conduit at the roof, THWN-2 typically carries the circuit the rest of the way.

USE-2

USE-2 (underground service entrance) is a single-conductor, sunlight-resistant, 90°C wet-rated wire.

• Rated to 600 V — lower than PV Wire.
• Sunlight-resistant and usable in free air or underground.
• Sometimes used for exposed runs, but PV Wire's higher voltage rating and durability ratings make it the preferred choice for module interconnections.

Sizing still applies

Choosing the right insulation type is only half the job — the conductor must also be sized for ampacity with the correct temperature and conduit-fill corrections. That process is walked through in wire ampacity and NEC 310.16. The wiring methods that dictate where conduit is required are covered in NEC 690.31 PV wiring methods.

Match the conductor to its environment: PV Wire for exposed DC, THWN-2 inside conduit, USE-2 where a 600 V sunlight-rated single conductor fits. Confirm every rating against the manufacturer datasheet, and have a licensed electrician or PE review the final design. Browse more in the engineering reference.

Educational reference, reviewed 2026-07. A design aid, not a substitute for a licensed electrician or PE. Confirm the enforced NEC edition and local amendments with your AHJ.