Building Electrical Design Principles--The Building Electrical System

There are many possible electrical systems that can be used to distribute power in a building. Typically, system design begins with selection of a building system voltage, which is dependent on sizes and types of the connected loads, utilities near the building, local codes and ordinances, economics, and safety.

It’s more economical to distribute power at high voltage.

Amperage determines conductor size, and higher voltages mean lower amperage, which allows use of smaller conductors. Use of high voltages, however, introduces more complex safety issues, so use of high voltages in buildings is reserved for equipment with heavy loads. Larger installations have greater power requirements and thus require higher building system voltages. Smaller installations such as residences use lower voltage systems to avoid ex posing occupants to safety issues related to higher voltages.

Typical types of building electrical systems are introduced in the following sections.

.Residential Systems Normally, a 120/240 V, three-wire, single-phase service en trance serves a residence. In some cases where it’s the only service available, a 120/208 V, three-wire, single-phase service entrance is used. Power from a ground- or pole-mounted utility transformer located outside the building is brought to the building service equipment through underground or overhead service entrance lines.

From a single panelboard rated from 100 to 200 A or more, power is distributed throughout the residence through branch circuits. These branch circuits originate at the panelboard, are protected by overcurrent protection in the panelboard, and terminate at outlets serving appliances, equipment, and lighting. In large residences, a set of feeders may extend from the main panelboard and bring power to one or more sub-panelboards located at a re mote area of the building. Branch circuits originating at the sub panelboard feed outlets in these outlying areas.

Multifamily Dwellings In multifamily dwelling units (e.g., apartments, condominiums), power is brought from a utility transformer to the building service equipment. It’s then divided at a main distribution panel, passes through individual meters, and is distributed to the individual dwelling units through feeders. Each dwelling unit is served by a separate panelboard located in the dwelling unit. Branch circuits extend from a panelboard to feed outlets within the unit. A house panelboard has branch circuits that serve common areas (e.g., corridors, laundry rooms, lobbies, and so forth).

Imgs **1 through **22 show examples of residential electrical installations.

Img.4 Power pedestals prepared for installation. (--)

Img.5 Underground service entrance for a multifamily dwelling unit.

Img.6 A 120/240 V-150 A, underground service entrance for a single-family residence. (--)

Img.1 Underground electrical distribution lines serving a residential development being buried in a trench.

Img.2 Underground distribution lines readied for connection to exterior transformer. (--)

Img.3 A power transformer (right) steps down distribution voltage to building system voltage (120/240 V). The power pedestal (at center right) is where the connection is made before being fed to the residence through the service entrance conductors. ( --)

Img.7 A service entrance with the electrical meter and main disconnect (below meter and above circuit breakers).

Circuit breakers are properly marked on the panelboard face.

Img.12 Branch circuit cables extend through the inside of the wall (behind the panelboard).

Img.8 Panelboard with face removed. ( --)

Img.9 A close-up view of the inside of the panelboard.

Conductors from the meter connect to the two lugs in the top of the photo. Power is fed through two bus bars connected to the lugs and then through each circuit breaker and the dark-colored ungrounded conductors (wires). On the right side of the panelboard, the white grounded (neutral) conductors and bare grounding conductors connect to the neutral/grounding bus. (--)

Img.10 A main ground extends from the panelboard to the water service pipe and to a grounding rod (not shown).

Img.11 Branch circuit cables shown before installation of the panelboard.

Img.13 Branch circuit conductors are fed to outlets. (--)

Img.14 Circuit cables can pass through floor and ceiling joists. (--)

Img.18 Exposed lighting fixture.

Img.19 Roughed-in receptacle box.

Img.20 Exposed box in finished drywall with unattached receptacle and faceplate.

Img.21 Covered convenience receptacle.

Img.15 Rough wiring of branch circuits extends through framing to outlets.

Img.16 Roughed-in lighting box. (--)

Img.17 Roughed-in recessed light fixture. (--)

Img.22 Ganged switches with faceplate removed. (--)

Img.23 Switchboard for a manufacturing facility.

Img.24 Service entrance conductors and feeder conductors encased in conduit and a wire gutter. Note the grounding wire (bottom wire).

Img.25 Ground wire connection to grounding rod that extends into earth on other side of foundation wall.

Commercial/Industrial Systems

Large commercial and industrial facilities have large and varied power requirements that necessitate different types of systems for different building occupancies. Essentially there are so many variations that there is no standard type of system.

In a typical large building, electrical power is provided to a transformer located outside the building or it enters a trans former vault located at the service level in the building. A transformer vault is a basement- or ground-level structure or room in which power transformers, network protectors, voltage regulators, circuit breakers, meters, and so on are housed.

Three-phase power normally enters the building with three un grounded (hot) conductor lines sharing a common neutral conductor. A transformer steps down power to voltages below 600 V (e.g., 480 Y/277 V, three phase). At the stepped-down level, the electrical power passes out of the transformer to the main switchboard serving the building.

The switchboard is located in a separate room, near the transformer or adjacent to the transformer vault. At the switch board, power passes through meters and circuit breakers or fuses as it’s divided into primary circuits and is then carried by feeder busways or sets of large conductors into the different parts of the building. Several feeder circuits provide power to power distribution panelboards throughout the building, where it’s used to run large motors powering mechanical equipment (e.g., HVAC fan motors and plumbing pumps) and elevators.

Others feeder circuits feed lighting and appliance panelboards where the power is divided to smaller circuits serving lighting and convenience outlets.

Primary feeders are separated into high-power and low power bus risers, busways, or sets of conductors that supply different system voltages to the many levels or areas of the building. As the bus riser or busway extends to the electrical room of each floor or area, a feeder circuit is split off to supply the high-power panelboard (480/277 V). In the high-power panel, additional branch circuits are divided out. For example, a three-phase circuit may be established to drive air-handling units in the mechanical room. Single-phase, 277 V circuits pro vide power to overhead fluorescent fixtures. Another feeder circuit is taken off of the high-power panel by a local three-phase step-down transformer every three floors, which steps down voltage to a 208/120 V for low-power circuits. Low power is then distributed from a panelboard through branch circuits to outlets in the individual spaces (e.g., offices, classrooms, and so on). Closet transformers may be located throughout the building to accommodate localized power requirements. Imgs **23 through **38 show various components of commercial and industrial systems.

Img.26 Distribution panels with fused disconnects.

Img.27 Open distribution panels exposing two sets of fuses.

Img.28 A close-up of a single distribution panel with fused disconnect.

Img.29 Open distribution panel with three cartridge fuses protecting a three-phase feeder (three ungrounded conductors).

Img.30 A 480 Y/277 V-400 A, three-phase panelboard providing three-phase power to equipment (e.g., milling machines, lathes, machining centers, and so on) or 277 V single-phase power for lighting.

Img.31 Three feeders from the main distribution panel (see Img **26) connected to lugs in the panelboard (see Img **30).

The lugs connect to the three bus bars to which circuit breakers are connected. The fourth conductor (left side of photograph) is the grounding conductor, which connects the panelboard ground to the main distribution panel near the service entrance.

Img.32 The panelboard with circuit breakers installed.

Img.33 A close-up view of the three bus bars.

Img.34 An individual three-phase breaker protects three conductors on a three-phase circuit.

Img.35 Conductors for each circuit extend through conduits from the main panelboard (under construction).

Img.36 Circuits to equipment are controlled by heavy-duty switches.

Img.37 Armored cable carrying circuiting from panelboard to electrical boxes; a commercial installation.

Img.38 Electrical boxes receiving conductor-carrying armored cable.

Prev: System and Circuit Grounding
Next: Conductor Requirements

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