Understanding electrical principles is vital to
designing safe and efficient architectural electrical systems
Electric power is generated from several sources of energy:
wind, water, nuclear, fossil fuel, solar (photovoltaic), and solar
energy directly into an electric current. All other energy sources
are harnessed to produce a rotary mechanical motion that drives
electrical generators. The generators convert movement into
electricity. Transformers are used to "step up" (increase) the
electrical power to very high voltages (hundreds of thousands of
volts) for transmission by wires over long distances. Wherever the
transmission lines enter an industrial or residential community
for local power distribution, large transformers are used to "step
down" the voltage to a few thousand volts. Smaller transformers
set on poles or in underground vaults are used for final
distribution to small groups of ho8uses or individual factories.
Usually 110 and 220 volts are delivered to residences.
Electrical properties can be measured with instruments. The
terms used to describe units of electricity -- volt, ampere, and
watt-- are used in both metric and customary systems.
A volt is the unit of electrical pressure or
potential. This pressure makes electricity flow through a wire.
For a particular electrical load, the higher the voltage, the
greater will be the amount of electricity that will flow.
The term for flow of electricity is current. An
ampere, or amp, is the unit used to measure the magnitude of
an electric current. An ampere is defined as the specific quantity
of electrons passing a point in one second. The amount of current,
in amperes, that will flow through a circuit must be known in
order to determine proper wire sizes and the current rating of
circuit breakers and fuses.
The amount of power required to light lamps, heat water,
turn motors, and do all types of work is measured in watts.
Wattage depends on both potential and current. Current (in
amperes) multiplied by potential (in volts) equals power (in
watts).
amperes x volts = watts
The actual energy used (the watts utilized) for work performed
is the basis for figuring the cost of electricity. The unit used
to measure the consumption of electrical energy is the
kilowatt-hour. A kilowatt is 1000 watts. An hour, of course,
is a unit of time. A 1000-watt hand iron operating for one hour
consumes one kilowatt-hour (1 kWh). The device used to measure the
kilowatt-hours consumed in the watt-hour meter.
Electricity flowing through a material always meets with some
resistance. Materials such as wood, glass, and plastic have a high
resistance. They are good insulators. Copper, aluminum, and
silver have low resistance and are therefore good conductors of
electricity. Most electrical
I = E or
= IR or R =
R =
R = 
R = 12 ohms
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Power is supplied to a building through a
service entrance. Three heavy wires, together called the drop,
extend from a utility pole
or an underground source to the structure. These wires are twisted
into a cable. At the building, overhead wires are fastened to the
structure and spliced to service entrance wires that enter a
conduit through a service head, as shown in Figure 31-1
In planning overhead service
drop paths, minimum height requirements for connector lines must
be carefully
followed. See Figure 31-2. If these distances cannot be
maintained, rigid conduit, electrical metallic tubing, or busways
(channels, ducts) must be used.
If the service is supplied underground, three
wires are placed in a rigid conduit. An underground service
conduit is brought to the meter socket. An underground service
entrance includes a watt-hour meter, main breaker, and lightning
protection. Automatic brownout equipment is also required by many
codes for new construction. All electrical systems must be
grounded through the service entrance.
Electrical current is delivered throughout a
building through a distribution panel, or service panel.
See Fig. 31-3.
The
size of a distribution panel (in amperes) is determined by the
total load requirements (watts) of the entire building. Watts can
be converted to amperes by dividing the total (and future) watts
needed by the amount of voltage delivered to the distribution box:
Formula:
= amperes
W = metric symbol for watts
V = metric symbol for volts
A = metric symbol for amperes
Example:
= 145 A
Most residences require a distribution panel with
a capacity of 100 to 200 amps. The National Electrical Code (NEC)
minimum for new residential construction is 60 amps. To compute
the total load requirements, the watts needed for each circuit
must first be determined.
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From the distribution panel, electricity is routed
to the rest of the building through branch circuits. A
circuit is a circular path that electricity follows from the
power supply source to a light, appliance, or other electrical
device and back again to the power supply source. See Fig. 31-4.
If
the electrical load for an entire building were placed on one
circuit, overloading 3would leave the entire building without
power. Thus branch circuits are used. Each circuit delivers
electricity to a limited number of outlets or devices.
Each circuit is protected with a circuit
breaker. A circuit breaker is a device that opens
(disconnects) a circuit when the current exceeds a certain amount.
Without a circuit breaker, excessive electrical loads could cause
the wiring to overhead and start a fire. When a breaker opens, or
"trips," the power to the branch circuit is disconnected.
Similarly, if the sum of the current drawn by the branch circuits
exceeds the rating of the main circuit breaker, the main breaker
will trip. This protects the service-entrance wires and equipment
from overheating and damage. Older homes often have ruses instead
of circuit breakers. They serve the same purpose, but overloaded
fuses must be replaced. Circuit breakers that trip can be reset.
Branch circuits are divided into three types by
the National Electrical Code: lighting circuits, small-appliance
circuits, and individual circuits.
LIGHTING CIRCUITS: Lighting circuits are
connected to lighting outlets for the entire building. Different
lights in each room are usually on different circuits so that if
one circuit breaker trips, the room will not be in total darkness.
In all dwellings other than hotels, the NEC
requires a minimum general lighting load of 3 watts per square
foot of floor space. However, the amount of wattage demanded at
one time (demand factor) is calculated at 100 percent only for the
first 3000 watts; 35 percent is used for the second 17,000 watts;
and 25 percent is used for commercial demands over 120,000 watts.
Thus, the general lighting load planned for a 1500 sq. ft. house
would be 3525 watts, not the full 4500 watts. It is calculated as
follows:
|
1500 ft. x 3 W = 4500 W
(uncalculated amount) |
| First 3000 W |
x |
100% |
= |
3000 W |
| Next 1500 W |
x |
35% |
= |
525 W |
| TOTAL 4500 W |
|
|
|
3525 W |
If each branch circuit can supply on 2400 watts
9120 V x 20 A = 2400 W), a 1500 sq. ft. house should have two
1860-watt general lighting circuits. See Fig. 31-5.
Lighting
circuits are also used for small devices such as clocks and
radios. However, since all lights and other items on the circuit
are probably not going to be used at the same time, it is not
necessary to provide a service capable of supplying the full load.
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SMALL-APPLIANCE CIRCUITS: These circuits
provide power to outlets wherever small appliances are likely to
be connected. Small appliances include items such as toasters,
electric skillets, irons, electric shavers, portable tools, and
computers. Appliance circuits are not designed to also support
lighting needs. See Fig. 31-6 (a & b). The NEC requires a minimum
of two small appliance circuits in a residence. Each circuit is
usually computed as a 1500-watt load.
INDIVIDUAL CIRCUITS: Individual dedicated
circuits are designed to serve a single large electrical appliance
or device, such as electric ranges, automatic heating units,
built-in electric heaters, and workshop outlets. Large
motor-driven appliances, such as washers, garbage disposals, and
dishwashers, also use individual circuits. These circuits are
designed to provide sufficient power for starting loads. When a
motor starts, it needs an extra surge of power to bring it to full
speed. This is called a starting load.
A separate circuit (20 amps) is required in a
laundry area to provide power for the washing machine and the
dryer. Because of the danger of water leakage, a ground-fault
circuit interrupter receptacle is recommended.
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A CFCI receptacle must be located wherever there
is a possibility for people to ground themselves and be shocked by
the electrical current flowing through their body to the ground.
The purpose of a GFCI receptacle is to cut off the current at the
outlet. When the GFCI receptacle senses any change of current, it
immediately trips a switch to interrupt the current. It operates
faster and is safer than the circuit breaker switch or fuse at the
power entry panel. A GFCI valve will trip in 1/40 second when an
extremely small current variation (ground fault) of 0.005 amps is
reached.
In new construction GFCI receptacles must be
located with each convenience outlet near water sources and/or
pipes in the bathroom, kitchen, garage, laundry and outdoors. Any
receptacle located within 10' or within 15' of the inside of a
permanently installed swimming pool must also be wired through a
GFCI. GFCIs are also required if outlets are placed in unfinished
crawl spaces below grade level.
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Wires used to conduct electricity are classified
by the type of wire material, the insulation ma6terial, and the
wire size. The size of the wire used in a circuit depends on the
current to be carried by the circuit. See Fig. 31-7.
Although
the meter voltage is 120V and 241V, wiring resistance reduces the
voltage at the receptacles to approximately 110Va and 220V. Sizes
6 through 2/0 are used for 240-vold (240V) service entrance and
circuits. The exact size depends on the capacity of the service
panel. Sizes 10 through 14 are used for 120V and 240V lighting and
small appliance circuits. Sizes 16 and 18 are used for low voltage
items such as thermostats and doorbells.
A low-voltage switching system may be used to turn
on or off any fixture, appliance, or light. Because of the low
voltage, extremely small wires are used to hook up the switch to
the fixture.
Wire size is critical. If a wire is too small for
the current applied, excessive resistance (overload) can result.
This may cause the insulation to overheat and break down, causing
a potential fire hazard. When selecting or preparing to use
appliances, it is important to check the UL (Underwriter's
Laboratories) ratings to learn the proper wiring requirements.
Aluminum wire is lighter and less expensive than
copper, but many codes apply stricter rules to the use of aluminum
for residential work. Insulation is available in flexible metal
armored or nonmetal sheathed worm. For underground or exterior
exposed wiring, wires must be encased in rigid or flexible metal
or PVC (plastic) conduits.
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The installation of the proper size of service
entrance equipment and branch circuits are dependent upon the
square footage of the residence, number of appliances, lighting,
and future expansion allowances. To find the total amp service
needed for an entire building, first determine the total number of
watts needed for each circuit. Add these to find the total watts
needed for the building. For example, to calculate the size of the
service entrance for a 2000 sq. ft. residence, list the amount of
wattage to be used as follows:
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