The secondary distribution system is that part of the electrical power system between the primary system and the customer’s service entrance. This system includes distribution transformers, secondary circuits (secondary mains), customer services (consumer drops), and watthour meters to measure customer power consumption. Secondary voltages are provided by distribution transformers that are connected to the primary system and sized for the voltages required for specific parts of the service area.
Heavy industries or mines, which require the most power, are usually supplied with three-phase power by privately owned or corporate industrial substations. They are typically located on land owned by those companies and close to the equipment being served. These substations are capable of providing a wide range of voltages from the 12.47- to 13.8-kV transformers located there.
Factories, high-rise buildings, shopping centers, and other large power consumers are furnished with three-phase power from load substations in the 480-V to 4.16-kV range. Many commercial and light industrial customers are supplied by 208Y/120-V or 480Y/277-V three-phase, four-wire systems.
The most reliable service in densely populated urban business and commercial areas is provided by grid-type secondary systems at 208Y/120 V or by spot networks, usually at 480Y/277 V. Spot networks are usually located in urban areas near high-rise office buildings, factories, hospitals, and dense commercial properties such as shopping malls, which have high load densities. In these networks the transformers and their protective equipment are typically placed adjacent to or within the properties being served.
Secondary network systems are used in about 90 percent of all cities in the United States with populations of 100,000 or more and in one-third of all cities with populations between 25,000 and 100,000. Despite the generally high reliability of these systems, many facilities such as hospitals, computer centers, and chemical or pharmaceutical industries performing critical processes that cannot tolerate power outages have backup power sources. These include standby or emergency generators and/or storage batteries together with automatic switching so that service to critical loads can be maintained if the normal utility supply is interrupted. Some of these facilities have automatic switching that puts alternative utility power sources online without human intervention.