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Difference Between High, Medium and Low Voltage Classifications and How They Relate to Industrial Generators

Voltages Divided into Classifications

Switching and Distribution Equipment Room
High, medium, and low voltage are the terms we hear the most when talking about voltage classifications. From an international standpoint, these classifications and ranges change depending on where you live. In the United States, the National Electrical Code (NEC) and the National Electrical Manufacturer's Association (NEMA) have guidelines and standards that cover all voltage classifications. The American National Standards Institute (ANSI) oversees the creation, promulgation, and use of thousands of guidelines and standards that affect businesses. Each industry complies with applicable regulations. 

Both the ANSI and the NEC code are publications that are purchased. The Electrical Engineering Portal (EEP) supplies a breakdown of ANSI standards C84.1-1989. This document divides voltages into five classifications. These classifications can be combined into the categories below:
  • High (HV), Extra- High (EHV) & Ultra-High Voltages (UHV) - 115,000 to 1,100,000 VAC
  • Medium Voltage (MV) - 2,400 to 69,000 VAC
  • Low Voltage (LV) - 240 to 600 VAC
Generac issued a white paper titled Medium Voltage On-Site Generation Overview. The white paper compares NEC to ANSI Standards. It sites the following NEC voltage standards:
  • High Distribution - 1000 to 4160 volts
  • Medium Distribution - 50 to 1000 volts
  • Low Distribution - 0 to 49 volts
The above lists illustrate the classification of voltage level changes depending on the governing authority. Generac states that generators less than and equal to 600 volts are medium-voltage and generators greater than 600 volts are considered high voltage. Generators producing 4160 volts are common in many industries for large motors that require high voltage. The backup generator supplies voltage to an individual grid. 

Commonly stocked generator voltages are 4160 VAC, 480 VAC, 12,470 VAC, and 13,800 VAC, When power fails to an industrial facility, the backup generator supplies power to distribution and control panels for continued operations. The higher voltages from the generator are stepped down with transformers. The below content supplies information on each category of information.

The content in this document is for informational use only. Always consult with a certified professional when designing and working on electrical equipment. Never work on energized circuits or perform duties that you are not qualified for.

High, Extra-High and Ultra-High Voltages

High and extra-high voltages are associated with supply transmission from the power plant. The reason for transmitting power at high and extra-high voltage levels is to increase efficiency. The lower current accompanying the high voltage transmission allows for the use of thinner lighter-weight cables. This reduces the cost in the tower and electrical line construction. High voltages range from 115,000 to 230,000 VAC and Extra-High voltages range from 345,000 to 765,000 VAC.Caterpillar 13,800 VAC Generator

The United States transmits up to 500,000 volts on the high voltage grid. High voltages require specialized switching and distribution panels. The control rooms have redundant switching capabilities. They can be controlled remotely or placed in a manual for maintenance and testing of individual supply systems. Sub-stations provide stepped down voltage distributed to localized areas. Ultra-High voltages are voltages that are over 765,000 to 1,100,000 VAC. China is using the highest voltage transmission at 800,000 VAC. They are developing a 1,100,000 VAC system using cables rated at 1,200,000 VAC today.

Medium Voltages and Industry 

Large industrial complexes and factories that require a substantial amount of power often utilize medium supply voltages. Electrical variational analysis dictates that the voltage is inversely proportional to amperage. This means that when the voltage is increased amperage is decreased to complete the operation. 

Motors and electrical equipment designed to operate with higher voltages use less electricity and are more economical to operate. Most primary sub-stations do not receive more than 35,000 VAC from utility supply. The primary sub-station can supply stepped down power to secondary sub-station(s) or to a single building.

The secondary sub-station distributes power received from the primary sub-station. Secondary sub-stations can have step-down transformers to further step down the power for distribution to a control panel for distribution throughout the facility. The sub-stations are generally located in areas that can serve one or more buildings on the property.

Aluminum Company of America (ALCOA) Warrick Operations is an example of a large industry that consumes massive amounts of power. They are located in Southern Indiana and boast a self-contained power plant. They generate electricity by use of coal-fired power plant located on the Ohio River. They process aluminum ingots into rolled aluminum sheets to be used by factories that require aluminum can stock. The ingots are melted in large electric melting furnaces and then are processed through a series of operations to obtain the correct stock thickness.

Any factory that uses medium voltage supply to a sub-station requires emergency or backup power supply. It is not uncommon to see generators that supply 13,800 VAC. The voltage supply is perfect for small and medium voltage sub-stations and secondary sub-stations. With proper generator support, the complex can continue to operate during power outages. Offered in a variety of design styles including installed, sound attenuated enclosure, and portable units. Portable units are enclosed in sound attenuated enclosures on a trailer pulled by a semi-tractor. 

Low Voltage Supply and Controls

Generator with Voltage Warning Signs
Low voltage has multiple meanings in the electric/electronic world. A common rule of thumb is that anything below 600 volts is considered low voltage. Factories that use automation can use multiple voltages. Dividing the electrical use into supply and controls aids in understanding the usage. Each division performs a mission critical to the operation to the factory. Both must be working for production.

Factories that require medium or high voltage supply from the electric utility can have a dedicated sub-station. These substations step down voltages levels and distribute to buildings throughout the property.

However, not all factories require high or medium voltages. Some require low voltages of 240, 480, or 600 VAC from utilities. In this instance, power is routed directly the distribution system of the plant. 

A system or machine that uses low voltage to operate higher voltage equipment are the basics for a control system. Programmable Logic Controller (PLC) is commonplace in these systems. The PLC receives inputs from sensors via the Input portion of the I/O. Outputs are calculated and sent out through the output section of the I/O. Both inputs and outputs are 12 or 12 VDC depending on system design.

The output can be routed to a relay with DC coil and AC contacts. When the relay receives the DC signal, its contacts close. This energizes the equipment or component until the trigger signal is removed by the I/O. 

All factories require power. When utility power is lost, the industry is shut-down without the properly sized backup generator. We offer a wide range of styles of generators that can satisfy most needs. Our pre-owned generators pass a 31-point inspection prior to sales. Go to Inventory for a list of in stock generators. We can often ship a generator within 24-hours of purchase.




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