Secondary distribution systems provide electrical energy at the utilization voltage levels from distribution transformers to customer meters. Typical secondary voltages in the United States include 120/240 V for residential use, 208Y/120 V for residential and commercial use, and 480Y/277 V for industrial and high-rise commercial use.
In residential areas, 120/240 V single-phase, three-wire service is commonly used for lighting, outlets, and large appliances. Urban areas with high-density loads often use 208Y/120 V three-phase, four-wire service for lighting and motor loads. High-rise buildings and industrial areas typically use 480Y/277 V three-phase, four-wire service to support heavy electrical equipment. Proper design of distribution systems ensures that customer voltage levels stay within American National Standards Institute (ANSI) limits. Load-tap-changing transformers and voltage regulators are employed to maintain voltage within these limits.
The individual distribution transformer per customer is common in rural areas. This system uses a dedicated transformer for each customer, suitable for long distances between customers. A common secondary main system connects multiple services to a primary feeder through one or more transformers. It takes advantage of diversity among customer demands, allowing for a smaller transformer capacity compared to individual transformer systems. Secondary networks or grids are used in urban areas with high load densities. They are highly reliable and consist of interconnected feeders from multiple transformers. They operate continuously; faults are isolated by network protectors. A spot network consists of a secondary network that serves a single, concentrated load, such as a high-rise building. Spot networks are fed by two or more primary feeders through network transformers. They ensure high service reliability and flexibility.
Secondary distribution systems are crucial for maintaining reliable and efficient power delivery, catering to diverse customer needs across various regions and applications.
