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Chapter 25: PRINCIPLES OF ELECTRICITY
Summary
This chapter is the first of several that deal with building electrical systems. As noted by the authors,
electricity is the most prevalent form of energy in today’s buildings and knowledge of its properties,
delivery, and usage is beneficial to the building designer.
Electricity is characterized as a form of energy that is not terribly useful in its natural forms (lightning and
galvanic action), but which has proven invaluable in its manufactured forms. Electricity is comprised of
electrons flowing in a circuit. The flow is called a current (measured in amperes). Direct and alternating
current (dc and ac) systems are available, with alternating current predominating in building applications.
DC systems are encountered when batteries or photovoltaic cells are used. The tendency for electrons to
flow is termed potential or voltage (measured in volts). Resistance to current flow is termed resistance (in
dc circuits) or impedance (in ac circuits) and is measured in ohms. Ohm’s Law defines the relationship
between voltage, current, and resistance in a circuit.
Circuits may be arranged in series (with elements connected one after another) or in parallel (with
elements connected via branches to/from the same points). Parallel circuits are standard for all building
wiring. In a parallel circuit loads are additive with respect to current, and each load experiences the same
voltage. A short circuit occurs if an inadvertent connection appears across the circuit, with potentially
damaging results. AC electricity can be generated at different frequencies; 60 Hertz (cycles per second) is
the standard in North America.
AC power generation is briefly discussed. The difference between power and energy is emphasized and
noted as a commonly misunderstood distinction. Units of measurement for power and energy are
described and the concept of power factor introduced. Sample calculations for power, energy, and
electricity costs are provided. Load factor and demand charge are explained. Typical electric demand
control approaches are reviewed. The distinction between demand control and energy management is
emphasized. Meters and measurements for electric systems are discussed.
Chapter Outline
25.1 Electric Energy
25.2 Unit of Electric Current—The Ampere
25.3 Unit of Electric Potential—The Volt
25.4 Unit of Electric Resistance—The Ohm
25.5 Ohm’s Law
25.6 Circuit Arrangements
(a) Series circuits
(b) Parallel circuits
25.7 Direct Current and Alternating Current
25.8 Electric Power Generation—DC
25.9 Electric Power Generation—AC
25.10 Power and Energy
25.11 Power in Electric Circuits
25.12 Energy in Electric Circuits
25.13 Electric Demand Charges
25.14 Electric Demand Control
(a) Level 1—load scheduling and duty-cycle control
(b) Level 2—demand metering alarm
(c) Level 3—automatic instantaneous demand control
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(d) Level 4—ideal curve control
(e) Level 5—forecasting systems
25.15 Electrical Measurements
Key Concepts
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electricity (as the preeminent energy source for buildings)
electric current (describing the flow of electricity)
alternating and direct current (as two forms of electricity)
voltage or potential (as the driving mechanism for electricity)
conductor or insulator (as opposing sets of properties)
Ohm’s Law (as it relates current, voltage, and resistance)
parallel and series circuits (as distribution options)
energy and power (as critical concepts)
demand control (as a design strategy—and distinct from energy management)
Terminology and Metrics
Important Terminology
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electric current (direct or alternating; dc or ac)
electric potential or voltage
electric resistance
conductor
insulator
Ohm’s Law
electric circuit (parallel, series)
short circuit
cycle
frequency
wavelength
battery
generator (dc; ac = alternator)
commutator
kWh meter
energy
power
reactance
power factor (pf)
vectorial sum
overall load factor
demand charge
electric demand control
time-of-day (variable) utility rate
interruptible and uninterruptible loads
demand control (shedding, automated load, peak demand, programmable load controls)
duty cycle
load comparator controller
rate control
ideal curve
forecasting
meter (amperage, voltage, watt-hour)
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Important Metrics
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A (ampere; electric current)
V (volt; electric potential)
R (resistance; DC resistance, in ohms)
Z (impedance; AC resistance, in ohms)
Hz (Hertz; same as cycles per second)
electron-volt (a measure of energy at the atomic scale)
W (watt; electric power; also kW)
PF (power factor; dimensionless)
volt-ampere (wattage equivalent in a resistive circuit)
Wh (watt-hour; electric energy; also kWh)
load factor (a measure of peak power demand versus average power demand; dimensionless)
Links to Resources
Why is electricity so hard to understand? [not recommended for the faint of heart]:
http://www.eskimo.com/~billb/miscon/whyhard1.html
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