Chapter 25

Electrical Power Systems

Objectives

• After reading the chapter and reviewing the materials presented the students will be able to:

• Summarize the process for generating and distributing electrical power.

• Identify various types of cables, boxes, switches, and receptacles.

• Identify electrical wires based on color.• Outline the steps for installation of an electrical power

system in a structure.

Electrical Power Subsystems• A power system supplies electrical energy needed for lights, heating, and cooling units, appliances, and other

mechanical equipment in buildings.• Power generating plant is a facility for manufacturing electrical power.• Transformers at the power generation plant increase the voltage of the electricity so it can be transmitted

efficiently over large distribution lines (fig 25-2, page 429).• Distribution stations convert high voltage electricity from cross country distribution lines to lower voltages for

safe distribution within a city.• A service transformer is an electrical device that lowers the voltage to what is needed in the structure.• A service entrance is that part of the electrical distribution system that connects the electric meter with the

service panel.• A service cable is a heavily insulated wire that carries electricity from the service transformer to the meter base.• The electrical power meter is a device that measures the electrical power used at a specific location.• The service panel is a steel box installed inside a building that contains a main breaker and circuit breakers for

each of the branch circuits within a building.• The main breaker is a switch that shuts off all power to the building.• A branch circuit is an electrical device that consists of all the wiring, electrical receptacles, and lights controlled

by one fuse or circuit breaker.• A general purpose circuit is an electrical circuit for lights and receptacles.• A small appliance circuit is an electrical circuit installed in locations where small appliances are located.• A fixed appliance circuit is an electrical circuit that supplies only one appliance.

Circuit Breakers and Fuses

• Circuit breakers and fuses protect wiring from electrical overload.• Electrical overloads can overheat wires and cause fires.• When too much current is drawn through a fuse, the wire in the fuse

melts and stops the flow of electricity. The fuse has to be replaced.• If excess current flows through a circuit breaker, the breaker

switches off. Power is restored to the breaker by flipping the breaker switch.

• A ground fault circuit interrupter (GFCI) circuit breaker senses current leakages and the circuit breaker disconnects the electricity. GFCI circuit breakers are required on circuits that power receptacles that are near water.

• Circuit breakers or fuses are located in the service panel. Always remove the fuse or turn off the circuit breaker when working on a circuit.

Conductors

• Conductors allow electric current to move freely through them. Copper is commonly used as a conductor.

• Cables consist of two or more insulated wires bundled together and covered by an additional layer of insulation.

• Insulators impede the flow of electrical current and protect electrical conductors. Rubber and plastic are used to insulate wires.

• Insulation on each conductor is color coded based on the conductor’s function (fig 25-9, page 434). Typically white is a neutral wire. No fuses or switches are connected to the neutral wire. Green or bare wire is the ground wire. Black and red wires are the hot wires. Switches and fuses are connected to these lines.

Conductor Protection and Support

• Conductors must be supported and protected.• Wires in residential construction are supported by the framing members

and protected by the plastic cover that encases the wires.• Conduit is metal or plastic tube or pipe used to encase and protect electrical

wire.• Electrical boxes are used to mount devices, conduit, and cables.• Junction boxes are used to house wire connections.• Receptacles or outlets are used to disconnect and connect electrical

equipment to a circuit.• A single pole switch opens and closes one wire in a circuit.• A three way switch controls a load from two points.• A dimmer switch varies current in a circuit to raise and lower the brightness

of the light bulb.• A load is an electrical device that uses electrical energy, such as a light bulb,

motor, or computer.

Installing and Electrical Power System

• The power company is responsible for installing the wire from the pole to the meter.

• An electrical subcontractor installs the meter base, service mast, service head, service panel, and branch circuits.

• The electrical code in most communities is based on the National Electrical Code (NEC).

• Fish tape is a long, flexible metal strip with a hook on one end that is used to pull wire through conduit.

• Cables pass through holes bored in the center of framing members (fig 25-17, page 438).

Summary• A power system supplies electrical energy needed for lights, heating, and cooling units, appliances, and other

mechanical equipment in buildings.• A branch circuit is an electrical device that consists of all the wiring, electrical receptacles, and lights controlled

by one fuse or circuit breaker.• Circuit breakers and fuses protect wiring from electrical overload. Circuit breakers or fuses are located in the

service panel. Always remove the fuse or turn off the circuit breaker when working on a circuit.• A ground fault circuit interrupter (GFCI) circuit breaker senses current leakages and the circuit breaker

disconnects the electricity. GFCI circuit breakers are required on circuits that power receptacles that are near water.

• Insulation on each conductor is color coded based on the conductor’s function. Typically white is a neutral wire. No fuses or switches are connected to the neutral wire. Green or bare wire is the ground wire. Black and red wires are the hot wires.

• Conduit is metal or plastic tube or pipe used to encase and protect electrical wire.• Electrical boxes are used to mount devices, conduit, and cables.• Junction boxes are used to house wire connections.• A load is an electrical device that uses electrical energy, such as a light bulb, motor, or computer.• The electrical code in most communities is based on the National Electrical Code (NEC).• Fish tape is a long, flexible metal strip with a hook on one end that is used to pull wire through conduit.

Home Work

• 1. What are the functions of a power system?• 2. What is the function of circuit breakers and

fuses?• 3.