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Direct Burial vs. Duct Installation for Solar Farm MV and DC Cables: A Total Cost Comparison
Both methods run cables underground. But the difference in ampacity, installation speed, and 25-year maintenance cost is bigger than most project teams realise.
★ TCO Analysis — Solar Farm CablingI’ve worked on enough solar projects to know that the choice between direct burial and duct (conduit) is rarely a straightforward one. On paper, both methods put the cable in the ground. But once you factor in thermal derating, installation speed, and what happens when something goes wrong 12 years in, the two paths diverge dramatically.
Here’s the headline: direct burial typically costs less upfront and delivers better ampacity, while duct installation costs more but makes future cable replacement far easier. Which one wins on total cost of ownership depends on your soil, your layout, and how much cable replacement risk you’re willing to carry.
Let me break it down with the numbers I’ve seen in real projects.
Technical Requirements for Each Method
They both go in a trench, but the engineering requirements are not the same.
Direct Burial — What the Cable Must Handle
Direct burial means the cable is in direct contact with the soil. There’s no secondary mechanical barrier. So the cable itself must provide everything:
- Mechanical protection — armoured construction (SWA for AC cables) or robust double-sheath construction for DC cables
- Moisture resistance — water-blocking layers or a fully water-resistant jacket design
- UV resistance during storage — cables sit on the reel before installation; UV stabilisation is still needed
- Rodent and termite protection — in certain regions, additional armouring or repellent additives are required
- Sand bedding — typically 100 mm of sand below and above the cable, plus a protective slab or warning tape above that
- Minimum burial depth — per NEC 690: 18 inches (457 mm) for PV cables in conduit, or 24 inches (610 mm) for direct burial without conduit; per GB 50797-2024: ≥600 mm general; ≥1000 mm in frequently ploughed areas
Duct (Conduit) Installation — What the System Provides
In a duct installation, the conduit takes the mechanical load. The cable can be a standard construction (though it still needs to handle pulling tensions):
- Conduit material options — PVC Schedule 40/80 (most common in US), HDPE (flexible, good for directional boring), or rigid steel (rare for solar farms due to cost)
- Pull boxes / handholes — required at intervals (typically every 100–150 m) for cable pulling access
- Conduit sizing — per NEC Chapter 9, Table 1, conduit fill must not exceed 40% for 3 or more conductors; this drives duct size selection
- Spare ducts — industry best practice is to install 20–30% spare conduits for future expansion
- Bend radius — conduit sweeps must respect the cable’s minimum bending radius (typically 6–8× OD for armoured cables, 12–15× OD for unarmoured MV cables)
💡 My take: If your soil is soft, well-drained, and free of rocks, direct burial is almost always the right call. If you’re dealing with rocky ground, chemically aggressive soil, or a site where you know cables will need to be replaced (brownfields, temporary installations), duct is worth the premium.
Installation Time and Equipment Cost
This is where the two methods really separate. Direct burial is faster and cheaper to install. But the gap narrows when you look at the whole lifecycle.
▼ Direct Burial
~$13–39/m Trenching + installation (US market)- No conduit material cost
- Single-pass excavation
- No pull boxes or handholes needed
- Faster in open corridors
- ~38 RMB/m (Chinese PV rates)
■ Duct / Conduit
~$25–60/m Trenching + conduit + cable pulling- PVC conduit: ~$3–10/m additional
- Two-step process (lay conduit, then pull cable)
- Handholes every 100–150 m
- More labour and inspection steps
- Bend radius constraints require careful routing
Installation Process Comparison
| Step | Direct Burial | Duct Installation |
|---|---|---|
| Trench excavation | Same for both | Same for both |
| Sand bedding | Lay 100 mm sand | Lay 100 mm sand + concrete encasement (if required) |
| Cable/conduit placement | Lay cable directly | Assemble and lay conduit; wait for joint glue to cure |
| Pull boxes | None needed | Install at intervals (±100–150 m) |
| Cable pulling | Not required (cable laid in trench) | Pull cable through conduit — requires pull line, lubricant, tension monitoring |
| Cover/protection | 100 mm sand + protection slab + warning tape | Backfill around conduit |
| Backfill | Same for both | Same for both |
| Testing | IR test after backfill | IR test after pulling (before backfill) + after backfill |
| Estimated crew-hours per km | ~200–300 | ~350–500 |
Equipment Cost Breakdown
| Item | Direct Burial (per km) | Duct Installation (per km) |
|---|---|---|
| Trenching (0.6 m × 0.8 m, soft soil) | $13,000–$39,000 | $13,000–$39,000 |
| Sand bedding material | $1,500–$3,000 | $1,500–$3,000 |
| PVC conduit (2 × 4" schedule 40) | — | $10,000–$20,000 |
| Concrete encasement (if required) | — | $8,000–$25,000 |
| Pull boxes / handholes | — | $3,000–$8,000 |
| Protection slab + warning tape | $2,000–$4,000 | — |
| Cable laying labour | $8,000–$15,000 | $15,000–$30,000 |
| Cable pulling equipment | — | $2,000–$5,000 |
| Total estimated (per km) | $24,500–$61,000 | $52,500–$130,000 |
| Cable material cost | ~30% more copper (vs free-air, see Section 3) | ~40% more copper (vs free-air; more derating than direct burial, requiring larger cross-section) |
| Costs are indicative ranges for US market in soft soil. Rock removal adds 20–50% to excavation. Chinese market prices are approximately 40–60% lower for labour-driven costs. Cable material costs vary with copper prices. | ||
⚠ Read the fine print on cable cost: The ampacity advantage of direct burial (~6–24% higher) means you can sometimes use one size smaller cable than you’d need in conduit. But the derating for direct burial is still substantial — you’ll be oversizing cables for both methods vs. free air. The real TCO comparison has to include the cable sizing difference, which is site-specific.
Long-Term Thermal Performance and Fault Accessibility
The two biggest long-cost items that don’t show up in the installation budget: thermal derating (which affects cable sizing) and fault repair cost (which affects O&M).
Thermal Performance — How Heat Dissipation Differs
Here’s the physics: both methods embed the cable in soil, which is a poor thermal conductor. But conduit adds another layer of thermal resistance — the air gap inside the conduit and the PVC or HDPE wall itself.
A detailed IEEE study comparing the two methods found that for a typical MV cable at a soil thermal resistivity of 1.2 °C·m/W:
| Parameter | Direct Buried | In Conduit | Difference |
|---|---|---|---|
| Ampacity (single circuit, trefoil, at rho 1.2 °C·m/W) | ~204 A | ~155 A | −24% in conduit |
| Thermal bottleneck | Soil dry-out around cable | Trapped heat inside duct + soil | — |
| Impact of soil resistivity increase | Significant (−50% possible) | Even more significant | — |
| Multiple circuits (4 trefoil, touching) | ∼70% of single circuit | ∼60–65% of single circuit | Conduit derates more |
The IEEE paper concluded: “For the range of soil thermal resistivities found in North America, the cable buried in conduit has an ampacity lower than a direct buried cable at all locations.”
But here’s what that means in practical terms. If your design load is 180 A per circuit and you’re choosing between burial methods:
- Direct burial: You might size for a cable rated ~240 A base (~204 × 1.18 safety margin). That could be a 95 mm² cable.
- In conduit: You’d need a cable rated ~280 A base (~155 × 1.8 margin). That’s a 150 mm² cable — roughly one to two sizes larger.
- Extra copper cost: One to two sizes larger on a 10 km MV collector run adds $50,000–$120,000 in material cost.
💡 Pro tip: If you’re designing a large solar farm and want to use conduit (for protection or future replacement), consider using HDPE ducts with a smooth inner bore and corrugated outer wall. These have slightly better thermal performance than solid PVC because the corrugated profile reduces the contact area with the soil, and some manufacturers offer a “thermal-enhanced” duct that improves heat transfer by up to 15%.
Fault Accessibility — The 25-Year View
| Scenario | Direct Burial | Duct Installation |
|---|---|---|
| Cable fault repair | Dig to locate, excavate to access, splice cable, re-bury. 2–5 days per fault. Cost: €12,000–€17,000 per splice (industry estimate, European O&M data). | Identify section, pull out damaged cable section, pull in new cable via existing duct. 1–2 days. Cost: €5,000–€10,000. |
| Conduit/cable damage from external excavation | High risk — no physical barrier protecting the cable | Lower risk — conduit provides mechanical protection, but if struck, both conduit and cable may need repair |
| Water ingress | Low risk — cable in direct contact with soil; moisture is distributed | High risk if duct not sealed — water accumulates in conduit, submerged cable may fail prematurely |
| Rodent damage | Risk depends on armouring and local rodent activity | Lower risk — conduit adds a barrier |
| Cable replacement (full string) | Requires full re-trenching — disruptive and expensive | Old cable pulled out, new cable pulled in — no excavation needed if duct is intact |
Here’s the honest engineering trade-off: in 15 years of solar farm O&M, I’ve seen roughly one cable fault per 50–100 km of buried cable per year, on average. Most are caused by third-party excavation, not insulation failure. The question is whether you’re willing to bet the long-term O&M budget on a low fault probability (direct burial saves installation cost) or whether you want to make every eventual repair cheaper (conduit costs more now, less later).
25-Year TCO Projection (Per km of Collector Cable)
| Cost Item | Direct Burial | Duct Installation |
|---|---|---|
| Initial installation cost | $30,000–$70,000 | $60,000–$150,000 |
| Cable material (MV, 3-core 95–240 mm²) | $50,000–$120,000 | $60,000–$150,000 (typically 1–2 sizes larger) |
| Expected cable faults (25 yr, ~0.3 faults/km) | $15,000–$25,000 (3 repairs × $5k–$8k each) | $6,000–$12,000 (3 repairs × $2k–$4k each) |
| Water ingress remediation | — | $5,000–$15,000 (duct pumping/sealing if needed) |
| Total 25-year TCO (per km) | $95,000–$215,000 | $131,000–$327,000 |
| TCO projection assumes soft soil, moderate fault probability. Rock, aggressive soil, or high third-party excavation risk shifts the balance toward conduit. Brownfields or sites with expected future modifications strongly favour duct. | ||
On straight TCO, direct burial wins in most stable-soil scenarios by roughly 25–35%. But that gap narrows significantly — and sometimes inverts — in challenging ground conditions or when future cable replacement is likely.
SORIVO Cables Rated for Direct Burial and Duct
Not every cable is built for underground service. Here’s how Sorivo’s product range maps to each installation method.
| Application | Installation Method | Recommended SORIVO Cable | Key Feature for Underground Service |
|---|---|---|---|
| PV DC strings | Direct burial (in conduit) | H1Z2Z2-K 4–25 mm² | EN 50618, tinned copper, 1500 V DC, UV + moisture resistant sheath |
| PV DC strings | Direct burial (no conduit) | H1Z2Z2-K in sand/concrete-encased trench + warning tape | Additional protection layer recommended; cable designed for outdoor exposure, not direct earth contact |
| AC collection (LV, 0.6/1 kV) | Direct burial | CU/XLPE/SWA/PVC | Steel wire armour for mechanical protection, PVC sheath for moisture resistance, BS 5467 / IEC 60502-1 |
| AC collection (LV, 0.6/1 kV) | In duct | CU/XLPE/SWA/PVC or CU/XLPE/LSZH/SWA/LSZH | SWA still recommended for pulling protection; LSZH variant preferred if ducts are in confined spaces |
| MV collection (11–33 kV) | Direct burial or duct | MV XLPE armoured cable (contact Sorivo) | TR-XLPE insulation, water-blocking, metallic screen for earth fault detection |
| BESS DC connection | Direct burial or duct | TÜV 2PfG 2642 ESS cable | Chemical-resistant sheath, 1500 V DC, TÜV certified for BESS environments |
How Cable Quality Affects Underground Performance
The difference between a premium solar cable and a generic economy cable shows up most clearly in underground installations — where moisture, soil chemistry, and thermal cycling are relentless over 25 years.
| Feature | Market Generic / Economy | SORIVO Premium Grade |
|---|---|---|
| Conductor | Bare copper — tarnishes in buried environments, increases contact resistance | Tinned copper per IEC 60228 Class 5/6 — corrosion-resistant, stable contact resistance over 25 years |
| Insulation | Variable XLPE cross-linking — may degrade faster in wet underground conditions | Verified cross-linking per EN 50618 / IEC 62930 — full 90 °C rating, tested wet insulation resistance |
| Moisture resistance | Standard jacket — water ingress likely over time in direct burial | Water-blocking options available — meets wet withstand test per IEC 60502-1 |
| Armour (where applicable) | Under-gauge galvanised wire — corrodes faster in buried environments | Full-gauge galvanised SWA per BS 7655 — proven corrosion resistance for underground service |
| Traceability | None — impossible to verify conductor size after burial | Metre-marked sheath, batch traceable — full material certification available before burial |
| Factory testing | Batch sample only | 100% factory tested: conductor resistance, IR, high-voltage withstand, spark test |
| Certification | Self-declared CE — no third-party verification for underground performance | TÜV / UL / BASEC certified — independently verified for direct burial and duct installation |
★ Recommendation: For direct burial, I strongly recommend specifying tinned copper conductors and armoured construction wherever the budget allows. The incremental cost is small compared to the cost of excavating a corroded bare copper fault 10 years in. For duct installations, tinned copper is less critical (the duct provides a drier environment), but armoured cable still helps during the pulling process.
Frequently Asked Questions
Can I direct-bury standard H1Z2Z2-K solar cable without conduit?
H1Z2Z2-K is rated for outdoor UV exposure, but it is not designed for direct earth contact — it lacks mechanical armour. For underground use, always run it in conduit (PVC or HDPE) for mechanical protection, or lay it in a sand bed with a protective slab above. In rocky soil or areas with digging activity, never direct-bury unarmoured PV cable. Some jurisdictions also require conduit for PV cables by code (NEC 690). Always check your local AHJ requirements.
Does conduit installation always mean lower ampacity than direct burial?
In almost all cases, yes. The air gap inside the conduit and the thermal resistance of the conduit wall itself add extra insulation around the cable. IEEE studies show conduit can reduce ampacity by 6–24% compared to direct burial for the same cable and soil conditions. The biggest difference is at higher soil thermal resistivities (dry or sandy soil). The exception is if the duct is water-filled (e.g., a submerged duct run) — water has much better thermal conductivity than air, which can partially offset the conduit penalty.
How many spare ducts should I install for future expansion?
Industry best practice is 20–30% spare capacity. For a 3-duct bank, that means 1 spare. For an 8-duct bank, 2–3 spares. The incremental cost of adding a spare duct during initial trenching is about 10–15% of installing it later as a retrofit (which requires re-excavation). In my experience, project teams that skip spare ducts to save 2% on installation cost always regret it when the plant is repowered or expanded 10 years later.
Is it better to use rigid PVC or flexible HDPE conduit for solar farm ducts?
Both have their place. PVC Schedule 40/80 is the most common choice for solar farms — it’s rigid, UV-resistant, cost-effective, and widely available. HDPE is better for directional boring (trenchless installation under roads or sensitive areas) because it’s flexible and comes in long continuous coils. HDPE also has slightly better impact resistance at low temperatures. For most open-trench solar farm installations, PVC is the standard. Use HDPE for road crossings, wetland crossings, or anywhere you need directional drilling.
What’s the best way to seal duct ends to prevent water ingress?
Water ingress into ducts is one of the most common long-term problems with conduit installations. The standard solution is duct sealing bags or mechanical duct plugs at both ends — at the combiner box and at the inverter/pull box. For additional protection, install the duct with a slight slope (1:100 minimum) toward a drain point or sump, and use water-dissipating cable compounds (swellable yarns or tapes inside the cable construction) that prevent longitudinal water migration. Per NEC 300.5(G), conduits must be sealed to prevent moisture from entering enclosures.
Need cable specifications for your solar farm installation?
Sorivo provides factory test data, ampacity calculations for direct burial and duct conditions, and full material certifications with every order. Contact our engineering team for project-specific cable sizing.
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