Power System Voltages

The System One-Line on Part-1, shows an Incoming utility primary service feeding different types of distribution equipment at each of the various utilization voltages necessary to power the actual loads. 

The One-Line illustrates a number of voltage transformations and is a good example of the types of choices and challenges a power systems design engineer faces today.

Voltage Classifications

ANSI and IEEET standards define various voltage classifications for single-phase and three phase systems. The terminology used divides voltage classes into:

■■ Low voltage

■■ Medium voltage

■■ High voltage

■■ Extra-high voltage

■■ Ultra-high voltage

Table 1.1-3 presents the nominal system voltages for these classifications.

Table 1.1-3. Standard Nominal System Voltages and Voltage Ranges (From IEEE Standard 141-1993)

The 2014 National Electrical Code has ushered in a change to the definition of low voltage. The NEC elevated the maximum voltage threshold for this category from 600 V maximum to 1000 V maximum. This was done to accommodate the growing solar market where voltages up to 1000 V are becoming more commonplace. 

In general, the voltage classes above medium voltage are utilized for transmission of bulk power from generating stations to the utilities substations that transform it to the distribution voltage used on their system.

A power system design engineer should attempt to familiarize themselves with the application of all equipment available in the various voltage classes. This is particularly true if they are involved in designing industrial facilities or campus arrangements that may be served by a utility at medium or high voltage.

Incoming Service Voltage

When designing a new power distribution system, the engineer needs to be knowledgeable of the local utility requirements including the service voltage that is available to be provided for their client. Meeting with the utility’s customer service representative responsible for the installation site, early in the design process, can help set expectations for both parties and avoid subsequent delays.

Most utilities will require a load letter when requesting a new service or upgrade to an existing utility service. The letter must include calculated values for the types of continuous and noncontinuous loads that will be served.

Article 220 of the NEC covers branchcircuit, feeder and service calculations. It also includes references to other articles that pertain to specific types of installations requiring special calculation considerations. 

The determination of the utility service voltage is driven by a combination of factors including the engineers initial load letter, prevailing utility standards and the type of facility being served. 

Excessively high megawatt loads such as those required by large wastewater treatment plants or complex process facilities like petrochemical refining will typically exceed the utility’s infrastructure to serve the end customer at low-voltage. In these instances, a medium voltage service at 34.5, 33 kV, 26.4 kV, 13.8 kV, 13.2 kV, 12.47 kV or 4.16 kV will be mandated. Extremely large loads may even involve a utility interconnect at the 69 kV or high voltage level. 

The System One-Line on Part-1 is an example of a power system for a hypothetical college campus with a design load over 8 megawatts at a 0.8 power factor. This would require a Utility service of over 400 A at 13.8 kV. 

The most common service voltage arrangements are in the low-voltage range (<600 Vac). Normal residential services are at 240/120 three-wire, (two phases each at 240 and a Neutral Conductor). Connection from each 240 V phase to neutral provides 120 V for the lighting and plug loads.

A three-phase, four-wire low-voltage service is generally provided for commercial customers. It includes a neutral and may be provided at 208/120 Vac wye, 240/120 Vac wye or 480 /277 Vac wye. 

Typical applications for the commercial category of three-phase low-voltage services are small commercial buildings, department stores, office buildings, kindergarten through 12th grade schools and light manufacturing facilities. 

There are a number of other older service configurations utilized in rural locations such as Delta Hi Leg. These were used as an inexpensive way to supply 240 V three-phase and 240 V or 120 V single-phase from a single pole mount transformer. 

As a general rule, the serving utility will offer a basic service option that is outlined in the tariff documents that have been approved by the governing authority or agency that regulates the utility. This basic service option is one that minimizes the utility costs and best accommodates their system requirements. 

The utility may alternately offer to upcharge the client for extending or reinforcing cable connections to a location on their overhead or underground grid where they can supply the service the user is requesting. In major cities where the serving utility utilizes underground spot networks, the option to select a voltage other than that available is either limited or extremely expensive. 

Utility metering requirements vary from one serving entity to another and are more complex for medium-voltage switchgear used as service equipment. 

Commercial low-voltage utility metering (<600 V) is more common and includes standardized designs that can be provided in various lowvoltage switchboard and drawout switchgear configurations.