Professional cable manufacturer
Solar Cable Burial & Conduit Requirements: NEC 690, Depth Tables & Best Practices
Burying solar cable isn't the same as running standard AC underground. Higher DC voltages, unique code rules, and specific cable types (PV Wire, USE-2) make solar burial a different game. This guide covers NEC 690.31 wiring methods, Table 300.5 burial depths, conduit specs, and trenching best practices for residential and utility-scale systems.
Why Solar Cable Burial Is Different
I've inspected more trenching jobs than I can count — and the most common mistake? Treating solar DC burial like standard AC underground service. Here's the thing: solar circuits operate at voltages up to 1500 V DC, often in ungrounded (non-grounded) arrays, with unique requirements for cable selection, burial depth, and conduit fill that don't apply to regular AC runs.
The governing codes break down like this:
- NEC Article 690 — The primary standard for PV solar installations in the US (2023 edition)
- NEC Table 300.5 — Minimum cover (burial depth) requirements for all wiring methods
- NEC 690.31 — Specific wiring methods for PV source and output circuits
- IEC 60364-5-52 / BS 7671 — Applicable standards for European/UK solar installations
Common Violation
Using standard THHN in underground conduit for solar DC — THHN is rated for dry locations only and is not sunlight-resistant. Per NEC 310.10(G) (wet location requirements), THWN-2 or XHHW-2 must be used for wet underground conduit runs. Better yet: USE-2 for direct burial or PV Wire (UL 4703) for the most flexibility.
Cable Types for Buried Solar Circuits
Choosing the right cable type is your first decision. Not all "solar cable" is rated for burial, and not all burial-rated cable is suitable for solar. Here's the breakdown:
| Cable Type | Voltage | Direct Burial | Conduit | UV Rated | Best For |
|---|---|---|---|---|---|
| PV Wire (UL 4703) | 600V–2000V | ✅ Yes | ✅ Yes | ✅ 720 hr | Both grounded & ungrounded arrays |
| USE-2 | 600V AC / ≤600V DC† | ✅ Yes | ✅ Yes | ✅ 720 hr‡ | Grounded arrays only |
| RHW-2 | 600V | ✅ Yes* | ✅ Yes | Limited | Conduit & direct burial in wet locations |
| THWN-2 | 600V | ❌ No | ✅ Yes | ❌ No | Conduit, indoor/underground duct |
| XHHW-2 | 600V (1000V for 8 AWG+) | ❌ No | ✅ Yes | ❌ No | Conduit, high-heat environments |
* RHW-2 is rated for direct burial per its UL 44 listing when dual-rated as USE-2. For solar PV source/output circuits, USE-2 or PV Wire is the preferred and more commonly listed option.
† USE-2 is 600V AC rated per UL 854; for DC solar use, it is limited to ≤600V DC grounded arrays only.
‡ Single-conductor USE-2 cables marked sunlight-resistant pass 720 hr UV test per UL 854 §1.6. The 300 hr requirement applies only to inner conductors beneath the overall jacket of multi-conductor cables.
Key Selection Rule
For direct burial (no conduit), you need USE-2 or listed PV Wire. For conduit underground, THWN-2 or XHHW-2 works — but remember, if the conduit transitions to an exposed rooftop or exterior wall, the cable must be sunlight-resistant. A dual-rated PV Wire (USE-2/RHW-2) gives you the most flexibility in a single cable.
1000V+ Systems
For solar systems operating above 600V DC (common in commercial — 1000V or 1500V), USE-2 is not permitted — it's only rated for 600V. You need PV Wire rated for 1000V or 2000V per UL 4703. Per NEC 690.31(G) (2023), conductors for 1000–1500V DC systems must have minimum 2000V insulation.
Burial Depth Requirements (NEC Table 300.5)
Burial depth is measured from the finished grade to the top of the cable or conduit. These are minimums — local amendments may require deeper.
| Wiring Method | General Depth | Under 2" Concrete Slab | Under Residential Driveway | Under Street / Parking |
|---|---|---|---|---|
| Direct Burial Cable | 24 in (600 mm) | 18 in (450 mm) | 24 in (600 mm) | 24 in (600 mm) |
| Rigid Metal Conduit (RMC) | 6 in (150 mm) | 4 in (100 mm) | 6 in (150 mm) | 6 in (150 mm) |
| PVC / HDPE Conduit (Sch 40) | 18 in (450 mm) | 12 in (300 mm) | 18 in | 24 in |
| PVC / HDPE Conduit (Sch 80) | 18 in (450 mm) | 12 in (300 mm) | 18 in | 24 in |
| 120V GFCI Protected Branch Circuit | 12 in (300 mm) | 6 in (150 mm) | 12 in | 24 in |
Solar-Specific Notes on Depth
- Residential solar feeder (240V AC): Use PVC conduit at 18" depth with THWN-2 conductors
- DC string circuits (600V–1500V): Direct burial PV wire at 24" depth, or PVC conduit at 18"
- Utility-scale solar farm (medium voltage): Cables above 600V up to 40 kV require 36 in (900 mm) minimum
- Under buildings / concrete slabs: Direct burial not permitted — must be in metal raceway or concrete-encased duct
Residential Scenario — 10kW Solar to House Main Panel
Situation: Running a 240V AC feeder from the inverter to the main panel, 50 ft trench across a lawn.
Recommended: PVC Sch 40 conduit at 18" deep, with 6 AWG THWN-2 copper conductors. Red warning tape at 12" below grade. Conduit sized per NEC Chapter 9 Table 1 (max 40% fill).
Conduit Specifications for Solar Underground Runs
Solar installations have unique conduit considerations. Let's walk through the key ones.
PVC Conduit (Schedule 40 vs 80)
Schedule 40 is sufficient for most residential underground runs. Schedule 80 is required where the conduit is subject to physical damage — exposed above grade, or under driveways and roads. For solar, this means:
- Underground under lawn: Sch 40
- Transition above grade at the inverter or combiner box: Sch 80 (exposed)
- Under driveway: Sch 80 or concrete-encased Sch 40
Conduit Sizing for Solar DC Circuits
PV DC circuits often require larger conduit than AC circuits at the same current — because the conductor insulation is thicker (PV Wire or XHHW-2), and you may be running multiple strings in parallel.
| System Size | Conductors (per string) | # of Strings | Recommended Conduit Size (PVC Sch 40) |
|---|---|---|---|
| 5–8 kW | 10 AWG PV Wire | 2 strings | 1 in |
| 8–12 kW | 8 AWG PV Wire | 2–3 strings | 1.25 in |
| 12–20 kW | 6 AWG PV Wire | 3–4 strings | 1.5 in |
| 20–50 kW | 4 AWG PV Wire | 4–6 strings | 2 in |
| 50–100 kW (commercial) | 2 AWG – 4/0 | 6–12 strings | 3–4 in (or parallel conduits) |
Pro Tip — Pull Boxes
For underground conduit runs exceeding 100 ft, install pull boxes or junction boxes at intermediate points. NEC 352.26 limits PVC conduit to 360 degrees of total bends between pull points. For solar farm applications, pre-install pull cords in all ducts — future-proofing saves enormous cost if a string needs replacement.
NEC 690.31 — PV Wiring Methods at a Glance
Article 690.31 contains the most important wiring rules specific to solar PV systems. Here are the critical sections:
690.31(C)(1) — Single-Conductor Cable Outdoors
- Permitted cables: PV Wire, USE-2, or single-conductor marked sunlight-resistant
- 8 AWG and smaller: Supported and secured at intervals ≤ 24 in.
- Larger than 8 AWG: Supported at intervals ≤ 54 in.
690.31(D) — DC Circuits Inside Buildings
Where PV DC circuits exceed 30V and are routed inside a building, they must be in metal raceways, MC cable, or metal enclosures (2023 NEC 690.31(D)). This is a fire safety consideration — solar DC is not self-extinguishing like AC.
690.31(B)(1) — Conductors of Different Systems
PV source and output circuits shall not occupy the same raceway or enclosure with non-PV circuits or inverter output circuits unless separated by a partition (NEC 690.31(B)(1)). Where conductors from different PV systems share a raceway, they must be grouped by cable ties at least every 6 ft. All conductors in a shared raceway must also be rated for the maximum voltage present per NEC 300.3(C)(1).
690.31(G) — Systems Over 1000V DC (New in 2023)
For 1000–1500V DC systems:
- Not permitted on or in one- and two-family dwellings
- Not permitted inside habitable rooms of any building
- On building exteriors: limited to ≤ 10 ft above grade and ≤ 33 ft of surface attachment length
- Conductor insulation: minimum 2000V rated
Trenching & Installation Best Practices
Over the years, I've seen the same problems crop up in solar trenching jobs. Here's what separates a 25-year installation from one that fails in 5:
Bedding & Backfill
- Use sand or fine-grained soil (< ½ in. stones) directly around cables
- Minimum 2 in. sand layer below and above cable
- Backfill free of rocks, debris, organic material
- Compact to 95% minimum — but no mechanical compaction directly over cables
Marking & Protection
- Red warning tape at 12 in. below grade: "CAUTION BURIED ELECTRIC LINE BELOW"
- Protective concrete board above direct-burial cables in rocky soil
- Bend radius: minimum 12× cable diameter for armoured, 8× for unarmoured
- Pull cords pre-installed in all ducts for future access
Installation Checklist
- Cable type verified for voltage rating (600V / 1000V / 2000V match system voltage)
- Wet-rated conductors used for underground conduit (THWN-2, XHHW-2, or PV Wire)
- Burial depth meets or exceeds Table 300.5 minimum for wiring method used
- Conduit sized per NEC Chapter 9 (max 40% fill)
- Warning tape installed 12 in. below finished grade
- Bonding and grounding per NEC 250 and Article 690.41–690.49
- Pull boxes installed where conduit runs exceed 100 ft or 360° of bends
- DC disconnect location labeled and accessible
- PV source and output circuits labeled at every termination
- Trench backfill compacted, surface restored for drainage
Direct Burial vs Conduit — Which to Choose?
Direct burial with USE-2/PV Wire: Lower labor cost, faster installation. Best for long runs in open areas (solar farms, large commercial ground-mounts). Conduit protection is absent — requires careful backfill and good soil conditions.
Conduit with THWN-2/PV Wire: Higher material cost but better protection, easier future cable replacement, and simpler code compliance. Best for residential, urban sites, and anywhere future changes are likely.
Frequently Asked Questions
Can I use standard PV wire (H1Z2Z2-K) for direct burial?
H1Z2Z2-K per EN 50618 is UV-resistant and designed for outdoor exposed use, but it is not typically rated for direct burial. For direct burial, you need a cable with mechanical armor (SWA) or dual rating (USE-2/PV Wire per UL 4703). Check the manufacturer's specific rating — some PV wires are dual-rated for both above-ground and direct burial.
What's the minimum burial depth for solar DC cable under a driveway?
Per NEC Table 300.5: 24 in (600 mm) for direct burial cable under a residential driveway, or 18 in for PVC conduit. For commercial driveways and roads subject to heavy traffic, 24 in is required for both direct burial and conduit.
Can solar DC and AC conductors share the same underground conduit?
Generally, no. NEC 690.31(B) prohibits PV source and output circuits from occupying the same raceway as non-PV circuits or inverter output circuits unless separated by a partition or barrier. The 2023 NEC added an exception for properly identified and grouped conductors sharing junction boxes — but underground conduit runs should still keep AC and DC separate for safety and troubleshooting.
Do I need a ground conductor in the underground conduit for solar?
Yes. Per NEC 690.41–690.49, PV systems require equipment grounding conductors (EGC) sized per 250.122. Grounding is especially critical for solar because DC arcs are self-sustaining — a ground fault can maintain an arc that AC protective devices won't quench. In buried runs, use a bare copper or insulated green wire, sized per code.
Is concrete-encased duct required for solar farm conduits?
For utility-scale solar farms, concrete-encased duct banks (3000 psi minimum) are standard for conduits under access roads, equipment pads, and anywhere subject to vehicle traffic. For less critical areas within the array field, direct burial with sand bedding and warning tape is typical. Always consult project specifications and the AHJ.
Need solar cable for your buried trench?
We stock H1Z2Z2-K solar cable (EN 50618) and UL-certified PV Wire & USE-2 cable for direct-buried and conduit applications. All sizes from 10 AWG to 500 kcmil with full third-party certifications.
Request a Burial Cable Quote