NEC 690.31 is the wiring methods section of Article 690 — it specifies which wire types, cable assemblies, and conduit configurations are permitted for solar PV source and output circuits.
The rules are more restrictive than standard building wiring: solar systems operate outdoors, at elevated DC voltages, with constant UV exposure and temperature extremes. Standard building wires (THHN, NM-B) that are acceptable for indoor AC circuits are not permitted in exposed PV circuits.
Permitted Wiring Methods
Listed PV Wire
Listed PV Wire is the most common wiring method for module-to-module connections and exposed roof surface wiring. Requirements:
- Must be listed specifically for PV use (marked “PV Wire” or “Photovoltaic Wire”)
- Available in 600V and 1000V DC ratings — use the voltage rating that matches the system
- Must be sunlight-resistant (UV-stabilized insulation)
- Typically 10 AWG or 12 AWG for residential source circuits
- Comes in black and red for polarity identification
PV wire is suitable for use:
- On module surfaces (module-to-module wiring)
- In exposed locations on rooftops
- In conduit (where it behaves like any other conductor)
- For short exposed runs between the array and the conduit entry point
USE-2 Cable
USE-2 (Underground Service Entrance cable, 2-wire, type 2) is a sunlight-resistant, 600V-rated cable that can substitute for PV wire in many applications:
- Sunlight-resistant by listing
- Rated for wet locations and direct burial (when listed for direct burial)
- Available in common solar wire sizes (10 AWG, 8 AWG, 6 AWG)
- Widely available and often less expensive than dedicated PV wire
USE-2 is not available in 1000V rating — for 1000V DC commercial systems, listed 1000V PV wire is required for exposed circuits.
Conductors in Conduit (THWN-2, XHHW-2)
For all conduit runs — rooftop conduit, exterior wall conduit, underground conduit — standard wet-rated conductors are permitted:
| Conductor Type | Temp Rating | Notes |
|---|---|---|
| THWN-2 | 90°C (wet) | Most common for PV conduit runs |
| XHHW-2 | 90°C (wet) | 90°C wet rated; suitable for all locations |
| RHW-2 | 90°C (wet) | Rubber insulation; less common |
| THHN | 75°C (dry) | NOT suitable — not wet-rated, not sunlight-resistant |
THHN vs. THWN-2
THHN wire is dry-location rated only. Many THHN/THWN-2 dual-listed wires are physically identical and meet both ratings. Check the wire labeling carefully — the jacket must say “THWN-2” (or be dual-listed THHN/THWN-2) for use in conduit in wet locations. Pure THHN without the wet rating is not permitted in PV conduit runs exposed to weather.
Rooftop Conduit: Temperature Derating
NEC 310.15(B)(3)(c) adds 22°C (40°F) to the maximum ambient air temperature for conductors in conduit on or above a rooftop within 7 inches (175mm) of the roof surface.
When the Derating Applies
- Conduit strapped directly to roof surface (within 7 inches)
- Conduit on rack supports up to 7 inches above the roof
- Both PVC conduit and metallic conduit on the roof
When It Does NOT Apply
- Conduit raised more than 7 inches above the roof surface (use standard ambient temperature)
- Conduit inside an attic or building interior
- Underground conduit
Practical Impact: Phoenix Example
Phoenix, AZ maximum design air temperature: 43°C (109°F) Rooftop conduit effective temperature: 43 + 22 = 65°C
For THWN-2 (90°C rated conductor):
- Temperature correction factor at 65°C: 0.58
- 10 AWG THWN-2 table ampacity (from NEC Table 310.16, 90°C column): 40A
- Allowed ampacity in Phoenix rooftop conduit: 40A × 0.58 = 23.2A
A source circuit with Isc = 9.5A needs minimum 125% × 9.5A = 11.875A. After derating, 10 AWG provides 23.2A — sufficient. But if conduit fill derating is also applied, 10 AWG may not be enough.
Connector Requirements (690.31(F))
Module connectors — the plug-and-socket connectors that connect module leads to source circuit wiring — must:
- Be listed for PV use (typically UL 6703 or equivalent)
- Field-installed connectors must be from the same manufacturer family, OR must be specifically tested and listed together by a recognized testing laboratory
The MC4 Compatibility Issue
The industry-standard MC4 connector is manufactured by Stäubli and licensed to other manufacturers. “MC4-compatible” connectors from other brands may physically mate with genuine MC4 connectors but have NOT been tested and listed together.
Using a genuine MC4 from Stäubli with an “MC4-compatible” connector from a different manufacturer creates a connector assembly that:
- Has not been tested for proper mating force
- Has not been tested for IP67 water ingress with the specific mating counterpart
- Creates a listed connector with an unlisted assembly → NEC violation
Practical Rule for Connectors
Specify connectors by brand and model in the permit package. Use the same brand throughout the system for all field-wired connections. Module pigtails come with the module manufacturer’s chosen connector — if it’s a Huawei PV connector and the installer is using standard MC4, a pigtail adapter from a listed supplier may be needed. Document all connector types and confirm they’re listed together.
Underground Wiring (690.31(E))
For underground circuits connecting ground-mounted arrays to buildings:
| Method | Direct Burial? | In Conduit? |
|---|---|---|
| USE-2 (listed for direct burial) | Yes | Yes |
| PV Wire | No (not direct burial listed) | Yes |
| THWN-2 | No | Yes |
| PVC conduit with THWN-2 | — | Yes (conduit is buried) |
| HDPE conduit with THWN-2 | — | Yes |
Minimum burial depth for solar DC circuits:
- In PVC conduit: 18 inches
- In metallic conduit: 6 inches
- Direct-burial cable: 24 inches (under driveways: 24 inches)
Flexible Wiring for Tracking Systems
Tracking systems (single-axis or dual-axis trackers) require flexible wiring at the pivot points. Use:
- Listed flexible PV cable assemblies rated for continuous flexing
- Extra-flexible PV wire (must be specifically listed as flexible)
- Standard PV wire is not rated for continuous repeated flexing — it can fail at bending stress points
Common 690.31 Mistakes
| Mistake | Code Issue | Fix |
|---|---|---|
| Using THHN in exposed roof conduit | Not wet-rated | Use THWN-2 or XHHW-2 |
| Using NM-B (Romex) for any PV circuit | Not rated for DC, not sunlight-resistant | Replace with listed PV wire or THWN-2 in conduit |
| Mixing MC4 connector brands | 690.31(F) violation | Use single connector brand throughout |
| Not applying 22°C rooftop adder to conduit derating | Undersized conductors | Add 22°C to ambient for all conduit within 7” of roof |
| Using USE-2 for 1000V DC commercial systems | USE-2 is only 600V rated | Use listed 1000V PV wire for exposed circuits |
Design NEC 690.31-Compliant Wiring Systems
SurgePV’s permit packages include wiring method notes on every circuit segment, ensuring AHJ plan reviewers can quickly verify 690.31 compliance without asking for clarification.
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Frequently Asked Questions
What wiring methods are allowed for solar PV systems under NEC 690.31?
Listed PV Wire (sunlight-resistant, 600V or 1000V), USE-2 cable, and conductors in conduit (THWN-2, XHHW-2). Standard indoor building wire (THHN, NM-B) is not permitted in exposed PV circuits.
Can I use regular THHN wire for solar DC circuits?
Only in conduit in dry locations. THHN is not wet-rated and not sunlight-resistant. Use THWN-2 (wet-rated) for conduit runs in outdoor or wet locations. Use PV wire or USE-2 for any exposed outdoor circuit not in conduit.
Can I mix MC4 connectors from different manufacturers?
No. NEC 690.31(F) requires field-installed connectors to be from the same manufacturer family or be specifically listed together. Mixing brands creates an unlisted connector assembly, which is a code violation and a potential safety and warranty issue.
