How the Distribution System Works: Technical Explanation Made Simple
The distribution system begins where the transmission system ends—at the distribution substation where voltage is stepped down from transmission levels (69-765 kV) to distribution levels (4-35 kV). From these substations, primary distribution feeders branch out like arteries, carrying three-phase power along main roads and corridors. These primary circuits typically operate at voltages between 4,160 and 34,500 volts, with 12,470 volts (often called "12.5 kV") being the most common in North America. At this voltage, the electricity is still far too dangerous for direct use but low enough to be carried on wooden poles through neighborhoods.
The three-phase primary system uses three separate conductors carrying alternating current with peaks spaced 120 degrees apart. This arrangement provides constant power delivery and creates the rotating magnetic fields that industrial motors require. Large commercial and industrial customers often connect directly to all three phases, while residential areas typically use single-phase taps from the primary system. These single-phase laterals branch off main feeders, carrying one phase plus a neutral conductor to serve groups of homes.
Distribution transformers, those ubiquitous gray cylinders on poles or green boxes in yards, perform the final voltage reduction. A typical pole-mounted transformer takes 7,200 volts (phase-to-neutral voltage on a 12.5 kV system) and steps it down to 240/120 volts for household use. Inside, the high-voltage winding connects to the primary distribution line through fused cutouts—protective devices that disconnect the transformer if internal faults occur. The secondary winding provides 240 volts between two "hot" conductors, with a center tap creating the neutral that allows both 240-volt and 120-volt service.
From the transformer, secondary distribution lines carry power to individual homes. In overhead systems, triplex cable (two insulated hot conductors wrapped around a bare neutral messenger wire) spans from pole to house. The neutral conductor is grounded at regular intervals and at each service entrance, creating multiple return paths for current and enhancing safety. Underground residential distribution uses direct-buried cables or conduits, with pad-mounted transformers serving multiple homes through underground secondary networks.
The distribution system must accommodate highly variable loads that change constantly throughout the day. A residential feeder might see morning peaks as people prepare for work, afternoon lulls, then evening peaks as air conditioners run and dinners cook. The system is designed with enough capacity to handle expected peaks plus reasonable growth, but this means conductors and transformers operate well below capacity much of the time. This inherent inefficiency is necessary to maintain reliability during high-demand periods.
Protection and switching equipment throughout the distribution system allows isolation of faulted sections while maintaining service to other customers. Reclosers—essentially circuit breakers that automatically attempt to restore service after detecting faults—protect main feeders. Since many distribution faults are temporary (like tree branches touching lines), reclosers try closing several times before locking out. Sectionalizers downstream of reclosers count operations and open during dead time if faults persist, isolating problem areas. Fuses provide economical protection for laterals and individual transformers.
Modern distribution systems increasingly incorporate automation and communication. Automated switches can reconfigure feeders to restore service to unfaulted sections within minutes of an outage. Fault indicators with radio communication help crews quickly locate problems. Smart meters at customer premises provide real-time consumption data and can report outages instantly. This evolution toward a "smart grid" improves reliability and operational efficiency while enabling new services like time-based rates and integration of distributed generation.