Medium Voltage Cable 101: Understanding the Basics and Common Applications

What Is Medium Voltage Cable?

Medium voltage (MV) cable is a type of power cable designed to transmit electricity in the "medium" voltage range, bridging the gap between low-voltage distribution and high-voltage transmission. In most applications, MV cable is used for systems above 1 kV (1,000 volts) up to about 35 kV (35,000 volts). So MV cables are typically rated up to 2.4kV, 5 kV, 15 kV, 25 kV, or 35 kV, depending on cable and system design.

As the bridge between building-level low-voltage wiring (for example, 600 V-class systems) and transmission-level high voltage (35 kV and above), medium voltage cable is used to move power efficiently over moderate distances, feed large loads such as motors and substations, and connect distributed generation sources such as solar farms or wind turbines to the grid.

Because MV systems carry higher energy and operate at higher electric field stresses than low-voltage circuits, they require specialized construction: shielded insulation, controlled stress grading, robust jackets, and carefully designed metallic shielding for grounding and fault current return. These features distinguish medium voltage cable from standard building wire and are essential for reliability and safety over the cable’s service life.

Medium Voltage Cable Construction and Key Components

MV cables are highly engineered. Each layer serves a specific electrical, mechanical, or environmental function. Understanding these layers helps with interpreting datasheets and seeing why so many different constructions exist.

Conductors: Copper vs Aluminum and Stranding Options

The conductor is the current-carrying core of the cable. Two main materials are used:

Copper

Higher electrical conductivity than aluminum
Smaller cross-sectional area for the same ampacity
Excellent mechanical strength and fatigue resistance
Often preferred for critical applications, tight spaces, or frequent terminations

Aluminum

Lower density, significantly lighter than copper
Lower material cost per amp carried
Larger cross section required for equivalent ampacity
Widely used for long feeders, utility distribution, and cost-sensitive projects

Conductor stranding determines how the individual wires (strands) of a conductor are arranged, which also affects cable performance. Most MV cables use one of three configurations:

Concentric Round: Strands are laid in layers and twisted around a central wire in a geometric pattern, which balances flexibility and mechanical strength.
Compressed Stranding: A concentric round conductor is compressed by running it through a die, reducing it's diameter by about 3% and creating a more solid cross-section within the conductor by shrinking gaps between strands.
Compact Stranding: Rather than starting with round strands, each wire is formed into a trapezoidal shape that creates a very tight fit when strands are twisted together, reducing the outer diameter by as much as 10% and creating a near-solid conductor cross-section. Compact stranding is most common in largest MV conductor sizes.

Conductor Shield and Insulation Shield

Medium voltage cables almost always use shielded insulation systems, which is one of the major differences from many low-voltage cables.

Conductor shield (semi-conductive layer)

Applied directly over the conductor, this extruded semi-conductive layer provides a smooth interface between the conductor and main insulation. It:

Eliminates air gaps and sharp edges from individual strands
Ensures a uniform electric field at the inner insulation surface
Reduces local stress concentrations that can trigger partial discharge

Insulation shield (outer semi-conductive layer)

Applied on top of the insulation, this layer:

Forms a controlled interface between the insulation and metallic shield
Allows the electric field to be contained within the insulation system
Provides a reference surface at ground potential when properly bonded and grounded

Together, the conductor shield, insulation, and insulation shield create a coordinated stress-control system. This structure is critical at medium voltage levels, where uncontrolled stress can lead to treeing, partial discharge, and insulation breakdown.

Insulation Materials for Medium Voltage Cables

Two primary insulation materials dominate medium voltage cable design:

Property	XLPE (Cross-Linked Polyethylene)	EPR (Ethylene Propylene Rubber)
Typical operating temperature	~90°C continuous (commonly)	~90°C continuous (commonly), sometimes higher
Dielectric strength	High, very good electrical properties	Very good, often more forgiving under stress
Flexibility	Moderately flexible	Generally more flexible, rubber-like behavior
Thermal aging	Strong performance, good long-term stability	Robust aging characteristics
Moisture resistance	Good, enhanced by design and jackets	Typically very good
Typical applications	Utility feeders, industrial, renewables	Industrial plants, marine, flexible installations

XLPE (Cross-Linked Polyethylene)

A thermoset polyethylene with cross-linking that improves thermal and mechanical properties. XLPE medium voltage cable is widely used for utility distribution, underground feeders, and industrial systems because it combines good electrical performance with a relatively compact diameter.

EPR (Ethylene Propylene Rubber)

A rubber-like thermoset material with excellent flexibility and good dielectric strength. EPR is often used where flexibility, bending, or vibration resistance is a concern, such as in industrial plants and certain marine or mobile applications.

The choice between XLPE and EPR depends on installation constraints, environmental exposure, expected thermal cycling, and preferences for flexibility versus diameter.

Metallic Shields and Concentric Neutrals

Outside the insulation shield, medium voltage cables typically include a metallic shield. This layer serves several functions:

Provides a return path for fault current in case of insulation failure
Keeps the electric field contained within the cable
Maintains the insulation shield at or near ground potential
Reduces electromagnetic interference and improves system grounding

Common metallic shield constructions include:

Copper tape shield

Helically wrapped copper tape around the cable. This provides a continuous metallic path and is often used in utility and industrial applications.

Copper wire shield

Multiple copper wires helically applied, sometimes with an overlying tape. This design can offer good flexibility and fault current capacity.

Concentric neutral (often in utility MV)

Multiple wires applied around the cable, sized to carry a defined portion of system fault current or neutral current. Concentric neutrals are common in distribution cables where the neutral function is critical.

The metallic shield must be properly grounded and bonded according to system grounding practices (solidly grounded, impedance grounded, or ungrounded) to ensure correct operation during both normal and fault conditions.

Outer Jackets and Environmental Protection

Beyond the metallic shield, medium voltage cables are typically finished with an outer jacket. The jacket’s primary role is environmental and mechanical protection.

Common jacket materials include:

PVC (Polyvinyl Chloride)

Flame-retardant formulations widely available
Good general-purpose mechanical and chemical resistance
Often used indoors or where moderate chemical resistance is needed

PE (Polyethylene)

Excellent moisture resistance
Common choice for direct burial and outdoor utility cables
Good dielectric properties and abrasion resistance

LSZH (Low-Smoke, Zero-Halogen) compounds

Reduced smoke generation during fire
Halogen-free, lowering corrosive and toxic gas emissions
Preferred in tunnels, transit systems, data centers, and other life-safety sensitive environments

Jacket selection considers moisture, chemicals, sunlight (UV exposure), mechanical impact, flame spread requirements, and the intended installation method (buried, duct, tray, indoor, outdoor).

Medium Voltage Cable Ratings and Specification Basics

Beyond physical construction, medium voltage cable selection depends on ratings such as voltage class, insulation level, ampacity, and environmental suitability.

Voltage Ratings: How a Medium Voltage Cable Is Rated for Up to Specific kV Classes

Medium voltage cables are specified by system voltage rating, typically in kV (kilovolts). Common classes include:

Nominal System Class	Typical Cable Voltage Rating	Typical Uses
5 kV	5 kV or 8 kV class	Short feeders, small substations, older plants
15 kV	15 kV class	Industrial plants, campuses, utility distribution
25 kV	25 kV class	Longer distribution feeders, medium-size networks
35 kV	35 kV class	Larger campuses, utility feeders, renewables tie-ins

A medium voltage cable is rated for up to a given phase-to-phase voltage, and standards also define the corresponding phase-to-ground and test voltages. Insulation design is coordinated so that the electric stress remains within safe limits under normal operating and overvoltage conditions.

Understanding the difference between system nominal voltage and the cable rating is essential. For example, a 15 kV-class cable may be used on systems with nominal voltages around 13.8 kV phase-to-phase, while being tested and rated to handle higher test levels.

Ampacity and Temperature Ratings

Ampacity is the maximum continuous current that a cable can carry without exceeding its temperature rating, assuming defined installation conditions.

Key aspects include:

Conductor temperature rating

Common ratings: 75°C, 90°C, or 105°C
Many modern MV cables use 90°C as the standard continuous operating temperature

Operating conditions

Soil thermal resistivity for buried cables
Ambient air temperature for tray or free-air installations
Grouping of multiple cables, which affects heat dissipation

Emergency and short-circuit ratings

Short-term overload capabilities for emergency conditions
Short-circuit thermal withstand, often for durations such as 1 second or 0.5 seconds

Ampacity values in datasheets or standards assume specific reference conditions. Real installations often require derating factors for temperature, grouping, and installation method. This is where more detailed questions like how to size medium voltage cable arise, tying system loading and routing conditions back to manufacturer ampacity tables.

Insulation Level and System Grounding

Medium voltage systems can be solidly grounded, impedance grounded, or ungrounded. This has a direct impact on cable insulation requirements.

Common insulation levels include:

100% insulation level

Intended for solidly grounded systems where line-to-ground overvoltages during faults are limited

133% insulation level

Used where system grounding or overvoltage conditions may impose higher stress
Often applied in resistance or reactance grounded systems

173% insulation level

Used for certain ungrounded or resonant grounded systems where line-to-ground voltage can approach line-to-line values under fault or abnormal conditions

Selecting the correct insulation level ensures that the cable can withstand expected overvoltages and fault conditions throughout its life, without excessive stress leading to insulation deterioration.

Environmental and Mechanical Ratings

In addition to electrical ratings, medium voltage cable must fit the installation environment:

Wet vs dry locations

Many modern MV cables are suitable for wet locations, including direct burial
Some insulation and jacket systems are specifically tested for long-term wet operation

Direct burial and duct installation

Cables for direct burial need robust jackets and possibly additional moisture barriers
Duct or conduit installations may emphasize smaller diameter and pulling characteristics

UV and sunlight resistance

Outdoor-exposed jackets require UV-resistant formulations
Markings often indicate “Sunlight Resistant” when tested to appropriate standards

Flame and fire performance

Flame tests determine suitability for specific building or tunnel environments
LSZH jackets may be selected where smoke and toxicity are critical concerns

Mechanical ratings such as impact resistance, crush resistance, and flexibility also influence cable behavior, especially in industrial environments or locations subject to mechanical abuse.

Common Medium Voltage Cable Applications and Use Cases

Medium voltage cables support a wide range of power systems, from public utilities to highly specialized industrial and technological facilities.

Utility Distribution and Substations

In utility networks, medium voltage cables are used to:

Feed underground distribution circuits in urban or suburban areas
Connect substation transformers to medium voltage switchgear
Build loop and ring systems that allow reconfiguration and improved reliability
Tie in reclosers, sectionalizers, and ring main units (RMUs)

Underground MV cable allows utilities to improve aesthetics, protect against weather-related outages, and serve dense load centers with high reliability.

Industrial Plants and Manufacturing Facilities

Industrial complexes often operate large motors, heavy process loads, and distributed substations. Medium voltage cable is commonly used to:

Connect main substations to unit substations across the plant
Feed large motors and variable frequency drives (VFDs) at 4.16 kV, 6.6 kV, 11 kV, or similar levels
Link backup generators or cogeneration units to facility distribution systems
Support critical process lines requiring robust, reliable power

Mechanical robustness, thermal performance, and ease of termination are major considerations in these environments.

Data Centers and Commercial Campuses

Large data centers and multi-building campuses often use medium voltage distribution to improve efficiency and reduce conductor sizes over distance. MV cables may be used to:

Feed unit substations or medium-voltage-to-low-voltage transformers located near load centers
Connect central utility plants to buildings across the campus
Route redundant feeders (A/B systems) for high availability

Data center environments may emphasize LSZH jackets, fire performance, and precise coordination with protection systems.

Renewable Energy and Microgrids

Renewable and distributed generation systems rely heavily on medium voltage cables:

Solar farms

Cables connect inverter skids to collection systems and main substations
Direct burial MV cables are often used in large solar fields

Wind farms

MV cables run from individual turbines to collection points and then to substation transformers

Battery energy storage and microgrids

Medium voltage links connect storage systems, generators, and loads
Cable selection must coordinate with high fault currents and sophisticated protection schemes

Soil conditions, trench layout, and long-term environmental exposure are central considerations in these applications.

Selecting the Right Medium Voltage Cable for the Application

Medium voltage cable designs represent a balance among electrical performance, mechanical strength, installation practicality, and long-term reliability. The wide range of available constructions and materials exists because different power systems and environments emphasize different aspects of this balance. Understanding the main options helps clarify why one project might use a compact copper XLPE cable in duct banks while another uses aluminum EPR with a rugged jacket for buried feeders.

Conductors: Copper vs Aluminum and Their Trade-Offs

The choice between copper and aluminum is one of the most fundamental distinctions in medium voltage cable design.

Copper conductors

Higher conductivity: For the same current, a copper conductor can be smaller in cross-sectional area than aluminum, which can reduce overall cable diameter and ease installation in tight spaces.
Mechanical robustness: Copper handles bending, vibration, and terminations very well. Lugs, connectors, and terminations often experience repeated thermal cycling, and copper’s mechanical properties support reliable long-term connections.
Weight and cost: Copper cables are heavier and typically more expensive per foot, especially at larger sizes. Handling and support structures must account for this weight.

Aluminum conductors

Lower density: Aluminum is much lighter, which can simplify pulling long runs and reduce structural loads on supports or risers.
Cost efficiency: For a given ampacity, aluminum often provides a lower material cost, particularly over long feeder distances or in large-scale projects such as utility distribution and solar or wind collection systems.
Larger size: The larger cross section required for aluminum results in bigger cable diameters. Duct sizing, bending radius, and tray space must accommodate these dimensions.

Different projects may prioritize space-saving and mechanical strength (favoring copper) or reduced weight and material cost (favoring aluminum). Both materials are widely used in medium voltage systems when installed and terminated correctly.

Insulation Systems: XLPE vs EPR

Insulation materials influence electrical behavior, thermal performance, flexibility, and response to environmental stresses.

XLPE (Cross-Linked Polyethylene)

Electrical characteristics: XLPE offers high dielectric strength and low dielectric losses, supporting efficient operation at medium voltage levels.
Compact design: XLPE insulation can achieve required electrical performance with a relatively thin layer, which keeps overall cable diameter smaller.
Thermal and aging behavior: Cross-linking improves thermal stability and resistance to deformation at elevated temperature. XLPE has a strong track record in underground distribution, industrial feeders, and renewable energy projects.

EPR (Ethylene Propylene Rubber)

Flexibility: EPR’s rubber-like nature provides greater flexibility, which is helpful where cables must bend around tight routing constraints or experience vibration and movement.
Tolerance to electrical stress: EPR is often considered more forgiving in the presence of certain field irregularities or minor defects, though proper manufacturing and installation remain essential.
Thermal performance: EPR typically supports similar continuous operating temperatures to XLPE, with some formulations allowing higher short-term or emergency ratings.

In practice, XLPE is frequently selected where compact size and established utility practice dominate, while EPR becomes attractive in environments that value flexibility, vibration resistance, or particular dielectric and thermal characteristics.

Shields and Neutrals: Tape Shields, Wire Shields, and Concentric Neutrals

The metallic shield system in a medium voltage cable is more than a protection layer; it is an integral part of the electrical design.

Copper tape shields

Provide a nearly continuous cylinder of copper around the insulation, forming a smooth, uniform path for electric field control.
Offer predictable fault current capability, as the cross-sectional area of the tape is sized for defined duty.
Create a robust overall structure, though flexibility can be somewhat less than that of wire-based shields in some configurations.

Copper wire shields

Use multiple small wires helically wrapped around the cable to perform the shielding function while improving flexibility.
Allow the number and size of shield wires to be tailored to meet different fault current and shielding requirements.
Are sometimes combined with an overlying tape to blend flexibility with continuity.

Concentric neutrals

Serve as both the return path for unbalanced current and the fault current path in many distribution cables.
Are sized according to utility requirements for neutral and fault current capacities.
Are common in medium voltage cables used in overhead-to-underground transitions and underground distribution networks.

These variations exist so that cable designs can match the way a power system handles ground faults, unbalanced currents, and electromagnetic compatibility. Industrial systems, campus networks, and utility feeders may therefore be built with different shield and neutral strategies, even at similar voltages.

Jackets and Protective Layers: PVC, PE, LSZH, and Beyond

The outer jacket protects the electrically active parts of the cable from the environment and from mechanical damage. Different materials emphasize different protection priorities.

PVC (Polyvinyl Chloride) jackets

Flame-retardant behavior is readily achievable, which is important in many building and industrial settings.
Provide a balance of mechanical protection, chemical resistance, and cost.
Are often used where direct sunlight and severe weather are limited, or where conduit and duct provide additional physical protection.

PE (Polyethylene) jackets

Offer strong moisture resistance, making them common in direct-buried and outdoor utility cables.
Provide good abrasion resistance for installations in ducts or trenches.
Can be UV stabilized for long-term outdoor exposure.

LSZH (Low-Smoke, Zero-Halogen) jackets

Are engineered to release minimal smoke and no halogen acids when exposed to fire, reducing risks to personnel and equipment.
Are often specified in tunnels, transit systems, certain data center areas, offshore facilities, and enclosed public spaces.
May have different mechanical characteristics than standard PVC or PE jackets, which is reflected in handling and installation practices.

By varying jacket materials, medium voltage cables can be adapted to buried solar fields, industrial refineries, tunnels, high-density data halls, and many other environments, each with specific combinations of fire, moisture, chemical, and mechanical requirements.

Insulation Levels and System Behavior

Beyond material type, insulation level is another dimension of MV cable design that reflects the way a system behaves during faults and abnormal conditions.

100% insulation level

Intended for solidly grounded systems where line-to-ground overvoltages are limited.
Represents a balance between sufficient dielectric strength and economical insulation thickness.

133% insulation level

Provides additional margin where ground-fault currents or overvoltage conditions may be more severe.
Often used on systems with impedance grounding or where clearing times may be longer.

173% insulation level

Supports systems in which line-to-ground voltage can approach line-to-line values under fault or abnormal operating conditions, such as some ungrounded or resonant grounded systems.

These gradations exist so that cable insulation can be aligned with expected electrical stresses. The same nominal voltage (for example, a 15 kV-class system) might use different insulation levels depending on grounding philosophy and protection practices.

Taken together, choices in conductor metal, insulation material, shield construction, jacket type, and insulation level illustrate how medium voltage cables are engineered for specific electrical demands and environmental conditions. Each option reflects a set of trade-offs between size, flexibility, thermal performance, fault behavior, fire properties, and long-term durability, which is why medium voltage cable catalogs contain such a variety of constructions rather than a single “one-size-fits-all” design.