MV-105 Medium Voltage Cable Overview (5kV–35kV)

Medium voltage (MV) cable carries power at voltages from 2,001 volts to 35,000 volts — the range between standard building wire and high-voltage utility transmission lines. MV cable is a fundamentally different product from low-voltage wire like THHN or NM-B. Every medium voltage cable requires multiple engineered layers — conductor shielding, rated insulation, insulation shielding, and a metallic shield — to safely contain the electric field and prevent partial discharge that would destroy the cable over time. This guide covers the construction, voltage classes, insulation types, and selection criteria for MV-105 rated medium voltage cable from 5 kV through 35 kV.

What Does "MV-105" Mean?

The designation MV-105 is a UL® cable type defined in UL 1072 (Standard for Medium-Voltage Power Cables). The "MV" stands for medium voltage and the "105" indicates the cable's maximum continuous conductor temperature rating: 105°C (221°F). This temperature rating applies to normal operating conditions — emergency overload and short-circuit ratings are higher.

UL 1072 also defines other MV cable types based on temperature rating. MV-90 is rated for 90°C continuous operation. MV-105 cables use insulation materials — typically EPR (ethylene propylene rubber) — that can withstand the higher 105°C operating temperature, giving them an advantage in high-ambient-temperature environments or applications with frequent overload cycling, such as oil and gas facilities, industrial plants, and power generation stations.

NEC® Article 311 (formerly Article 328 prior to the 2023 edition) governs the installation of Type MV cable. NEC® Table 310.60 provides ampacity ratings for medium voltage conductors at various installation conditions.

MV Cable Construction: Layer by Layer

Medium voltage cable construction is significantly more complex than low-voltage wire. Each layer serves a specific electrical or mechanical purpose, and every layer must be properly specified and manufactured to ensure reliable long-term performance. From the center outward, here is what makes up an MV-105 cable:

1. Conductor

The central current-carrying element. MV cables use either copper or aluminum conductors, typically in Class B stranded construction per ASTM B8 (copper) or ASTM B231 (aluminum). Compact stranding (Class C per ASTM B496) is also available and reduces the overall cable diameter by approximately 10%, which can be significant in conduit fill calculations. Conductor sizes for MV cable typically range from #2 AWG through 1,000 KCMIL, though larger sizes are available for special applications.

2. Conductor Shield (Strand Shield)

An extruded semiconducting layer applied directly over the conductor. This layer fills the gaps between individual strands and creates a smooth, uniform surface that distributes the electric field evenly around the conductor. Without the strand shield, the electric field would concentrate at the tips of individual wire strands, creating corona discharge (partial discharge) that degrades the insulation over time.

3. Insulation

The primary dielectric barrier that contains the voltage. Insulation thickness is determined by the cable's voltage class and insulation level (100%, 133%, or 173%). Common MV insulation materials include EPR (ethylene propylene rubber), XLPE (cross-linked polyethylene), and TR-XLPE (tree-retardant XLPE). Each material has distinct properties — detailed in the insulation section below.

4. Insulation Shield

An extruded semiconducting layer applied over the insulation. Like the conductor shield, its purpose is to create a smooth, uniform interface so the electric field terminates evenly across the insulation surface rather than concentrating at imperfections. The insulation shield must be strippable (removable without damaging the insulation) at splice and termination points. This layer is critical — a missing or defective insulation shield is one of the most common causes of premature MV cable failure.

5. Metallic Shield

A grounded metallic layer that serves three functions: it confines the electric field within the cable, provides a ground reference for the insulation shield, and may carry fault current depending on system design (though the equipment grounding conductor typically carries the majority of fault current). Common metallic shield types include:

The metallic shield must be properly grounded at both ends (or at one end with the other end isolated, in single-point grounding configurations) to function correctly. An ungrounded or floating shield defeats the cable's ability to contain the electric field and creates a shock hazard.

6. Overall Jacket

The outermost layer provides mechanical protection, moisture resistance, and chemical resistance. PVC (polyvinyl chloride) is the most common jacket material for MV-105 cable, offering good general-purpose protection, sunlight resistance (when formulated with carbon black), and flame retardance. LLDPE (linear low-density polyethylene) jackets offer better moisture resistance for direct burial. CPE (chlorinated polyethylene) jackets provide superior chemical and oil resistance for industrial environments.

7. Armor (Optional)

Some MV cables include an interlocking galvanized steel or aluminum armor layer between the metallic shield and the jacket (or as the outermost layer). Armored MV cable is specified where the cable requires mechanical protection — direct burial without conduit, areas subject to physical damage, or installations where rodent protection is needed. Ramcorp stocks armored and unarmored MV-105 cable in single-conductor and 3-conductor configurations.

Voltage Classes: 5 kV Through 35 kV

Medium voltage cable is manufactured in discrete voltage classes defined by industry standards. Each voltage class specifies the insulation thickness, test voltages, and BIL (basic impulse insulation level) the cable must withstand. The voltage class you need is determined by the system operating voltage — not the cable-to-ground voltage.

15 kV is the most commonly specified voltage class because it covers the 12.47 kV and 13.8 kV distribution systems used by most electric utilities and large commercial/industrial facilities in North America. Ramcorp stocks 15 kV MV-105 cable in sizes from #2 AWG through 750 KCMIL in both single-conductor and 3-conductor configurations.

Insulation Levels: 100%, 133%, and 173%

Within each voltage class, MV cable is available at different insulation levels that determine the insulation wall thickness. The insulation level is selected based on how quickly the system's protective relaying clears a ground fault.

133% insulation level is the most widely specified for industrial and commercial applications because it provides additional dielectric margin for systems where ground fault clearing time may exceed one minute — a common scenario in industrial plants using high-resistance grounding. The cost difference between 100% and 133% insulation is modest relative to the total installed cost of medium voltage cable, so many engineers specify 133% as a standard practice.

Insulation Thickness by Voltage Class (EPR or XLPE)

1 mil = 0.001 inch. Values per ICEA S-93-639 / NEMA WC-74 and AEIC CS8.

Insulation Types: EPR vs. XLPE vs. TR-XLPE

The three primary insulation materials used in MV-105 cable each offer different performance characteristics. The choice of insulation affects flexibility, moisture resistance, electrical properties, and long-term reliability.

EPR (Ethylene Propylene Rubber)

EPR is the most common insulation for MV-105 rated cables. It is a thermoset elastomer that provides excellent flexibility, moisture resistance, and resistance to electrical treeing (a degradation mechanism where microscopic tree-shaped channels grow through the insulation under sustained voltage stress). EPR's inherent flexibility makes it easier to handle, pull, and terminate in the field — a significant advantage on large conductor sizes where cable stiffness becomes a practical concern. EPR insulation supports a continuous operating temperature of 105°C, which is why most MV-105 cables use EPR.

XLPE (Cross-Linked Polyethylene)

XLPE is a thermoset insulation created by cross-linking polyethylene under heat and pressure. It has a lower dielectric constant and lower dielectric loss than EPR, making it slightly more electrically efficient — particularly in long cable runs and high-voltage applications. However, standard XLPE is more susceptible to water treeing (moisture-driven insulation degradation) than EPR, which limits its expected service life in wet environments unless a tree-retardant formulation is used. XLPE is stiffer than EPR, which can make field handling more difficult on larger sizes. Standard XLPE is typically rated for 90°C continuous operation (MV-90), though some formulations support higher temperatures.

TR-XLPE (Tree-Retardant XLPE)

TR-XLPE addresses the water-treeing vulnerability of standard XLPE by incorporating additives that retard the growth of water trees. It retains XLPE's favorable electrical characteristics (low dielectric constant and loss) while significantly improving wet-aging performance. TR-XLPE has become the dominant insulation choice for utility distribution cables that will be direct-buried or installed in wet environments. It is typically rated for 90°C continuous (MV-90), though 105°C formulations exist.

Conductor Options: Copper vs. Aluminum

Both copper and aluminum conductors are widely used in medium voltage cable. The choice depends on ampacity requirements, space constraints, weight considerations, and total installed cost.

Copper conductors offer higher ampacity per unit area (approximately 1.6 times that of aluminum for the same cross-section), smaller cable diameter for a given ampacity, better corrosion resistance, and easier termination with standard compression connectors. Copper is the standard choice for industrial and commercial installations where conduit space is limited or where the higher reliability of copper terminations justifies the premium.

Aluminum conductors cost significantly less per ampere of current-carrying capacity and weigh roughly one-third as much as copper for equivalent ampacity. The trade-off is larger cable diameter (typically two AWG sizes larger than copper for equivalent ampacity), which requires larger conduit, and the need for aluminum-rated connectors and proper surface preparation at terminations. Aluminum is the standard for utility distribution, where the cost savings on long runs outweigh the larger cable diameter.

Stranding Classes

MV cable conductors are available in several stranding configurations per ASTM standards:

Single-Conductor vs. Multi-Conductor MV Cable

Single-conductor MV cable consists of one insulated and shielded conductor per cable. Three individual cables are pulled together to form a 3-phase circuit. This is the most common configuration for utility distribution and large industrial feeders because it allows each phase to be independently sized and routed, and it simplifies splicing and termination.

Three-conductor (3/C) MV cable bundles three individually insulated and shielded conductors — and often a ground conductor — under a single overall jacket or armor. The 3/C configuration reduces the number of cable pulls, saves conduit space, and provides a more compact installation. Armored 3/C MV cable is commonly specified for industrial and commercial applications where the cable will be installed in cable tray, direct buried, or routed through areas subject to mechanical damage.

Ramcorp stocks both single-conductor and 3-conductor MV-105 cable in 5 kV, 8 kV, and 15 kV configurations with EPR insulation and PVC jacket.

Standards and Compliance

Medium voltage cable manufacturing and testing is governed by multiple overlapping standards. Understanding which standards apply ensures you specify a cable that meets both the code authority's requirements and the utility or facility owner's performance expectations.

Cables listed to UL 1072 and manufactured to ICEA S-93-639 meet the baseline requirements for most applications. Utility and critical-infrastructure projects often require compliance with the additional AEIC CS8 (EPR) or AEIC CS9 (XLPE/TR-XLPE) specifications, which impose tighter manufacturing tolerances and additional factory testing.

Common Applications

Medium voltage MV-105 cable is used wherever electrical power must be distributed at voltages above 2,000 volts. The most common applications include:

Utility primary distribution: Underground residential distribution (URD), commercial/industrial service entrance, and feeder circuits from substations to transformers typically operate at 12.47 kV or 13.8 kV (15 kV class cable). Utilities installing underground distribution have largely standardized on 15 kV cable with 133% insulation level.

Industrial facilities: Refineries, chemical plants, steel mills, and manufacturing plants use MV cable to feed large motors, transformers, and switchgear. EPR-insulated MV-105 is the preferred choice in industrial environments because the 105°C rating provides margin for high ambient temperatures and cyclic loading, and EPR's flexibility simplifies routing through congested industrial cable tray systems. Oil and gas facilities are particularly heavy users of MV-105 cable due to the high-temperature and hazardous-area requirements.

Power generation: Power plants — fossil fuel, nuclear, and renewable — use MV cable extensively for generator leads, station service distribution, and auxiliary power circuits. IEEE 1580 provides specific guidance for MV cable installation in generating stations.

Mining: Surface and underground mining operations require MV cable for draglines, shovels, conveyors, and portable substation feeders. Mining MV cable is typically specified with more flexible stranding (Class M) and ruggedized jackets to withstand the demanding physical environment.

Renewable energy: Wind farms use MV cable (typically 34.5 kV / 35 kV class) for collector circuits connecting individual turbines to the project substation. Solar farms use MV cable for inverter-to-transformer connections and AC collection systems.

Installation Considerations

Minimum Bend Radius

MV cable has larger minimum bend radii than low-voltage cable due to the thicker insulation and shield layers. Exceeding the bend radius can crack the insulation shield, create voids in the insulation, or damage the metallic shield — any of which can lead to premature failure. General guidelines per ICEA and IEEE 1580:

Pulling Tension

Maximum pulling tension is limited by both the conductor's mechanical strength and the cable's sidewall bearing pressure (SWBP) in conduit bends. For copper conductors, the maximum pulling tension is typically 0.008 pounds per circular mil of conductor area. For aluminum, it is 0.006 pounds per circular mil. Exceeding pulling tension limits can stretch the conductor and damage the insulation system. Always calculate pulling tension before the pull, especially for long or complex conduit runs.

Splicing and Termination

Every MV cable splice and termination must properly manage the electric field at the point where the cable's insulation shield is removed. This requires a stress cone or geometric stress relief device — typically a pre-molded or cold-shrink component — to grade the voltage transition from the shielded cable section to the unshielded termination point. Improperly terminated MV cable will experience partial discharge at the shield cutback point, leading to insulation failure. MV splicing and termination should only be performed by qualified personnel using manufacturer-approved kits.

Shield Grounding

The metallic shield must be grounded at each termination and splice point. For short cable runs, both ends of the shield are typically grounded (solid grounding). For longer runs, single-point grounding (one end grounded, the other end isolated with a shield voltage limiter) may be used to eliminate circulating currents in the shield that reduce ampacity. The grounding method should be specified by the system engineer based on cable length, load current, and system configuration.

Quick Selection Guide

1. Determine the voltage class — Match the cable voltage class to your system operating voltage. For a 12.47 kV system, specify 15 kV class cable. For a 4.16 kV system, specify 5 kV class cable.
2. Select the insulation level — 100% for solidly grounded systems with fast fault clearing. 133% for resistance-grounded systems or where fault clearing may exceed 1 minute. 173% for ungrounded delta systems.
3. Choose the insulation type — EPR (MV-105) for industrial and high-temperature applications. TR-XLPE for utility distribution in wet soil. XLPE for dry environments where low dielectric loss matters.
4. Size the conductor — Use NEC® Table 310.60 ampacity tables for the installation method (direct buried, duct bank, cable tray, or conduit), applying correction factors for ambient temperature, mutual heating, and number of conductors.
5. Specify the metallic shield — Copper tape for general industrial. Wire shield or LC shield for direct burial or where higher fault current capacity is needed.
6. Select the jacket — PVC for general purpose and sunlight resistance. LLDPE for direct burial moisture resistance. CPE for chemical/oil exposure.
7. Determine if armor is needed — Specify armored cable for direct burial without conduit, cable tray in areas subject to physical damage, or environments with rodent concerns.

Frequently Asked Questions

What is the difference between MV-90 and MV-105?

The difference is the continuous conductor temperature rating. MV-90 cable is rated for 90°C continuous operation, while MV-105 is rated for 105°C. The higher temperature rating of MV-105 provides greater ampacity for a given conductor size and more thermal margin in high-ambient-temperature environments. MV-105 cables typically use EPR insulation, which inherently supports the higher temperature, while MV-90 cables may use XLPE or TR-XLPE insulation.

Can I direct-bury MV cable without conduit?

Yes, provided the cable is rated for direct burial and meets NEC® cover (depth) requirements per Article 300.50. Single-conductor MV cable rated for direct burial can be installed without conduit at the minimum depths specified — typically 30 inches for 0–22 kV circuits and 36 inches for 22–40 kV circuits, though local amendments may require more. Armored cable provides additional mechanical protection for direct burial installations. The cable's jacket must be rated for wet locations and direct earth contact.

What is the minimum conductor size for medium voltage cable?

NEC® Article 311 requires a minimum conductor size of #2 AWG copper or #1 AWG aluminum for MV cable. However, many manufacturers and specifications begin at #4 AWG or #2 AWG for 5 kV cable and #2 AWG or #1/0 AWG for 15 kV and above, depending on the mechanical requirements of the cable construction.

Why does MV cable need a metallic shield?

The metallic shield is required to safely confine the electric field within the cable and provide a ground reference for the insulation shield. At voltages above 2,000 volts, the electric field around the conductor is strong enough to cause partial discharge (corona) in any air gaps or surface imperfections. The semiconducting insulation shield, in contact with the metallic shield, ensures the entire outer surface of the insulation is at ground potential, eliminating the voltage gradient that would cause corona. The metallic shield also provides a path for fault current during a ground fault — though in most systems, the equipment grounding conductor carries the majority of fault current, with the shield assisting depending on system design and shield sizing.

How long does MV cable last?

Properly manufactured, installed, and maintained MV cable has a design life of 30 to 40 years. Some installations have exceeded 50 years of service. The primary factors that shorten service life are moisture ingress (particularly with non-tree-retardant XLPE insulation), excessive operating temperature, physical damage during installation, and poor splicing or termination workmanship. EPR insulation generally has superior wet-aging characteristics compared to standard XLPE.

What is the difference between 5 kV and 15 kV cable in a 4.16 kV system?

In a 4.16 kV system, 5 kV cable is the technically correct voltage class. However, some engineers specify 15 kV cable for 4.16 kV systems when they anticipate a future voltage upgrade, when they want extra dielectric margin, or to standardize on a single voltage class across a facility. The cost premium for 15 kV cable over 5 kV cable is modest for small conductor sizes but increases with conductor size due to the thicker insulation wall.

Do I need shielded cable below 5 kV?

NEC® generally requires shielding on all insulated conductors operating above 2,000 volts, with a few exceptions for non-shielded MV cable listed under specific conditions (typically 5 kV and below, in dry locations, where the cable is not directly buried). In practice, virtually all MV cable installed today above 2 kV is shielded. The risk of running unshielded MV cable — partial discharge, unreliable fault detection, and surface voltage gradients — makes shielding the default choice even where code technically permits an exception.

Related Resources

• How to Choose the Right Cable for Your Project
• AWG Wire Gauge Guide: Sizes, Ampacity & Selection
• How to Read a Cable Print Legend: Markings, Codes & What They Mean
• UL Listings for Cable: What They Mean & Why They Matter
• Wire & Cable for Oil & Gas

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Disclaimer: This guide is provided for informational purposes only and is not installation advice. It does not constitute professional electrical, engineering, or code-compliance advice. Installing wire & cable can be dangerous and pose a risk of possible electric shock or other hazards. Building codes, NEC editions, and local amendments change periodically. Always consult a licensed electrician and your local authority having jurisdiction (AHJ) before specifying or installing cable. Images are for illustration purposes and may not reflect actual installed products.

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