MV cable is medium voltage cable, a type of power cable designed to carry electricity at voltages greater than 1,000 volts and less than 100,000 volts (100 kV). These cables are the workhorses behind electrical distribution systems that connect utility substations to commercial buildings, industrial facilities, hospitals, and large residential developments. If you’ve ever seen thick cables running underground or through conduit in a large building’s electrical room, there’s a good chance you were looking at MV cable.
How MV Cable Is Defined
The ANSI C84.1 standard defines medium voltage as anything greater than 1,000 volts and less than 100 kV. The National Electrical Code (NEC) similarly classifies medium voltage equipment at 1,000 volts and higher. Before the 2014 edition of the NEC, that threshold started at 600 volts, so older references and documentation may use a different cutoff.
In practice, most MV cables you’ll encounter are rated at common distribution voltages: 5 kV, 15 kV, 25 kV, or 35 kV. A 15 kV rated cable, for example, is one of the most widely used for commercial and industrial power distribution.
What’s Inside an MV Cable
MV cables are significantly more complex than standard low-voltage wiring. Each cable is built from several distinct layers, and every layer serves a specific purpose.
At the center is the conductor itself, typically copper or aluminum. Surrounding the conductor is the conductor shield, a thin semiconducting layer that smooths out the electrical field between the metal conductor and the insulation. Without it, tiny air gaps and surface irregularities would create concentrated electrical stress points that could damage the insulation over time.
Next comes the insulation layer, which is the thickest component and prevents the high voltage from escaping the conductor. Outside the insulation sits the insulation shield, another semiconducting layer that performs the same smoothing function on the outer surface of the insulation. A metallic shield, usually copper tape or concentric wires, wraps around the insulation shield. This metallic layer must be grounded, and it serves several critical functions: it confines the electrical field within the cable, eliminates shock hazards on the cable surface, and reduces the risk of fires, explosions, and radio interference. Finally, an outer jacket protects everything from moisture, chemicals, and physical damage.
This shielding system also prevents corona discharge, a phenomenon where electrical energy ionizes the air around the cable and produces ozone. Left unchecked, ozone accelerates the aging of the insulation and can eventually cause the cable to fail.
XLPE vs. EPR Insulation
The two dominant insulation materials in MV cables are cross-linked polyethylene (XLPE) and ethylene propylene rubber (EPR). Your choice between them affects the cable’s durability, flexibility, and performance in specific environments.
XLPE is widely used and generally less expensive. However, it has a well-documented vulnerability called water treeing, where moisture slowly penetrates the insulation in a branching, tree-like pattern. Over time, this lowers the insulation’s electrical strength and can cause premature failure. Newer formulations called TR-XLPE (tree-retardant XLPE) were developed to address this, but water treeing remains a consideration for cables installed in wet or underground environments.
EPR insulation is largely immune to water treeing. It’s also more chemically and thermally stable, and it offers far greater resistance to corona and partial discharge damage. Where XLPE has very poor discharge resistance, some EPR formulations are nearly completely discharge-resistant. EPR cables can also handle higher temperatures, with ratings reaching 140 to 150°C compared to the lower limits of XLPE-based cables.
EPR also provides greater high-frequency loss characteristics, which sounds like a drawback but is actually protective. It helps absorb fast electrical transients generated by variable speed drives and power switching equipment, reducing the risk of premature failure in connected motors and transformers. The tradeoff is that EPR cables tend to cost more and have a slightly larger diameter than XLPE cables of the same voltage rating.
MV-90 and MV-105 Ratings
When shopping for MV cable, you’ll see designations like MV-90 and MV-105. These refer to the maximum continuous conductor temperature the cable can handle during normal operation. MV-90 cable is rated for 90°C, while MV-105 cable handles 105°C with an emergency rating that can reach 140°C.
The higher temperature rating of MV-105 cable means it can carry more current for a given conductor size, or it can operate in hotter environments without derating. MV-105 cables typically use EPR insulation, while MV-90 cables are more commonly insulated with XLPE.
You’ll also see insulation level percentages listed alongside the voltage rating, such as “15 kV 100%” or “15 kV 133%.” The 100% insulation level is standard for systems where a ground fault will be cleared quickly (within one minute). The 133% level provides a thicker insulation wall for systems where faults may persist longer before being cleared, giving the cable extra margin.
NEC Requirements for Installation
The National Electrical Code added Article 311 in its 2020 edition specifically to cover the use, installation, construction, and ampacity (current-carrying capacity) of Type MV cables. Before that, medium voltage cable requirements were scattered across different parts of the code. Article 311 brought them together and established clear standards for insulation thickness, jacket specifications, and minimum conductor sizes for both shielded and nonshielded cables.
In subsequent code cycles, the NEC moved these requirements to Article 315, expanding coverage to include specific rules for cable joints and terminations. These are critical connection points where improper installation can lead to insulation failure, so the added requirements address how to properly join and terminate MV cables to maintain the integrity of the shielding system.
All metallic insulation shields on MV cable must be connected to a grounding electrode conductor, grounding busbar, equipment grounding conductor, or grounding electrode. An ungrounded shield defeats its purpose and can create a serious shock hazard.
Where MV Cable Is Used
MV cable shows up anywhere electricity needs to travel at distribution-level voltages. Common applications include underground utility feeders running from substations to neighborhoods, power distribution within large commercial buildings and campuses, industrial plants where heavy equipment requires higher voltages, and renewable energy installations connecting solar arrays or wind turbines to the grid.
These cables can be installed in conduit, direct-buried in the ground, run through cable trays, or pulled through underground duct banks. The installation method affects which jacket type and cable construction you need. Cables rated for direct burial, for instance, require more robust moisture protection than those installed in dry indoor cable trays. Look for designations like “Sun Res” for sunlight resistance or “TC-ER” for exposed-run cable tray applications on the cable’s listing to confirm it’s suitable for your specific installation.