Key Takeaways
- MV cables operate between 1kV and 35kV, bridging inverter output and grid interconnection
- Common voltage classes for solar include 15kV, 25kV, and 35kV
- Cable sizing depends on current capacity, voltage drop limits, and thermal derating
- Proper installation requires tested splices, terminations, and shielding
- Trenching depth and spacing follow NEC and local utility standards
- MV cable selection directly affects system losses and long-term reliability
What Is an MV Cable?
An MV cable (medium-voltage cable) is an electrical cable rated for voltages between 1kV and 35kV. In solar energy systems, MV cables carry power from central or string inverter output to the point of interconnection with the utility grid. They are standard on solar farms, commercial rooftop installations with pad-mounted transformers, and any project where the distance between the inverter and the grid tie-in makes low-voltage wiring impractical.
MV cables consist of a conductor (typically copper or aluminum), semiconducting layers, insulation (commonly cross-linked polyethylene or XLPE), a metallic shield, and an outer jacket. The shielding contains the electric field within the cable, preventing interference and reducing safety hazards.
In utility-scale solar, MV cable runs can account for 2–5% of total project cost. Correct sizing and routing decisions made during solar design software modeling directly reduce both material spend and lifetime line losses.
How MV Cables Work in Solar Installations
MV cables serve a specific role in the power delivery chain from panel to grid. Here’s how they fit into a typical solar farm layout:
Inverter Output
Central inverters or groups of string inverters output AC power, typically at 480V or 600V in North America, or up to 800V in some configurations.
Step-Up Transformer
A pad-mounted or skid-mounted transformer steps the inverter output up to medium voltage — commonly 12.47kV, 13.8kV, or 34.5kV depending on utility requirements.
MV Cable Run
MV cables carry the stepped-up power through underground trenches or overhead runs to the project substation or point of interconnection. Cable lengths can range from hundreds of feet to several miles.
Switchgear and Protection
At the collection point, MV switchgear provides overcurrent protection, fault isolation, and sectionalizing capability for the MV cable network.
Grid Interconnection
The MV cable network delivers power to the main project substation, where it may be stepped up again for high-voltage transmission or fed directly into the distribution grid.
Vdrop = I × (R × cos(θ) + X × sin(θ)) × L × 2Where I is current in amps, R is resistance per unit length, X is reactance per unit length, θ is the power factor angle, and L is cable length.
Types of MV Cable
Different insulation and construction types suit different solar installation conditions.
XLPE Insulated (Cross-Linked Polyethylene)
The industry standard for solar MV applications. XLPE offers high temperature tolerance (90°C continuous, 250°C emergency), moisture resistance, and a 30–40 year service life. Available in single-conductor and three-conductor configurations.
EPR Insulated (Ethylene Propylene Rubber)
More flexible than XLPE, making it easier to install in tight conduit runs. Slightly higher dielectric losses but better performance under repeated bending. Used where routing flexibility matters more than maximum efficiency.
Armored MV Cable
Includes an additional metallic armor layer (steel wire or aluminum interlocking) for direct burial without conduit. Common in solar farms where trenching through rocky or unstable soil is required.
TR-XLPE (Tree-Retardant XLPE)
Modified XLPE formulation that resists water treeing — a degradation mechanism in wet environments. Preferred for direct-burial MV cable runs in areas with high water tables or flood-prone sites.
When modeling MV cable layouts in solar software, account for thermal derating based on soil conditions, burial depth, and cable grouping. A cable rated for 300A in free air may only carry 200A when direct-buried at 36 inches with adjacent circuits.
Key Metrics & Calculations
Correct MV cable specification requires balancing multiple electrical and physical parameters:
| Parameter | Unit | What It Determines |
|---|---|---|
| Voltage Class | kV | Must match or exceed the system’s medium-voltage level |
| Ampacity | A | Maximum continuous current the cable can carry safely |
| Conductor Size | AWG / kcmil | Cross-sectional area — larger conductors reduce resistance and losses |
| Insulation Level | % (100% or 133%) | 100% for grounded systems; 133% for ungrounded or high-impedance grounded |
| Voltage Drop | % | Must stay within utility and code limits (typically under 3%) |
| Thermal Resistivity | °C·cm/W | Soil thermal properties affect cable ampacity derating |
Power Loss = I² × R × L × Number of PhasesPractical Guidance
MV cable decisions affect project cost, reliability, and long-term performance. Here’s role-specific advice:
- Run voltage drop calculations early. MV cable sizing is often driven by voltage drop rather than ampacity, especially on long runs. Use solar design software to model cable routes and validate drop limits.
- Optimize trench routing. Shorter cable runs reduce material cost and line losses. Centralizing inverter pads and planning efficient collection circuits can save thousands of feet of MV cable.
- Specify the correct insulation level. Use 100% insulation for solidly grounded systems and 133% for resistance-grounded or ungrounded configurations. Getting this wrong causes premature cable failure.
- Account for future expansion. Slightly oversizing MV cables or conduits for future capacity additions is far cheaper than re-trenching later.
- Follow manufacturer splice and termination procedures exactly. MV cable failures almost always occur at splice and termination points. Use factory-trained technicians and test every joint.
- Perform hi-pot testing before energization. High-potential testing verifies insulation integrity after installation. This catches damage from installation handling before the cable is energized.
- Maintain minimum bend radius. Exceeding the minimum bend radius damages insulation and shielding. For most MV cables, the minimum is 12 times the cable diameter.
- Document trench depth and as-built routing. Accurate as-built drawings prevent future dig-ins and simplify maintenance and troubleshooting.
- Include MV cable costs in project budgets. MV cable and associated civil works (trenching, backfill, conduit) represent a significant line item. Omitting them leads to budget overruns.
- Explain line loss implications. Customers care about delivered energy, not just panel output. Show how proper MV cable sizing minimizes losses and maximizes their return.
- Highlight reliability and lifespan. Quality MV cable installations last 30–40 years with minimal maintenance, matching or exceeding the expected life of the solar array itself.
- Differentiate on design quality. Optimized MV cable routing from professional solar design software reduces both upfront cost and lifetime losses — a tangible competitive advantage in proposals.
Design Optimized MV Cable Layouts
SurgePV models cable routing, voltage drop, and line losses so you can optimize MV infrastructure during the design phase.
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Real-World Examples
Utility-Scale: 50 MW Solar Farm
A 50 MW ground-mount project in Texas uses 34.5kV MV cable to connect 20 inverter pads to a central substation. Total MV cable length is 4.2 miles using 500 kcmil aluminum XLPE cable. Optimized routing through centralized inverter placement reduced cable length by 18% compared to the initial layout, saving $340,000 in material and trenching costs.
Commercial: 2 MW Carport Installation
A 2 MW carport system at a distribution center uses 15kV MV cable to connect the inverter pad to the utility meter, a distance of 800 feet. The 1/0 copper XLPE cable handles the 95A load with a voltage drop of 1.2%, well within the 3% limit.
Solar Farm with Challenging Terrain
A 20 MW project in the Appalachian region required armored MV cable for direct burial through rocky soil. The armored construction eliminated the need for conduit, reducing civil works cost by $280,000 despite the higher cable price per foot.
Impact on System Design
MV cable selection interacts with several design decisions that solar software must account for:
| Design Decision | Short Run (under 500 ft) | Long Run (over 2,000 ft) |
|---|---|---|
| Conductor Material | Copper (higher ampacity per size) | Aluminum (lower cost per foot) |
| Sizing Driver | Ampacity | Voltage drop |
| Insulation Type | Standard XLPE | TR-XLPE if wet conditions |
| Cost Sensitivity | Low — cable is small portion of BOS | High — cable becomes major cost driver |
| Loss Impact | Minimal (under 0.5%) | Significant (1–3%) |
Request a soil thermal resistivity (thermal rho) survey before finalizing MV cable sizing on large projects. Sandy dry soils can have thermal resistivity values 3–4 times higher than assumed defaults, requiring significant ampacity derating.
Frequently Asked Questions
What voltage is considered medium voltage for solar cables?
Medium voltage for solar applications typically ranges from 1kV to 35kV. The most common voltage classes used in solar farms are 15kV (for projects interconnecting at 12.47kV or 13.8kV) and 35kV (for projects interconnecting at 34.5kV). The specific voltage depends on the utility’s distribution or sub-transmission voltage at the point of interconnection.
How do you size MV cable for a solar project?
MV cable sizing involves three checks: ampacity (the cable must handle the maximum continuous current with derating factors applied), voltage drop (must stay under 3% for most utilities), and short-circuit withstand (the cable must survive fault currents for the duration of protective device clearing time). The most restrictive of these three determines the final cable size.
What is the typical lifespan of MV cable in a solar installation?
Properly installed XLPE-insulated MV cable has a design life of 30–40 years, which aligns well with solar project lifespans. The most common causes of premature failure are improper installation (damaged insulation, poorly made splices), water ingress at termination points, and thermal overloading from undersized conductors or poor soil thermal conditions.
Should I use copper or aluminum MV cable for solar projects?
Aluminum is the more common choice for long MV cable runs in solar farms because it costs significantly less per foot than copper. While aluminum requires a larger conductor size for the same ampacity, the cost savings typically outweigh the increase in cable diameter. Copper is preferred for short runs, space-constrained installations, or when conduit size is limited.
About the Contributors
Content Head · SurgePV
Rainer Neumann is Content Head at SurgePV and a solar PV engineer with 10+ years of experience designing commercial and utility-scale systems across Europe and MENA. He has delivered 500+ installations, tested 15+ solar design software platforms firsthand, and specialises in shading analysis, string sizing, and international electrical code compliance.
CEO & Co-Founder · SurgePV
Keyur Rakholiya is CEO & Co-Founder of SurgePV and Founder of Heaven Green Energy Limited, where he has delivered over 1 GW of solar projects across commercial, utility, and rooftop sectors in India. With 10+ years in the solar industry, he has managed 800+ project deliveries, evaluated 20+ solar design platforms firsthand, and led engineering teams of 50+ people.