Solar Wire and Cable Guide: PV Wire vs USE-2 vs THHN — Sizing, Selection, and NEC Compliance
Reading time: ~15 min read
Selecting the correct wire and cable for solar photovoltaic systems is one of the most critical decisions an installer makes. The harsh outdoor environment, high DC voltages, elevated temperatures on rooftops, and stringent NEC requirements demand conductors that are properly rated, sized, and installed. This comprehensive guide compares the wire types approved for PV systems, explains NEC Article 690.31 requirements, provides temperature derating and conduit fill calculation methods, presents voltage drop tables, and covers connector standards and grounding conductor sizing.
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1. Wire Types for Solar PV Systems
The NEC permits several wire types for use in PV systems, each with specific insulation ratings, temperature capabilities, and application limitations. Understanding the differences is essential for proper selection and code compliance.
1.1 PV Wire
PV Wire (photovoltaic wire) is the most commonly used conductor in modern solar installations. Key characteristics include:
- Standard: Listed to UL 4703, Standard for Photovoltaic Wire.
- Insulation rating: 90°C (194°F) or 105°C (221°F) or 125°C (257°F) wet and dry.
- Voltage rating: 600 V, 1000 V, or 2000 V (select based on system maximum voltage).
- Sunlight resistance: Rated for sunlight resistance and UV exposure (720-hour weather resistance test).
- Direct burial: Permitted for direct burial where listed.
- Flame resistance: Listed for flame resistance.
- Application: Permitted in all locations where RHW-2 is permitted, and as single-conductor exposed cable in PV array dc circuits.
NEC 690.31(C) specifies that single-conductor PV wire or cable shall be listed. The informational note references UL 4703 for PV wire and UL 3003 for distributed generation (DG) cable ([NFPA NEC P04 Second Draft](https://docinfofiles.nfpa.org/files/AboutTheCodes/70/70_A2025_NEC_P04_SD_PCSubmittals.pdf)). PV wire is the preferred choice for exposed array wiring because of its superior UV, moisture, and temperature resistance.
1.2 USE-2 (Underground Service Entrance)
USE-2 wire has historically been used for PV array wiring and remains permitted under specific conditions:
- Insulation rating: 90°C (194°F) wet and dry.
- Voltage rating: 600 V.
- Sunlight resistance: Must be marked "sunlight resistant."
- Application: Permitted as single-conductor exposed cable in PV system DC circuits within the PV array, but must also carry Type RHW-2 marking.
- Limitation: Not permitted inside buildings in exposed locations (unlike PV wire, which can transition into buildings in raceways).
The NEC requires that single-conductor cable in exposed outdoor locations within the PV array shall be either PV wire or cable, or single-conductor cable marked sunlight resistant and Type USE-2 and Type RHW-2. This dual marking requirement ensures the cable passes the 720-hour weather resistance test ([NFPA NEC P04 First Draft](https://docinfofiles.nfpa.org/files/AboutTheCodes/70/70_A2025_NEC_P04_FD_PIResponses.pdf)).
1.3 RHW-2
RHW-2 (moisture and heat-resistant thermoset insulation, 90°C wet or dry) is often combined with USE-2 as a dual-rated cable. PV wire is permitted in all locations where RHW-2 is permitted, making PV wire the more versatile choice. RHW-2 is also used for underground runs and in conduit where USE-2 is insufficient or PV wire is not required.
1.4 THHN / THWN-2
THHN (thermoplastic insulation, high heat, nylon jacket) and THWN-2 (thermoplastic, water-resistant, nylon jacket, 90°C wet/dry) are general-purpose building wires used primarily for AC wiring in PV systems:
- THHN: 90°C dry locations only. The nylon jacket provides mechanical protection. Not suitable for wet locations.
- THWN-2: 90°C wet and dry locations. Suitable for conduit installations exposed to moisture.
- Application: Used for AC interconnection wiring, inverter output circuits, and grounding conductors in conduit. Not permitted for exposed DC array wiring.
- Limitation: Not sunlight resistant unless specifically marked. Not suitable for direct burial.
According to the Copper Development Association's recommended practices for designing and installing copper wiring, THHN is defined as a thermoplastic-insulated, nylon-jacketed conductor used in dry locations up to 90°C, while THWN-2 is suitable for wet or dry locations up to 90°C ([Copper Development Association](https://copper.org/applications/electrical/building/pdf/copper-wire-install-standard.pdf)).
1.5 Wire Type Comparison Table
| Property | PV Wire | USE-2 / RHW-2 | THHN | THWN-2 |
|---|---|---|---|---|
| UL Standard | UL 4703 | UL 44 / UL 854 | UL 83 | UL 83 |
| Max Temperature | 90–125°C | 90°C | 90°C (dry) | 90°C (wet/dry) |
| Voltage Rating | 600/1000/2000 V | 600 V | 600 V | 600 V |
| Sunlight Resistant | Yes (standard) | Yes (must be marked) | No (unless marked) | No (unless marked) |
| Direct Burial | Yes (if listed) | Yes | No | No |
| Flame Resistant | Yes | No | Yes (nylon jacket) | Yes (nylon jacket) |
| Exposed DC Array Wiring | Yes | Yes (with RHW-2 marking) | No | No |
| Inside Buildings (in raceway) | Yes | No (limited) | Yes | Yes |
| AC Wiring (in conduit) | Yes | Yes | Yes (dry only) | Yes |
| Typical Use in PV | DC array & home runs | DC array (legacy) | AC wiring (dry indoor) | AC wiring (wet/dry) |
2. NEC 690.31: Wiring Methods Requirements
NEC Article 690.31 contains the specific wiring method requirements for PV systems. The following subsections are most relevant to wire selection and installation.
2.1 Permitted Wiring Methods (690.31(A))
All wiring methods permitted by Chapter 3 of the NEC (Articles 310 through 392) are permitted for PV system wiring, subject to the additional requirements of Article 690. This includes conduit systems such as EMT, RMC, RNC (Schedule 40 and 80), FMC, and LFMC, as well as cable systems such as MC cable and TC cable.
2.2 Single-Conductor Cables (690.31(C))
Single-conductor cables used in exposed outdoor locations in PV system DC circuits within the PV array must comply with the following:
- PV wire or cable shall be listed.
- Single-conductor cable marked sunlight resistant and Type USE-2 and Type RHW-2 is also permitted.
- Exposed cables sized 8 AWG or smaller must be supported and secured at intervals not exceeding 24 inches (600 mm) using cable ties, straps, or hangers listed for outdoor use.
- PV wire or cable is permitted in all locations where RHW-2 is permitted.
- PV system DC circuits using single-conductor PV wire of all sizes are permitted in cable trays installed in outdoor locations, provided cables are supported at intervals not exceeding 12 inches (300 mm) and secured at intervals not exceeding 4.5 feet (1.4 m).
The 2026 NEC introduces Table 690.31(C)(4), which specifies minimum strand counts for PV wire used on tracking arrays, ensuring sufficient flexibility for the cyclical movement of tracking systems ([NFPA NEC P04 Second Draft](https://docinfofiles.nfpa.org/files/AboutTheCodes/70/70_A2025_NEC_P04_SD_PCSubmittals.pdf)).
| PV Wire AWG | Minimum Number of Strands |
|---|---|
| 18 AWG | 17 |
| 16–10 AWG | 19 |
| 8–4 AWG | 49 |
| 2 AWG | 130 |
| 1 AWG–1000 kcmil | Per manufacturer listing |
2.3 Wiring Inside Buildings (690.31(E))
PV system DC circuits that exceed 30 volts or 8 amperes and are installed inside buildings must be contained in metal raceways, Type MC metal-clad cable complying with NEC 250.118(A), or metal enclosures. This requirement ensures that DC wiring inside buildings is physically protected and does not present a fire hazard. PV wire in conduit (EMT, RMC, or RNC) is the standard method for routing DC circuits from rooftop junction boxes to the inverter or DC disconnect.
3. Temperature Derating
One of the most important and frequently overlooked aspects of PV wire sizing is temperature derating. Solar conductors routinely operate in environments significantly hotter than the 30°C (86°F) baseline used for NEC ampacity tables.
3.1 Rooftop Temperature Adder
NEC 310.15(B)(1)(1) (formerly 310.15(B)(3)(c)) requires a temperature adder for raceways or cables exposed to direct sunlight on or above rooftops where the distance from the roof surface to the bottom of the raceway or cable is less than 0.75 inches (19 mm). The adder is 33°C (60°F). For raceways or cables closer than 0.75 inches to the roof, even higher temperatures may apply.
Research by the Copper Development Association has shown that conduits on rooftops in direct sunlight can be significantly hotter than the surrounding air. The data indicates that a conservative 30°F (17°C) should be added to nominal outdoor temperatures when applying temperature correction factors for conduits in direct sunlight. When the conduit lies directly on a dark roof surface, more than 10°F additional can be added ([Copper Development Association](https://www.copper.org/publications/pub_list/pdf/Ambient_REPRINT.pdf)). In some cases, the temperature inside conduits can exceed 165°F (74°C) even with no current flowing.
The Copper Development Association also offers free guides for estimating temperatures inside conduits at various heights above rooftops for hundreds of U.S. cities, making it easier to apply the required ampacity adjustments ([CDA Press Release](https://copper.org/about/pressreleases/2007/pr2007_10_26.php)).
3.2 Ambient Temperature Correction Factors
NEC Table 310.15(B)(1)(1) provides correction factors for conductors at ambient temperatures other than 30°C (86°F). For PV wire rated at 90°C, the correction factors are:
| Ambient Temperature (°C) | Ambient Temperature (°F) | Correction Factor (90°C wire) |
|---|---|---|
| 21–25 | 70–77 | 1.04 |
| 26–30 | 79–86 | 1.00 |
| 31–35 | 88–95 | 0.96 |
| 36–40 | 97–104 | 0.91 |
| 41–45 | 106–113 | 0.87 |
| 46–50 | 115–122 | 0.82 |
| 51–55 | 124–131 | 0.76 |
| 56–60 | 133–140 | 0.71 |
| 61–70 | 142–158 | 0.58 |
| 71–80 | 160–176 | 0.41 |
The NFPA provides a free downloadable fact sheet including Tables 310.15(B)(1), 310.15(C)(1), and 310.16 extracted from the NEC, along with a flow chart to guide users through the ampacity table selection process ([NFPA Ampacity Charts](https://www.nfpa.org/forms/ampacity-workflow-lead-gen-fact-sheet)).
3.3 Temperature Derating Calculation Example
Consider a 10 AWG PV wire (90°C rated) with a base ampacity of 40 amps (from NEC Table 310.16, 90°C column). The conductor is installed in conduit on a rooftop in Phoenix, Arizona, where the design ambient temperature is 47°C (117°F). With the 33°C rooftop adder, the adjusted ambient temperature is 80°C (176°F). However, since the wire insulation rating is 90°C, the operating temperature is limited to 90°C.
Using the correction factor table for 90°C wire at an ambient of 80°C, the correction factor is approximately 0.41. The corrected ampacity is:
Corrected Ampacity = 40 A × 0.41 = 16.4 A
This dramatic reduction illustrates why temperature derating is critical in rooftop installations. In practice, installers often use higher-rated PV wire (105°C or 125°C) and/or increase conductor size to compensate for rooftop temperature effects. The Copper Development Association's research on solar heating of conductors provides detailed guidance on calculating these adjustments ([CDA Solar Heating Guide](https://copper.org/environment/sustainable-energy/renewables/solar/education/paschal_solar_heating.pdf)).
4. Conduit Fill Calculations
When PV conductors are installed in raceways, the NEC limits the number and size of conductors based on conduit fill to prevent overheating and ensure safe installation. NEC Chapter 9, Table 1 specifies maximum conduit fill percentages:
| Number of Conductors | Maximum Conduit Fill |
|---|---|
| 1 | 53% |
| 2 | 31% |
| 3 or more | 40% |
| 1 (lead-sheathed) | 55% |
4.1 Conduit Fill Calculation Method
- Determine the cross-sectional area of each conductor using NEC Chapter 9, Table 5.
- Multiply the area of each conductor type by the number of conductors of that type.
- Sum all conductor areas to get the total conductor area.
- Divide the total conductor area by the applicable fill percentage (e.g., 40% for 3+ conductors).
- Select a conduit size from NEC Chapter 9, Table 4 whose area equals or exceeds the required area.
4.2 Conduit Fill Example
Calculate the minimum EMT size for three 10 AWG PV wires (stranded) and one 10 AWG equipment grounding conductor (4 total conductors, 40% fill):
- 10 AWG stranded PV wire area: 0.0211 in² each (Chapter 9, Table 5).
- Total conductor area: 4 × 0.0211 = 0.0844 in².
- Required conduit area: 0.0844 ÷ 0.40 = 0.2110 in².
- From Chapter 9, Table 4: ½-inch EMT has 0.122 in² (insufficient); ¾-inch EMT has 0.213 in² (sufficient).
- Minimum conduit size: ¾-inch EMT.
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5. Voltage Drop Tables and Calculations
Voltage drop is a critical consideration in PV system design. Excessive voltage drop reduces energy production, increases conductor heating, and can cause inverter under-voltage trips. The NEC recommends (but does not require) a maximum voltage drop of 3% for branch circuits or feeder conductors, and 5% for branch circuit and feeder conductors combined ([Copper Development Association](https://copper.org/applications/electrical/building/pdf/copper-wire-install-standard.pdf)). For sensitive electronic loads, circuits should be designed for a maximum of 1.5% voltage drop for branch circuits.
5.1 Voltage Drop Formula
For single-phase DC and AC circuits:
Voltage Drop (V) = (2 × L × I × R) ÷ 1000
Where:
- L = one-way circuit length in feet.
- I = current in amps.
- R = conductor resistance in ohms per 1000 feet (from NEC Chapter 9, Table 8).
For three-phase AC circuits:
Voltage Drop (V) = (√3 × L × I × R) ÷ 1000
5.2 Voltage Drop Table for Common PV Wire Sizes (Copper, DC)
The following table shows the voltage drop per 100 feet of copper PV wire at the NEC maximum current (Isc × 1.25 × 1.25 = 1.56 × Isc) for typical module string currents. Values assume 90°C insulation at 75°C ambient:
| Wire Size (AWG) | Resistance (Ω/1000 ft) | Voltage Drop/100 ft @ 8 A | Voltage Drop/100 ft @ 12 A | Voltage Drop/100 ft @ 16 A | Voltage Drop/100 ft @ 20 A |
|---|---|---|---|---|---|
| 14 AWG | 3.07 | 4.9 V | 7.4 V | 9.8 V | 12.3 V |
| 12 AWG | 1.93 | 3.1 V | 4.6 V | 6.2 V | 7.7 V |
| 10 AWG | 1.21 | 1.9 V | 2.9 V | 3.9 V | 4.8 V |
| 8 AWG | 0.764 | 1.2 V | 1.8 V | 2.4 V | 3.1 V |
| 6 AWG | 0.491 | 0.8 V | 1.2 V | 1.6 V | 2.0 V |
| 4 AWG | 0.308 | 0.5 V | 0.7 V | 1.0 V | 1.2 V |
| 2 AWG | 0.194 | 0.3 V | 0.5 V | 0.6 V | 0.8 V |
| 1/0 AWG | 0.122 | 0.2 V | 0.3 V | 0.4 V | 0.5 V |
For a 600 V DC system, a 3% voltage drop equals 18 V. For a 200-foot run at 12 A, 10 AWG wire would drop approximately 5.8 V (2.9 V/100 ft × 2), which is under 3%. However, 12 AWG would drop 9.2 V, exceeding the 3% guideline.
6. PV Circuit Sizing per NEC 690.8
NEC 690.8 specifies the methodology for determining conductor ampacity in PV circuits. The process involves calculating the maximum current and then applying continuous load and correction factors.
6.1 Maximum Current Calculation (690.8(A))
- PV source circuits (monofacial): Isc × 1.25
- PV source circuits (bifacial): Bifacial Isc × 1.25
- PV output circuits: Rated inverter output current × 1.25
- For systems ≥100 kW: A documented, stamped design using an industry-standard method may be used.
6.2 Conductor Ampacity (690.8(B))
Conductor ampacity must be not less than the larger of:
- 690.8(B)(1): Maximum current (from 690.8(A)) × 1.25, after applying all correction and adjustment factors.
- 690.8(B)(2): Maximum current (from 690.8(A)), after applying correction and adjustment factors, without the additional 125% factor, but not less than the ampacity required by NEC 110.14(C) for equipment terminals.
In practice, this means PV conductors are typically sized at 1.56 × Isc (1.25 × 1.25) before applying temperature and conduit fill derating factors. This accounts for both the continuous nature of PV circuits and the potential for irradiance to exceed STC under enhanced conditions.
6.3 Sizing Example
A PV string of 12 modules in series, each with Isc = 10.5 A:
- Maximum current = 10.5 A × 1.25 = 13.13 A.
- Conductor ampacity required = 13.13 A × 1.25 = 16.4 A (before derating).
- If installed on a rooftop with 40°C ambient + 33°C adder = 73°C ambient, the 90°C correction factor is approximately 0.58.
- Required ampacity = 16.4 ÷ 0.58 = 28.3 A.
- From NEC Table 310.16 (90°C column): 10 AWG copper = 40 A. Corrected: 40 × 0.58 = 23.2 A (insufficient).
- 8 AWG copper = 55 A. Corrected: 55 × 0.58 = 31.9 A (sufficient).
- Select 8 AWG PV wire.
7. MC4 Connector Standards
The MC4 (Multi-Contact 4mm) connector is the industry-standard DC connector used in the vast majority of PV module and inverter interconnections. While "MC4" is often used generically, the original connector is manufactured by St&228;ubli (formerly Multi-Contact) and is listed to UL 6703.
7.1 Key MC4 Specifications
| Specification | Value |
|---|---|
| Standard | UL 6703 |
| Voltage Rating | 1,000 V or 1,500 V DC |
| Current Rating | 30 A (1,000 V) / 25–30 A (1,500 V) |
| Wire Size Range | 14 AWG to 10 AWG (standard MC4) |
| Locking Mechanism | Snap-in locking, disconnect requires tool |
| IP Rating (mated) | IP67 or IP68 |
| Operating Temperature | −40°C to +85°C (or +105°C) |
| UV Resistance | Yes |
7.2 Connector Compatibility and Safety
One of the most common installation errors is mixing connectors from different manufacturers. Although connectors may appear physically compatible, they are not interchangeable. Different manufacturers use different contact geometries, locking mechanisms, and material specifications. Mixing brands can cause:
- Increased contact resistance leading to arcing and fire.
- Reduced current-carrying capacity.
- Water ingress and corrosion.
- Failure to meet UL listing requirements, voiding warranties and code compliance.
NEC 690.33 requires that connectors be listed and identified for the application. Installers should only mate connectors from the same manufacturer and product line. When transitioning between connector types, use listed adapter cables or field-assembled connectors with the correct crimping tool specified by the manufacturer.
8. Equipment Grounding Conductor Sizing
NEC 690.45 specifies that equipment grounding conductors (EGC) for PV system circuits shall be sized in accordance with NEC Table 250.122, based on the rating of the overcurrent protective device (OCPD) in the circuit. Where no OCPD is used, an assumed OCPD rated in accordance with 690.9(B) (125% of the maximum circuit current) shall be used when applying the table.
8.1 Equipment Grounding Conductor Sizing Table (NEC 250.122)
| OCPD Rating (Amps) | Copper EGC (AWG) | Aluminum EGC (AWG) |
|---|---|---|
| 15 | 14 | 12 |
| 20 | 12 | 10 |
| 30 | 10 | 8 |
| 40 | 10 | 8 |
| 60 | 10 | 8 |
| 100 | 8 | 6 |
| 200 | 6 | 4 |
| 300 | 4 | 2 |
| 400 | 3 | 1 |
| 500 | 2 | 1/0 |
| 600 | 1 | 2/0 |
| 800 | 1/0 | 3/0 |
| 1000 | 2/0 | 4/0 |
NEC 690.45 also states that increases in equipment grounding conductor size to address voltage drop considerations shall not be required. However, best practice in the solar industry is to upsize the EGC proportionally with current-carrying conductors when they are upsized for voltage drop, to maintain equivalent impedance and ensure proper fault clearing.
8.2 Grounding Conductor Material
Equipment grounding conductors in PV systems may be copper, aluminum, or copper-clad aluminum. However, the NEC requires that aluminum or copper-clad aluminum EGCs not be smaller than 6 AWG when used as a separate conductor. Bare copper is commonly used for equipment grounding within PV arrays, while insulated conductors (typically green or green/yellow striped) are used in conduit and raceway systems.
9. Wire Selection Quick Reference Guide
| Application | Recommended Wire Type | Typical Size | NEC Reference |
|---|---|---|---|
| Exposed module-to-module (string) | PV Wire (UL 4703) | 12–10 AWG | 690.31(C)(1) |
| Array home run to combiner/junction box | PV Wire (UL 4703) | 10–6 AWG | 690.31(C) |
| DC wiring inside building (in conduit) | PV Wire or THWN-2 in raceway | 10–2 AWG | 690.31(E) |
| AC inverter output (in conduit) | THWN-2 or XHHW-2 | 10–2/0 AWG | 310.10, 310.16 |
| Equipment grounding (exposed array) | Bare copper or PV Wire (green) | 10–6 AWG | 250.122, 690.45 |
| Equipment grounding (in conduit) | THWN-2 (green) | 14–2/0 AWG | 250.122, 690.45 |
| Grounding electrode conductor | Copper (bare or insulated) | 8–2/0 AWG | 250.66, 690.47 |
| Battery interconnect (DC) | PV Wire or welding cable (listed) | 2/0–4/0 AWG | 706.30 |
10. Best Practices for Solar Wire Installation
- Use listed PV wire: Always use UL 4703 listed PV wire for exposed DC circuits. Avoid substituting USE-2 without the required RHW-2 and sunlight resistant markings.
- Keep connectors matched: Never mix MC4 connectors from different manufacturers. Use the same brand throughout each string.
- Support cables properly: Secure exposed PV wire at intervals not exceeding 24 inches (600 mm) with UV-resistant cable ties or clips listed for outdoor use.
- Avoid rooftop contact: Use conduit risers or standoffs to keep conductors off hot roof surfaces. Maintain minimum 0.75 inches clearance to avoid the 33°C rooftop adder where possible.
- Size for temperature: Always apply the ambient temperature correction factor and rooftop temperature adder when sizing rooftop conductors. Never rely on the 30°C base ampacity without correction.
- Verify conduit fill: Calculate conduit fill including all conductors, including equipment grounding conductors. Remember that equipment grounding conductors count toward fill when they are 6 AWG or smaller.
- Protect conductors from physical damage: Use appropriate raceway or MC cable where conductors are subject to physical damage, especially where they transition from rooftop to building interior.
- Use proper crimping tools: Field-assembled MC4 connectors must be crimped with the manufacturer-specified tool. Improper crimps are a leading cause of connector failures and arc faults.
Conclusion
Proper wire selection and sizing for PV systems requires a thorough understanding of NEC Article 690.31, temperature derating principles, conduit fill calculations, and voltage drop analysis. PV wire (UL 4703) has become the industry standard for DC array wiring due to its superior temperature rating, UV resistance, and flexibility across installation locations. By following the NEC requirements and best practices outlined in this guide, installers can ensure safe, code-compliant, and efficient PV system wiring that will perform reliably for the 25+ year system life.
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Frequently Asked Questions
What is the difference between PV Wire and USE-2?
PV Wire is a dual-rated (600V/1000VDC) sunlight-resistant wire with XLPE insulation rated for 90°C wet and 150°C dry. USE-2 is a 600V underground service wire rated for 90°C wet/dry. PV Wire is required for exposed rooftop DC wiring; USE-2 is acceptable for module interconnections but cannot be used in conduit.
Can I use THHN wire for solar DC circuits?
No. THHN is rated for AC applications only and does not meet the sunlight resistance or DC voltage requirements of NEC 690.31(C). Using THHN on a PV DC circuit is a code violation. Use PV Wire or RHW-2 for all DC source and output circuits.
What wire size do I need for a 6 kW solar array at 100 feet?
For a 240V AC inverter output at 25A (6 kW / 240V), use 10 AWG copper for runs under 50 feet. For 100 feet, voltage drop exceeds 3% with 10 AWG, so step up to 8 AWG (2.6% drop) or 6 AWG (1.6% drop). Always verify with NEC 310.16 ampacity tables and the 125% continuous load factor per NEC 690.8.
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