Low Voltage and Medium Voltage Power Distribution

Planning for the electrical power required to run a facility is a significant consideration for any project. Whether retrofitting an existing facility or selecting equipment for a new location, the components and configurations are chosen based on the client’s technical, economic, and performance goals.

Coordinating with the local power utility can affect many choices in the design phase, including proximity to major transmission lines to address capacity constraints. The energy required to power a facility often leads to a conversation about whether low voltage or medium voltage power distribution is the most suitable.

Understanding Low and Medium Voltage Power Distribution

Voltage is the pressure of energy in an electrical circuit that pushes the current through a conducting loop. It is typically expressed in volts or kilovolts (kV). The power plant generates electricity at high voltages and transports it over great distances through the power grid.

In the US, the National Electrical Manufacturers Association (NEMA) and the National Electrical Code (NEC) provide standards to determine voltage classifications:

• High voltage refers to voltages between 115 and 230kV, and extra-high voltages range up to 765kV. 500kV is the transmission standard for long-distance transmission on the high-voltage grid in the US. When it reaches its destination, electricity is stepped down to medium and low voltage using transformers.
• Medium voltage (MV) refers to voltages between 1 and 35kV and allows electricity to be carried from substations to transformer stations, which then supply power to specific destinations. Medium voltage installations also power large energy consumers in industrial and manufacturing operations.
• In the US, low voltage (LV) generally applies to voltages up to 1kV, supplying most household applications and commercial consumers. What is considered low voltage varies worldwide, as different countries use local standards.

Low Voltage Power Distribution vs Medium Voltage Power Distribution

As the amount of power consumed increases, the alternating current transmission at higher voltage levels is more efficient because there is significantly less loss in the lines when the current flow is kept at a minimum. Note the inverse relationship between voltage and current in the following examples, all at 2.58MW and 80% PF:

2.58mW @ 34,500V (MV) = 54A
2.58mW @13,800V (MV) = 135A
2.58mW @ 12,470V (MV) = 149A
2.58mW @ 4,160V (MV) = 448A
2.58mW @ 2,400V (MV) = 776A
2.58mW @ 480V (LV) = 3,879A

At the illustrated power level, connection and distribution at LV would require large conductors to handle the current. Distribution at MV levels requires much less copper throughout, with less distribution line loss at that given power.

Key Considerations in Electrical Power Distribution Design

Due to the lower currents, medium voltage power distribution conductors can be significantly smaller in gauge than LV-rated conductors. Long MV cable runs do not pose significant losses. Some applications can realize savings in copper costs for distribution runs due to the smaller gauge required at MV. Flexibility of cables and easier handling for transportable operations can also be part of the selection criteria.

Machine-specific requirements limit some options that may be available otherwise. Additional equipment for LV drops may be necessary for machinery available only with LV motors, controls, or other constraints. Likewise, power-intensive OEM equipment may be supplied only for medium voltage power distribution feeds. In all cases, optimal and safe operations require a coordinated assessment of plant resources and a system analysis in the design phase to prevent costly mistakes.

Historically, deciding between low-voltage vs. medium-voltage equipment in the 300 to 1000 HP range has posed a challenge due to a cost gap. The power consumption of LV solutions in this HP range can pose ongoing operating expenses due to inefficiencies in distribution and peripheral equipment for harmonic noise mitigation.

Medium voltage power distribution solutions for the range require more space, costly components, and additional support personnel training. However, innovation and increasing competition are two factors helping to bring MV options within reach to bridge this gap.

How Medium and Low Voltage Power Distribution Impact Design

Due to the voltage levels applied, internal construction specifications are more rigorous for MV designs. For a given HP, the MV motor is larger and more costly but may be projected to have a longer service life.

Price points for medium voltage power distribution systems, including variable frequency drives, are falling due to manufacturing efficiency, innovative designs, and vendor desire to open more markets. While LV VFD price points can be lower, peripheral costs can negate that advantage.

Several vendors offer MV VFD solutions with harmonic noise mitigation inherent in their design. Comparable LV solutions operate at higher current levels and require more complex configurations to mitigate harmonic distortion.

Benefits of Medium Voltage Power Distribution for Large Operations

Medium voltage power distribution suits industrial and manufacturing facilities with high energy demands. Many MV motors use a copper bar rotor design and are thought to be more serviceable than LV aluminum bar motor coils. At high power levels, MV systems reduce current draw significantly, which reduces conductor size, copper use, and energy loss.

Additional benefits include:

• Lower Line Losses and Greater Efficiency. Medium voltage power distribution systems are more efficient over long distances, reducing power loss through transmission and lowering overall energy waste across the facility.
• Longer Equipment Lifespan. MV motors and switchgear often have longer service lives due to lower heat buildup and reduced current stress.
• Smaller Conductor Size Means Material Cost Savings. Because MV systems operate at lower currents, they require smaller conductors. This reduces copper usage and installation costs, especially in large plants with extended cable runs.

Benefits of Low-Voltage Power Distribution in Industrial Settings

Peripheral equipment and safety protocols for low-voltage systems are often simpler. LV systems may also be more suitable for facilities where power demand is relatively low or compatibility with existing equipment is a concern.

Additional benefits include:

• Lower Upfront Equipment Costs. LV components like breakers, transformers, and panels are typically less expensive than MV equivalents.
• Simplified Installation and Maintenance. LV systems are easier to install and maintain without requiring specialized gear or highly trained personnel.
• Wider Equipment Compatibility. Many industrial machines, controls, and OEM systems are readily available in low-voltage configurations.

What’s Next in Electrical Power Distribution Design?

One challenge the industry is currently working on is the incorporation of information technology advancements into a modernized power grid. New technology can improve medium voltage power distribution by delivering electricity using more sustainable sources in a more reliable and efficient system.

The National Institute of Standards and Technology (NIST) has worked with leaders in the industry, government, and academic institutions to develop a framework for moving forward with smart grid technology. Their latest release includes numerous updates, including guidance on cybersecurity practices and interoperability.

Expert Power System Analysis

From new equipment planning and installation to difficulties with the power company, having a trusted expert on your side ensures your interests are protected. Quad Plus specializes in low and medium-voltage power distribution systems and provides standard and custom power system studies to target and correct any issues you may have.

To speak with an experienced power systems expert, Contact Quad Plus now.