Service Entrance

Wires called service conductors extend from the main power line or transformer to the building's service equipment (Fig. 28-2). The portion of the overhead service conductors leading from the nearest utility pole to the building is called the service drop. In a residence, you may see three cables twisted together, or in older houses, running separated. In underground systems, the portion of the service conductor extending from the main power line or transformer to the building is called the service lateral. The section of the service conductor that extends from the service drop or service lateral to the building's service equipment is called the service entrance conductor. A grounding rod or electrode is firmly embedded in the earth to establish a ground connection outside the building.

The network of wires that carry electrical current through a building stretches out from a single center, the main service panel, which is usually located where the power line enters the building. In a residence, the

Transformer vault with switches, transformers, fuses

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Main building switchboard with switches, circuit breakers, metering

Distribution panels

Large motors

Lighting & appliance panels

Branch circuit wiring

Lighting & appliance panels

Branch circuit wiring

Lighting, receptacles, small motors & controls

Figure 28-1 How electricity is distributed through a building.

main service panel is usually located in the basement or in a utility room. In larger buildings, this equipment is usually located in a switchgear room near the entrance of the service conductors, and is mounted on a main switchboard. The main service panel is located as close as possible to the service connection to minimize voltage drop and to save wiring. Secondary switches, along with fuses and circuit breakers for controlling and protecting the electrical power supply to a building, are also in the main service panel.

To protect firefighters, the main service panel has a main disconnect (or service) switch. The disconnect switch must be in a readily accessible spot near where the service enters the building. Access to the main disconnect switch must not be blocked. In a residential building, the main disconnect is usually the main switch or breaker of the panel board. This may be a lever disconnect, with an external handle controlling contact with two main fuses in a cabinet. When the lever is pulled to the "off" position, the main power supply is shut off. Some residential systems have a pullout block arrangement. In this type of disconnect switch, the main cartridge fuses are mounted on one or two nonmetallic pullout blocks, and pulling firmly on the handgrips removes the blocks from the cabinet and disconnects the power. Other systems use a single main circuit breaker, which shuts off all power when switched to the "off" position. Some homes are not required by the National Electrical Code (NEC) to have a single main disconnect, and use a multiple breaker main. All breakers in the main section, up to a maximum of six, must be switched off to disconnect service. It is important to maintain easy access to the main disconnect.

When the voltage used by the building is different from the service voltage, a transformer is used to transform alternating current (AC) of one voltage to AC of another voltage. Transformers are not used with direct current (DC), which can only be used at its original voltage. Transformers may be pole or pad mounted outside a building or in a room or vault within the building. Step-down transformers lower voltage, and step-up transformers do the opposite. Typically, a transformer will step down incoming 1460V service to 480V for distribution within the building. Another transformer then steps down 480V to 120V for receptacle circuits. Low or secondary voltages used in buildings include 120, 208, 240, 277, and 480 volts.

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