PV Wire vs. USE-2 Cable: The Critical NEC Compliance Guide for US Solar Projects
- Vicky
- May 25
- 10 min read
If you’re designing or installing a US photovoltaic system and comparing PV wire vs USE-2 cable, the right choice determines both safety and inspection success. The key differences aren’t just marketing—they live in the listings (UL 4703 vs UL 44), voltage ratings (600/1000/2000 V), and NEC Article 690 wiring method rules that change depending on whether conductors are within the array, on or in buildings, or underground. This guide distills the code into practical decisions so you can specify correctly, pass AHJ reviews, and avoid rework.
Assumptions for clarity:
We focus on copper conductors (stranded copper) commonly used for solar DC circuits.
Code references align with NFPA 70 (NEC) 2017/2020/2023 structures. Always confirm the edition adopted by your Authority Having Jurisdiction (AHJ).
PV Wire refers to UL 4703 listed “Photovoltaic Wire;” USE-2 refers to UL 44 “Underground Service Entrance, Type USE-2.”
At‑a‑Glance: PV Wire vs USE‑2 Cable (Fast Answer)
For exposed module-to-module wiring within the array, especially on rooftops or for ungrounded/functional grounded systems at 1000–1500 Vdc: use PV Wire.
For ground‑mount arrays in free air (within the array) or direct buried home runs at ≤600 Vdc where permitted by the AHJ: USE‑2 can be acceptable.
On or in buildings from the array to disconnect/inverter: run conductors in raceway or metallic cable assemblies; exposed single conductors are restricted. Within the rooftop array boundaries, PV Wire is typically the only exposed single‑conductor allowed.
When in doubt (voltage >600 V, ungrounded arrays, rooftop installs, AHJ scrutiny): specify UL 4703 PV Wire or dual‑listed PV Wire with additional ratings (e.g., RHH/RHW‑2).
What Each Cable Really Is
PV Wire (UL 4703)
Purpose-built for photovoltaic source/output circuits.
Single conductor, sunlight resistant, 90°C wet rating, typically XLPO (cross-linked polyolefin) insulation with thicker dielectric and robust jacket.
Common voltage ratings: 1000 Vdc and 2000 Vdc (suitable for 1000 V and 1500 V array designs).
Often carries additional markings (e.g., RHH/RHW‑2) improving versatility in raceways and wet locations.
USE‑2 (UL 44)
Underground Service Entrance cable (thermoset insulation), 90°C wet, sunlight resistant, typically 600 V rated.
Designed for exterior and direct burial; not for interior building wiring above grade (except termination allowances).
Frequently XLPE-insulated, mechanically tough, but lacks the higher PV-specific voltage ratings and some flame performance attributes of PV Wire.
Standards and Code Articles That Govern Selection
Core Entities and Standards
NEC (NFPA 70) Article 690: Solar Photovoltaic (PV) Systems
UL 4703: PV Wire listing and construction
UL 44: Thermoset-insulated wires and cables (includes Type USE‑2)
Article 310 (conductors), Article 338 (Service-Entrance cable), Article 300 (wiring methods), Article 110.14 (terminals)
Why It Matters
The NEC treats PV DC circuits differently depending on location: within the array, on or in buildings, in raceways, or direct burial.
Voltage class (600 V vs 1000–1500 Vdc) and system grounding affect whether PV Wire is required or whether USE‑2 is acceptable.
Where Each Is Allowed by NEC
Note: Always verify your AHJ’s adopted NEC edition and any local amendments.
In the PV Array (module-to-module, string leads)
Ground‑mount arrays (not on or in buildings): Single‑conductor PV Wire is permitted in free air. Many AHJs also accept USE‑2 here for ≤600 Vdc systems, provided it’s sunlight resistant and routed/secured per 690.31.
Rooftop arrays (on buildings): Within the array boundary, single‑conductor PV Wire is the typical allowance for exposed conductors. USE‑2 is commonly not accepted on rooftops unless specifically permitted by the AHJ and code edition in force.
On or In Buildings (beyond the array boundary)
Conductors must be in a wiring method such as metal raceway or listed cable assembly with metallic sheath (e.g., MC‑PV, MC‑Cable) per 690.31(C). Exposed single conductors are restricted here.
PV Wire can be pulled in raceway; USE‑2 may be pulled in raceway where permitted, but be mindful of 600 V rating and interior use limitations of Type USE‑2.
Direct Burial (ground runs)
USE‑2: common and cost‑effective for 600 Vdc direct burial (check site voltage and derating).
PV Wire: many products are also listed for direct burial; preferred for 1000–1500 Vdc arrays or where extra flame/UV performance is desired.
Ungrounded or Functional Grounded PV Systems (690.41, 690.35)
PV Wire is the safe default. For ungrounded arrays and higher DC voltages, both conductors must carry equal insulation robustness against ground—a design assumption baked into UL 4703 PV wire.
Compliance and Capability: Side‑by‑Side Comparison
Attribute | PV Wire (UL 4703) | USE‑2 (UL 44) | Compliance/Use Notes |
Typical Voltage Rating | 1000 Vdc or 2000 Vdc | 600 V | USE‑2 unsuitable for 1000–1500 V arrays |
Sunlight Resistance | Yes (enhanced UV/ozone) | Yes | Both suitable outdoors |
Wet Location | 90°C wet | 90°C wet | Both wet‑rated |
Flame Performance | Often VW‑1 or better | Varies; not always VW‑1 | PV Wire typically has stronger flame ratings |
Direct Burial | Often listed | Listed | Verify product datasheet |
Within Rooftop Array (exposed) | Allowed | Often not allowed by AHJ | PV Wire is common requirement on rooftops |
On or In Buildings (beyond array) | In raceway or metallic cable | In raceway with limitations | Exposed single conductors restricted |
Ungrounded/Functional Grounded Systems | Preferred/Required | Generally discouraged | PV Wire listing addresses equal insulation to ground |
Typical Insulation | XLPO, dual/thick wall | XLPE thermoset | PV Wire insulation is purpose‑engineered for PV |
Common Markings | PV Wire, RHH/RHW‑2 | USE‑2, sometimes RHW‑2 | Dual‑listed PV Wire expands options |
Utility-Scale Fit (1000–1500 V) | Excellent | Not suitable | PV Wire wins for 1500 Vdc systems |
The Real Technical Differences That Matter
1) Voltage Rating and System Class
Modern commercial/utility PV fields commonly design at 1000 Vdc or 1500 Vdc to cut copper and losses. That alone rules out 600 V USE‑2 for exposed array conductors.
PV Wire listed at 1000 or 2000 V is engineered for these systems—safer creepage/clearance margins and thicker insulation wall.
2) Insulation/Jacket Chemistry
PV Wire typically uses XLPO with higher UV/ozone/thermal endurance and thicker dielectric—key for 25–30‑year exposure.
USE‑2 uses thermoset insulation (often XLPE). It’s tough and burial‑ready, but not optimized for high‑voltage PV stress and flame performance to the extent PV Wire is.
3) Mechanical and Environmental Durability
PV arrays vibrate, flex, and heat‑cycle daily. PV Wire’s thicker jacket and abrasion resistance resists clamp wear and wind‑induced movement.
For direct burial, both can be listed—but check the product’s actual “direct burial” and “oil/fuel” resistance if crossing industrial sites.
4) Compatibility and Terminations
Both are stranded copper. Use compression lugs rated for the strand class and temperature (90°C) and torque to spec. If you’re using MC4‑style connectors, ensure the crimp ferrule/die is approved for the wire OD and strand class.
PV Wire often coexists better with module whips, which are themselves PV Wire; mixing USE‑2 stubs to PV Wire leads invites mismatched OD/seal issues.
Application Scenarios and Recommendations
Residential Rooftop PV
Within array boundary: PV Wire (UL 4703) for all exposed single‑conductor runs.
From array to inverter/disconnect: metal raceway or MC‑PV cable; pull PV Wire or THWN‑2 conductors as required. Avoid exposed USE‑2 on rooftops.
Voltage class: Typically up to 600–1000 Vdc; PV Wire keeps you future‑ready.
Ground‑Mount Commercial
Within array in free air: PV Wire preferred, USE‑2 acceptable for ≤600 Vdc where AHJ permits.
Array home runs: For longer distances, conduit with THWN‑2 or PV Wire; for buried, direct‑burial PV Wire or USE‑2 (verify listing).
Combiner-to-inverter feeders: Raceway best practice; select conductor size by ampacity and voltage drop.
Utility‑Scale (1000–1500 Vdc)
Exposed array conductors: PV Wire (1000/2000 V listing) is the standard. USE‑2 is not appropriate at these voltages.
Trenching: Direct burial PV Wire listed for 2000 Vdc or MV‑rated PV cable systems for long feeders.
Battery‑Coupled/Hybrid Sites
DC bus and battery interconnects: Use cable types listed for the environment (e.g., RHW‑2, XHHW‑2, or fine‑strand battery cable with UL 1977/UL 1236 component compatibility), but for PV source circuits feeding charge controllers, retain PV Wire rules where exposed.
Step‑by‑Step: Choose the Right Solar DC Cable (NEC‑Aligned)
Identify location and routing
Within array only? On or in a building? Direct burial? Conduit?
Confirm system voltage and topology
600 Vdc, 1000 Vdc, or 1500 Vdc? Grounded, ungrounded, or functional grounded?
Select the wiring method
Within rooftop array: exposed single‑conductor allowed → choose PV Wire.
Beyond array/on buildings: raceway or metallic cable assembly.
Check listings and markings
Need UL 4703 (PV Wire) for exposed array conductors, especially >600 Vdc.
For burial: verify “direct burial” on the datasheet, not just marketing.
Size for ampacity and derating
Use Article 310 tables at the correct temperature column (often 75°C terminations). Apply ambient/conduit‑fill derates.
Verify voltage drop
Keep PV source/output circuits within practical limits (e.g., ≤2–3%), especially on long home runs.
Confirm terminations and connectors
Lugs accept the conductor gauge, insulation OD, and strand class. For MC4‑style, use the specified crimp tools and dies.
Document for the AHJ
Drawings should call out listing (UL 4703 or UL 44), wiring method (raceway/MC), voltage rating, and environment (wet, sunlight, direct burial).
Common Mistakes (and How to Avoid Them)
Using USE‑2 on a 1000–1500 V array
Fix: Specify PV Wire 1000/2000 V for exposed runs.
Exposed single‑conductors on rooftops outside the array boundary
Fix: Transition to metal raceway or MC‑PV at the array boundary per 690.31(C).
Assuming all PV Wire is direct‑burial
Fix: Check the product listing. Many are; some are not.
Mixing connector brands or improper crimp tooling (MC4)
Fix: Match brand/listed intermateable sets and use the manufacturer’s die.
Pulling USE‑2 inside buildings above grade
Fix: USE‑2 is not a general‑purpose interior wiring method; use THWN‑2 in raceway or listed MC cable.
Forgetting ambient/conduit derates
Fix: Apply temperature and bundling factors; rooftop ambients can exceed 50°C.
Case Example: Rooftop vs Ground‑Mount (What Passes Inspection)
9.6 kW residential rooftop (1000 Vdc design):
Within array: PV Wire 1000 V, UL 4703, sunlight resistant, secured to racking with UV‑rated clips, drip loops at module junction boxes.
Transition point: NEMA 4X junction box at array boundary; conductors enter EMT to inverter disconnect.
Result: Pass. Single‑conductor exposure confined to array; raceway used on building.
1 MW ground‑mount (1000 Vdc strings, long home runs):
Within array: PV Wire 2000 V, UL 4703.
Combiner outputs: Direct burial PV Wire 2000 V in trench with sand bed; warning tape at 12 in. above; or THWN‑2 in PVC Schedule 40.
Result: Pass. USE‑2 could have been considered for ≤600 Vdc; here 1000 Vdc dictates PV Wire.
Buying Checklist: What to Specify on Your PO
Cable type and listing: “UL 4703 PV Wire, 1000/2000 V” or “UL 44 Type USE‑2 (600 V), direct burial”
Insulation/jacket: XLPO/XLPE, 90°C wet, sunlight resistant, color and print legend
Environment: wet location, direct burial (if required), flame rating (e.g., VW‑1)
Conductor: stranded copper, AWG size, tinned copper if needed for corrosion
Packaging: reel lengths, pulling eyes, print footage
Documentation: UL file number, test report, spec sheet, RoHS/REACH if needed
Installation accessories: UV‑rated clips, MC4 connectors matched to wire OD, listed junction boxes, metallic raceway, bonding jumpers
Extended Comparison: PV Wire vs USE‑2 Cable (Capabilities, Use Cases)
Category | PV Wire (UL 4703) | USE‑2 (UL 44) |
Best fit | Rooftop arrays, ungrounded PV, 1000–1500 V, exposed array wiring | Ground‑mount arrays ≤600 V, direct‑burial home runs |
Conduit use | Excellent; often dual‑rated (RHH/RHW‑2) | Permitted; watch interior above‑grade limitations |
Rooftop exposure | Preferred/required in many jurisdictions | Often not accepted by AHJs for rooftop exposure |
Utility‑scale | Standard | Not suitable (voltage limit) |
Cost | Higher per foot | Lower per foot |
Durability | Superior UV/ozone/flame | Strong abrasion/burial, less PV‑specific |
Pro Tips for Rooftop Solar Wiring (Installer Notes)
Use UV‑rated stainless clips and maintain gentle bend radii—avoid tight bends at junction box strain reliefs.
Keep drip loops to prevent water tracking into connectors.
Segregate equipment grounding conductors (EGCs) and bond racking per 690.43; PV Wire/USE‑2 are not EGCs.
Label raceways and junction boxes per 690.31(G) with permanent, weather‑resistant markers.
For rapid shutdown (690.12), ensure conductor routing and device locations meet the “within the array” definitions in your adopted code cycle.
Frequently Asked Questions
What is the difference between PV wire and USE‑2 cable?
PV Wire is UL 4703 listed for photovoltaic circuits, typically 1000–2000 Vdc, with thicker XLPO insulation, superior UV/flame performance, and explicit allowance for exposed array wiring (especially on rooftops). USE‑2 is UL 44, 600 V, designed for underground/direct burial and exterior use but not optimized for higher PV voltages or rooftop exposure under many AHJs.
Is USE‑2 cable allowed for solar installations?
Yes, in specific contexts. Many jurisdictions allow USE‑2 within ground‑mount arrays and for 600 Vdc direct burial runs. On rooftops (on or in buildings), exposed single‑conductor allowances usually favor PV Wire within the array; beyond the array, use raceway or metallic cable assemblies.
Is PV wire required by NEC?
For exposed single‑conductor wiring within rooftop arrays and for ungrounded/functional grounded higher‑voltage DC systems, PV Wire is the de facto requirement in many NEC adoptions. The NEC permits single‑conductor PV cable within the array boundaries; elsewhere on or in buildings you must use raceway or metallic cable systems.
When should I use PV wire instead of USE‑2?
Any array above 600 Vdc (1000–1500 Vdc)
Ungrounded or functional grounded PV systems
Rooftop arrays where exposed single conductors are used within the array
When AHJ/utility specs call out UL 4703 explicitly
Can USE‑2 cable be used in photovoltaic systems?
Yes—for ≤600 Vdc applications and where routing and location comply with NEC and AHJ interpretation (commonly ground‑mount arrays and direct burial). It’s generally not used for rooftop exposed array wiring.
Which cable is better for rooftop solar in the US?
PV Wire. It’s purpose‑built for PV array exposure, higher voltages, and flame/UV performance, and it aligns cleanly with Article 690 array wiring allowances.
UL 4703 vs USE‑2 cable—what’s the practical voltage limit?
UL 4703 PV Wire is commonly 1000 or 2000 Vdc rated (suitable for 1000–1500 Vdc systems). USE‑2 is 600 V and therefore not appropriate for 1000–1500 Vdc arrays.
What solar cable meets NEC requirements on or in buildings?
Use metal raceways with conductors (e.g., THWN‑2 or PV Wire) or metal‑clad PV cable assemblies. Exposed single-conductor cable is generally restricted to within the array boundary.
Can I run PV Wire or USE‑2 inside a building?
PV Wire with appropriate dual ratings can be installed in raceway inside buildings. USE‑2 is not a general interior wiring method; it’s intended for exterior and underground applications with limited interior termination allowances—confirm with your AHJ.
Which cable is more durable outdoors?
Both are sunlight resistant. PV Wire typically offers thicker insulation, enhanced UV/ozone/flame performance tailored for decades of exposed PV service.
Conclusion: The Safe, Inspectable Choice in PV Wire vs USE‑2 Cable
When you balance voltage class, location, and NEC wiring methods, the pattern is clear: for exposed array wiring (especially on rooftops) and for 1000–1500 Vdc systems, FRCABLE PV Wire is the code‑clean, future‑proof option. USE‑2 has a role—chiefly in ≤600 Vdc ground‑mount arrays and direct burial runs where permitted—but it does not replace PV Wire for modern ungrounded arrays or rooftop installations. If you need a one‑line rule: when stakes include code compliance, higher voltage, and long service life, choose FRCABLE PV Wire—and put everything else in raceway.
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