A building's electrical system has three parts: service (where the electricity enters and is regulated), distribution (the conductors that carry the current to all parts of the building), and circuits (where the current is utilized); and each may be classed as small, medium, or large, as described in the sections that follow.
These systems typically have a service of 120/240 volts at 60-200 amps and one panel box of circuit breakers or fuses, with possibly an adjacent panel for electric heating.
A small electric system rarely has any large motors, three-phase wiring, or extensive distribution networks; and convenience outlets and lighting fixtures in each space are usually on the same circuit. Such systems are typically found in residences and small commercial buildings.
These systems are typically regulated by a main switchboard near the service entry and several smaller panel boxes elsewhere in the building. The service, which may be as high as 600 volts and 800 amps, may include three-phase wiring, large feeder conductors located in fireproof enclosures, convenience outlets and lighting fixtures on separate circuits, and a large motor or two having its own circuit. Some systems may have remote electrical closets with stepdown transformers and telecom switchboards.
Medium electrical systems are found in apartment complexes, motels, recreation centers, and small- to medium-sized commercial buildings.
These systems may serve acres of floor area. The primary service is usually 2,400, 4,160, 7,200, or 13,200 V, and it typically passes through a large stepdown transformer before entering extensive regulating equipment housed in a large mechanical room. Distribution systems include long feeders, extensive laterals, and large electrical closets containing small stepdown transformers and panelboards throughoutthe architecture; and large motors operate elevators, central HVAC equipment, and heavy business or industrial machinery. Circuitry wiring is typically laid above suspended ceilings, under access floor panels, or in wall-mounted raceways.
In large electrical systems, feeder and branch conductors are placed in protective enclosures that require ample chase space for full-length maintenance, heat removal, and future upgrading. Today's large systems also require extensive grounding and power conditioning devices to maximize clean electron flow in every conductor.
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