Understand how to size wire and cables in medium-voltage electrical systems

Electrical conductor insights

• The medium-voltage (MV) cable delivering power must be rated for its voltage level and demand.
• NFPA 70: National Electrical Code (NEC) defines conductor ratings, use, ampacity and many other aspects.

Medium-voltage (MV) electrical systems, which are considered to be around 1 kilovolt (kV) to 35 kV, are not typically seen at the consumer level. ANSI and IEEE standards define voltage classifications as follows:

• Low-voltage (LV): up to 600 volts (V).
• MV: between 600 V and 69 kV.
• High-voltage: between 69 and 230 kV.

MV systems are not typically seen at the consumer level. It is often used for larger buildings with significant power demand such as health care, manufacturing or data centers. It is also commonly seen on commercial and institutional campuses with multiple buildings where the owner wishes to own and maintain the power system. transmission or for bigger motors/loads.

This is because systems that have a higher power demand will have a lower current draw in a MV system than a LV system. This results in much smaller voltage drops and reduced sizes of cables, which reduces cost and distribution space needs. However, the MV cable delivering the power to the load must be rated for its voltage level and demand.

For applications requiring or benefitting from the use of MV, it is important to ensure proper MV wire sizing and compliance with codes and standards. Although the 2023 edition of NFPA 70: National Electrical Code (NEC) is not a design book, it contains minimum requirements to prevent overheating and fire. The minimum requirements for conductors rated from 2,001 V to 35,000 volts alternating current (Vac), per NEC Article 315, covers the use, installation and ampacities for MV conductors, cable, cable joints and cable terminations.

Construction specifications for MV cables

There are two types of Type MV cables: Type MV-90 and Type MV-105. The numbers following MV indicate the maximum operating temperature of the cable. Type MV-90 cables are rated for a maximum 90°C and Type MV-105 cables are rated for a maximum of 105°C. Type MV-105 cables have a higher current carrying capacity but tend to cost more than type MV-90 cables. These two types must have a thermoplastic or thermosetting insulation and an outer covering consist of jacket, sheath and/or armor.

Figure 1: Single conductor medium-voltage cable simple construction. Courtesy: CDM Smith

Type MV cables can be made up of the conductor, insulation, shield, armor and jacket as seen in Figure 1.

Conductor: The conductor carries the current. The conductor material is allowed to be aluminum, copper-clad aluminum or copper per NEC Article 315.12(B). Unlike LV systems, MV conductors are almost always required to be stranded.

Insulation: The insulation prevents the flow of current to any adjacent ground. The insulation must be thermoplastic or thermosetting per NEC Article 315.10(A), such as ethylene propylene rubber (EPR) or cross-linked polyethylene (XLPE). EPR is a copolymer of ethylene and propylene. It is typically used in the industry due to its flexibility and lower thermal expansion. XLPE is a thermoset insulation material. Compared to EPR, XLPE is slightly cheaper and has lower dielectric losses but is not as easy to install.

The thickness of the insulation is dependent on the protective device upstream of the cable and the materials of the MV cable.

For shielded insulated MV cables, there are three levels of insulation requirements: 100%, 133% and 173% insulation level. The application of each level of insulation are depenedent on the upstream relay protection. Cables with 100% insulation level, per NEC Article 315.10(C)(1), can only be used in situations where the system is provided with relay protection that can clear ground faults and de-energize the faulted section within a minute or less.

Cables with 133% insulation level can be used where the faulted section can be de-energized within one hour even if it cannot meet the clearing time for the 100% insulation level requirements.

Cables with 173% insulation level can be used only if:

• Qualified personnel maintain and supervise the condition of the cables in industrial facilities.
• The fault clearing time required for the 133% insulation level requirements cannot be met.
• The facility requires an orderly shutdown to protect equipment and personnel.
• The faulted section can be de-energized in an orderly shutdown.

The thickness, measured in millimeters and/or in mils (one-thousandth of an inch), of the insulation for shielded insulated MV cables can be found in NEC Table 315.10(C).

Shield: The shield confines the electrical field, symmetrically distributes the electrical stress, reduces electromagnetic interference (EMI) noise and protects the power system by conducting fault current to ground. The following lists the different types of shields. Often, these various shields are applied in combination to provide the facility with the best protection suited for the application.

• A metallic shield, i.e., foil shield, is typically a thin, light layer of either copper or aluminum wrapped around the cable core. The foil shield provides 100% cable core coverage. Due to its thinness, it can be harder to work with. In most cases, to ground the entire foil shield, a drain wire is used.
• A metallic tape shield involves wrapping the cable core with overlapping layers of aluminum or copper tape. The tape provides an effective shield against external EMI and can offer physical protection to the cable core. Often, metallic tape shield is used with another shield type to provide better overall cable protection.
• A wire braid shield is typically made of aluminum, tinned copper or bare copper wires braided into a mesh around the cable core. The braided shield does not provide 100% cable core coverage due to the gaps, but it does allow for better cable flexibility while offering high levels of shielding against EMI. However, it is made of small wires, meaning a cable with a braid shield tends to be bulkier and more costly.
• Extruded insulation shielding is a methodology for providing a shield by adding semiconductive materials to the cable insulation to provide the inherent shielding properties. This typically eliminates the need for additional shielding layers, which simplifies the cable construction process.
• A semiconductive shield is made of a layer of semiconductive material, typically in the form of tapes or screens, which is applied over the insulation. This type of shielding helps distribute the electric field evenly across the cable’s surface and reduces the risk of electrical stress concentration.

Not all MV cables are shielded. For nonshielded insulated MV cables, the conductors are limited to voltages rated 2,001 V to 5,000 V and can only be used in industrial establishments where maintenance and supervision is done only by qualified persons per NEC Article 315.44. These type of cables are much cheaper as there are less materials and typically smaller in size compared to shielded insultated MV cables.

Since it is nonshielded, the MV cable does not limit voltage stress on the insulation, which can quickly degrade the life span of the insulation making it dangerous to anyone who comes into contact with the energized cable if the insulation is damaged. The requirements for the thickness of the insulation and jacket of a nonshielded insulated MV cable can be found on NEC Table 315.10(B). Nonshielded insulated MV cables are not typically recommended to be used in water treatment plants.

Armor: The armor is a layer of metal, such as steel, wrapped around the outside of the cable. The armor physically protects the cable and is often used in applications where the MV cable is potentially exposed to physical damages. It is not to be confused with the jacket or shield. Typically the armor is not necessary, especially if the MV cable will be protected by a metal conduit.

Jacket: The jacket is the outermost layer that provides environmental protection to the cable. The jacket is often made of semiconductive material that can be flame retardant, oil- and sunlight-resistant.

Medium-voltage cable markings

Figure 2: MV single insulated copper cable in a pad mounted transformer. Courtesy: CDM Smith

Like LV conductors, all Type MV cables are required to have markings. Per NEC Article 315.16, all Type MV cables must have the following markings:

• The maximum rated voltage.
• Letter(s) indicating the type of wire or cable.
• The manufacturer’s name, trademark or identifiable marking by the organization responsible for the product.
• The conductor AWG size or circular mil area. Figure 2 shows an example of markings on a MV cable in a pad mounted switch. Type MV cables can be marked with optional additional information such as the cable materials or whether the cable is oil- or sunlight-resistant.

If the MV conductor is intended to be used as ungrounded conductors, the cable needs to be clearly distinguishable from grounded or grounding conductors per Article 315.14. The distinguishable markings shall not conflict with any of the four required markings as listed previously.

There are four different allowable methods to mark Type MV cables. They can be used alone or in combination. These are:

• Surface markings: Where the cable is durably marked on the surface. The conductor size needs to be repeated in intervals of 24 inches. All other required markings can be repeated in 40-inch intervals.
• Marker tape: Where tape containing the required markings is located within and along the complete length of the cable.
• Tag marking: Where a printed tag with the required markings is attached to the cable reel.
• Optional marking of wire size: Applicable only for multiconductor type metal clad cables (i.e., MV cables with armor). The wire size of the conductor can be marked on the surface of the individual insulated conductors in the multiconductor.

Conductor ampacities

The minimum size of the MV cable is limited by the voltage level per NEC Article 315.12(A). Refer to Table 1 for the minimum size requirement for MV cables per typical voltage levels seen in a water treatment plant. For a complete list of minimum sizes, refer to NEC Table 315.12(A).

Figure 3: Medium-voltage switchgear at a water treatment plant in Georgia. Courtesy: CDM Smith

This is a case where the NEC must not be used as a design guide. Even though the NEC has a minimum size requirement, some manufacturers do not make MV cables as small as the size listed in the table. For a complete design, availability of cable sizes must be checked and confirmed with MV cable manufacturers to ensure availability of the MV cable.

Per NEC Article 315.32, Type MV cables, with different requirements and exceptions, can be installed in wet/dry locations, raceways, cable trays (per NEC Article 392), exposed runs (per NEC Article 305.3), corrosive conditions, direct buried (per NEC Article 315.36) or paralleled (per NEC Article 310.10[G]).

How and where the MV cable is to be installed influences the ampacity of the cable. In situations where there is more than one calculated or tabulated ampacity for the given length of the circuit, the lowest value shall be used. The ampacity for a conductor rated 2,001 to 35,000 V can be found in the NEC Table 315.60(C)(1) to Table 315.60(C)(20) or calculated under engineering supervisions per NEC Article 315.60(B).

The ampacity in the NEC Table 315.60(C)(1) to Table 315.60(C)(20) are provided with the following considerations. Any changes to these considerations will require adjustment.

• Grounded shields: The ampacities detailed in NEC Tables 315.60(C)(3), 315.60(C)(4), 315.60(C)(15) and 315.60(C)(16) only apply for cables with shields grounded at one point. If the shield is grounded at multiple locations for these cables, the ampacity needs to be adjusted with consideration to the temperature increase caused by the shield currents.
• Ampacity in air: The following is mainly applicable for NEC Table 315.60(C)(1) to Table 315.60(C)(10). The ampacities in those tables are based on the ambient air temperature being 40°C. In the cases where the ambient temperature is rated anything other than 40°C, the ampacity needs to be adjusted based on ambient temperature.
• Ampacity in underground (ducts or direct buried): The following is applicable for Table 315.60(C)(11) to Table 315.60(C)(20). The ampacities in those tables are based on ambient earth temperature as 20°C, load factor as 100%, soil thermal resistance (i.e., rho) as 90, the minimum burial depth to the top of the electrical ducts or cable is in accordance with NEC Article 305.15 and the maximum burial depth to the top of the electrical ducts is 30 inches and for direct buried cable, 36 inches.

Adjustment factors for conductors

Temperature greatly affects the ampacity of the conductors. Ambient temperatures different from the considerations listed previously will need to consider ampacity adjustment factors as listed in NEC Table 315.60(D)(4). Temperature increase caused by paralleling conductors per NEC Article 310.10(G) will also require adjustment factors. With multiple current carrying conductor(s) in a raceway, the temperature in the raceway will increase, which will decrease the ampacity level of the conductor. This means that the conductor will need to be increased to the next conductor size.

Figure 4: A one-line diagram shows the electrical system at a water treatment plant. The switchgear feeds power into two utility switches with a radial loop connection, providing power to several utility transformers powering various buildings and well sites. Courtesy: CDM Smith

The burial depths of all direct-buried cables rated 2,001 to 35,000 V shall be in accordance with NEC Article 305.15, which states that direct buried cables shall be a minimum of 30 inches deep for circuit voltages over 1,000 to 22,000 V and 36 inches deep for circuit voltages over 22,000 to 40,000 V.

Depending on the conduits and location of the raceway, the minimum burial depth may vary per NEC Table 305.15(A). If the burial depths of the raceway are increased in part(s) of the total run, given that the part(s) is only 25% of the total run, then an adjustment in ampacity is not required.

However, if the part(s) of the total run is more than 25% and the burial depths are deeper than 30 inches for electrical ducts or 36 inches for direct buried cables, then an ampacity derating factor of 6% per 1 foot increase in depth needs to be considered for all values of rho. Although NEC Annex B is not a part of the NEC requirements, it provides more details on ampacity calculation and considerations for varying values of rho and load factor should a more specific engineering calculation be required. In most cases, the tables provided in the NEC will cover all areas in the United States, as the rho for the average soil is 90.

Do you have experience and expertise with the topics mentioned in this content? You should consider contributing to our WTWH Media editorial team and getting the recognition you and your company deserve. Click here to start this process.