Solar PV Wire: Photovoltaic Wire, 600V Options

Solar PV wire is not just "wire" — it is a purpose-built conductor rated for 90°C wet or dry, UV exposure, and 600V or 1,000V DC operation in outdoor environments that would destroy standard building wire in months. If you are building a solar array, the wire selection is as critical as the panels and inverter, and getting it wrong creates fire risks, code violations, and production losses that show up years later. I've replaced three runs of improperly spec'd THHN in rooftop arrays where the insulation had crystallized from UV exposure — two of them were less than four years old. This guide covers PV wire types, ampacity, temperature derating, voltage drop, conduit fill, and NEC 690 compliance so you spec the right conductor the first time. For bulk wire orders, browse our AWG 6 wire and AWG 4 wire collections.

Solar PV Wire: Photovoltaic Wire and 600V Options

The NEC recognizes three common conductors for solar DC circuits, and they are not interchangeable. PV wire (Photovoltaic Wire, RHW-2) and USE-2 (Underground Service Entrance) are both rated for 90°C wet/dry and direct burial or exposure. THHN/THWN-2 is rated for 90°C in conduit but is not UV-resistant — it fails catastrophically when exposed to sunlight. Romex (NM-B) is 60°C rated and explicitly prohibited for PV circuits by NEC 690.31(B).

The cost difference between PV wire and THHN is roughly $0.15–$0.25 per foot for #10 AWG. On a 200-foot residential array, that's $30–$50. The callback cost to replace failed THHN exposed on a roof — labor, lift rental, permit — starts at $800. The math is not close. We've seen three installations where the original installer used THHN in cable tray on a commercial flat roof; all three had insulation cracking within three years.

NEC 690.8(A) requires PV circuit ampacity to be calculated at 125% of Isc (short-circuit current) before any correction factors. Then you apply the temperature correction from 310.16 based on the actual conductor temperature. For rooftop arrays, NEC 310.15(B)(3)(c) adds 17°C to ambient for circuits within 36 inches of the roof deck — so a 90°F (32°C) day becomes 49°C for derating purposes.

The 49°C derating factor is 0.80 for 90°C insulation. That means a #10 AWG PV wire rated at 40A in free air at 30°C drops to 32A on a hot roof. For a string with Isc of 10.2A, 125% gives 12.75A — well under the 32A derated limit, so #10 is comfortable. But if you have three strings in parallel hitting a combiner at 30.6A (3 × 10.2A), you're at 96% of the derated ampacity — I would upsize to #8 AWG for that homerun.

Voltage drop is the most under-calculated parameter in residential solar wiring. A 3% drop on a 400V string is 12V — acceptable. A 3% drop on a 48V battery circuit is 1.44V, which can push the charge controller out of MPPT range or cause the battery to never fully charge. The formula is straightforward: VD = 2 × L × I × R / 1000, where L is one-way length in feet, I is current in amps, and R is conductor resistance in ohms per 1000 feet.

For a ground-mount array 150 feet from the house with a 48V battery system, #10 AWG gives a 4.3% drop at 10A — too high. #8 AWG drops to 2.7%, which passes. The cost difference for 300 feet (150 ft × 2 conductors) is about $230. That $230 buys you 4–6% more production over the life of the system — a 10-year payback on the wire upgrade alone. Last year I spec'd #6 AWG for a 200-foot run to a barn array in Yamhill County; the homeowner reported 8% higher production versus his neighbor's identical array with #10 AWG at 180 feet.

PV wire is manufactured in voltage classes: 600V, 1,000V, and 1,500V. The voltage rating must exceed the maximum system voltage of the array, which is the open-circuit voltage at the lowest expected temperature. NEC 690.7(A) specifies that temperature-corrected Voc be used — and the correction factor is significant: a module with 41.3V Voc at STC can reach 49.5V at -10°F.

For residential systems with 60-cell or 72-cell modules in strings of 10–12, 600V wire technically meets code if the cold-weather Voc stays below 600V. But 1,000V wire is only marginally more expensive and gives headroom for system expansion, module replacement with higher-Voc bifacial panels, and inspector comfort. We spec 1,000V as standard on all residential jobs now — the price delta is roughly 8% and it eliminates any voltage-class questions at inspection.

Once the PV wire leaves the array, it usually transitions to THHN in conduit for the run to the inverter or charge controller. NEC Chapter 9, Table 1 gives the fill limits: 53% for one conductor, 31% for two, and 40% for three or more. But solar has a twist — DC circuits often run two conductors (positive and negative) plus an equipment ground, which is three conductors at 40% fill. Here's the practical table for EMT:

Field rule: never fill conduit past 35% — pulling wire through a packed conduit damages insulation and creates future shorts. For a two-string residential array with #10 THHN (2 positive, 2 negative, 1 ground = 5 conductors), 3/4-inch EMT is the minimum and 1-inch is comfortable. For commercial arrays with multiple homeruns, upsize to 1-1/4 or 1-1/2 inch and use pull boxes every 100 feet.

NEC 690.31(C)(2) requires that PV system DC conductors be identified at all termination points. Standard practice:

• Positive (+): Red or marked with red tape at terminations
• Negative (-): Black or marked with black tape at terminations
• Equipment Ground: Green or bare copper
• System Ground (if used): White or gray (only in grounded systems)

PV wire typically ships in black with a red stripe on the positive conductor — verify polarity before cutting. We label every conductor at both ends with a Brady label: "String A1 POS", "String A1 NEG", etc. An unlabeled combiner box with eight black conductors is a troubleshooting nightmare that costs two hours to sort out. We've walked away from maintenance bids because the prior installer didn't label anything — it's not worth the liability.

THHN is approved for PV circuits only when installed in conduit and protected from UV exposure. It is not rated for direct outdoor use. For exposed rooftop jumpers or module-to-module connections, you must use PV wire (RHW-2) or USE-2. Using THHN outside conduit is a code violation (NEC 690.31) and a fire hazard.

The insulation thickness and dielectric strength. 1,000V wire has thicker insulation (typically 45 mils vs 30 mils for 600V) and is tested to withstand 3,500V AC for five minutes. For residential systems with string voltages under 500V, 600V wire meets code. We recommend 1,000V wire for all new installs because it costs only 5–10% more and provides margin for future expansion or higher-voltage modules.

Use the formula VD = (2 × L × I × R) / 1000, where L is one-way run length in feet, I is operating current in amps, and R is conductor resistance per 1,000 feet. For 10A through 150 feet of #10 AWG: VD = (2 × 150 × 10 × 1.02) / 1000 = 3.06V. On a 400V string, that's 0.8% — fine. On a 48V battery circuit, that's 6.4% — unacceptable. Always check drop at your actual system voltage.

Yes — PV wire and USE-2 are both rated for direct burial. For underground runs, USE-2 is more common because it has a thicker jacket. PV wire can be buried but benefits from sand bedding and warning tape at 12 inches depth per NEC 300.5. For runs under driveways or in rocky soil, use PVC conduit with THHN inside — it's easier to repair than digging up direct-burial cable.

PV wire is rated for exposed use, so conduit is not required for the rooftop jumpers between modules. However, most AHJs require conduit for the transition through the roof deck, for attic or soffit runs, and for any conductor within 10 feet of grade. EMT is standard for exposed runs; PVC Schedule 40 is used underground. Always verify local amendments.

For a typical 5 kW residential array with 12–14 panels in two strings: use #10 AWG PV wire for the string homeruns if under 100 feet. For the combiner-to-inverter run, size based on combined current — two strings at 9.5A Isc each give 19A combined; 125% = 23.75A, so #8 AWG THHN in 3/4-inch EMT is appropriate. For ground-mount arrays over 150 feet from the inverter, upsize to #6 AWG to keep voltage drop under 3%.

Aluminum is permitted by NEC 690.31 for conductors 4/0 AWG and larger. Below that, copper is strongly preferred because aluminum's oxidation properties and thermal expansion make small-terminals unreliable for DC connections. If you use aluminum, you must use connectors rated for AL-CU transition, apply antioxidant compound, and torque to manufacturer specifications. For residential solar, copper is the standard.

Understanding Solar PV Wire

Buying Guide for Solar PV Wire

• Solar Panels — modules from 300W to 700W
• Charge Controllers — MPPT controllers for every voltage
• Inverters — string, micro, and hybrid inverters
• Battery Storage — lithium and lead-acid banks
• Circuit Breakers — DC-rated breakers for combiner boxes
• Residential Solar Starter Kits — complete systems with wire

Wire Gauge: Understanding AWG

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