Electricity Unit Notes
Agriculture Mechanics I
Principles of Electricity
Electricity is a form of energy that can
produce light, heat, magnetism,
chemical changes
Resistance: tendency of a material to
prevent electrical flow
Conductor: if electricity flows easily
Insulator: material that provides great
resistance
Amps, Volts, Watts
Amperes: measure of the rate of flow of
electricity in a conductor
Volts: measure of electrical pressure
Watts: measure of the amount of
energy or work that can be done
Ohms: measure of electrical resistance
to flow
Ohm’s Law
Ohm = R
Volts = E
Amps = I
Ohm’s Law: E = IR
I=E/R
R=E/I
Electrical Safety
Shock and Fire
Never disconnect any safety device
Don’t touch electrical items with wet hands
or feet
Don’t remove ground plug prong
Use GFI in wet areas
Discontinue use of extension cord that feels
warm
Don’t put extension cords under carpet
Electrical Safety
Install wiring according to NEC
Blown fuse or breaker, determine cause
Don’t replace fuse with larger fuse
Don’t leave heat producing appliances
unattended
Heaters & lamps away from combustibles
Don’t remove back of TV (30,000v when
off)
Electric motors lubricated, free of grease etc.
Electrical Safety
Keep appliances dry
Don’t use damaged switches, outlets,
fixtures, extension cords
Follow manufacturer’s instructions for
installation and use of electrical
equipment
Service Entrance
Power from from power company
Transformer: drops volts from 25,000 volts
to 240 volts
Service drop: wires etc from transformer to
house
Entrance head: weather-proof at house
Meter: $$$
Service Entrance Panel (SEP): box with
fuses or breakers
Electric Meter
Kilowatt-hours: how electricity is sold
Kilo = 1000
Watt-hour = use of 1 watt for one hour
n
100 watt light bulb for 1 hour - 100 watthours
Kilowatt-hour = 1000 watts for one
hour
Electricity is priced and sold by the
Kilowatt-hour. (kilo-1000)
If you use 1000 watts of electricity for 1
hour, you use 1 kwh
The price of a KwH of electricity is about
8 cents ($.08)
How does it add up?
If you run ten 100 watt light bulbs for one
hour:
10 * 100 – 1000 watts
1000 watts = 1 kwh
1*.08 = $.08 (8 cents)
How much does it cost?
Assume that you have three 300 watt
bulbs on the outside of your house.
You turn the lights on at 9:00 p.m. and
turn them off the next morning at 7.
How much does it cost to run the bulbs
through the night?
How long did the bulbs run? From 9 until 7 is
10 hours
How many total watts? 3*300 = 900 watts
Watt-hours = 10 hrs * 900 watts = 9,000 watthours
9000/1000 = 9 kwh * $.08 = $.72
Try this one!
A 5000 watt clothes dryer will dry a load
of clothes in 1 hour. How much does it
cost to dry the load of clothes?
5000 watts * 1 hr. = 5000 watt-hours
5000 watt-hours/1000 = 5 kilowatt-hours
5 kwh * .08 = $.40 (40 cents)
If you can do this one, you’ve
got it!!
A football field has 36 bulbs rated at 500
watts. The coach turns on the lights at
5:00 p.m. and turns them off after the
game at 10:00 p.m.
How much does it costs to run the lights
during the game?
36 bulbs x 500 = 18000 watts
Time = 5 hours
18000*5=90000 watt-hours
90000/1000 = 90 kwh
900 * $.08 = $7.20
Branch Circuits
usually begin at SEP
branch out into a variety of places
only 1 motor or;
series of outlets or;
series of lights
use correct size wire and fuse or
breaker
Types of Cable
Nonmetallic sheathed cable: copper or
aluminum wire covered with paper,
rubber, or vinyl for insulation
Armored cable: flexible metal sheath
with individual wires inside. Wires are
insulated
Conduit: tubing with individually
insulated wires
Wire Type and Size
copper
No 14 (14 gauge) = 15 amp circuits
No 12 = 20 amps
No 10 = 30 amps
aluminum use one size larger
lower gauge number = larger wire
No 8 and larger use bundles of wires
current travels on outer surface of wire, so
a bundle of smaller wires can carry more
Voltage Drop
loss of voltage as it travels along a wire
lights dim, motors overheat
larger wires have less voltage drop for a
given amount of current
longer wire = greater problem
must increase wire size as distance
increases
Wire Identification
Type of outer covering, individual wire
covering, cable construction, number of
wires
Wire type stamped on outer surface
Wire Types
Type T - dry locations
Type TW - dry or wet
THHN - dry, high temps
THW and THWN - wet, high temps
XHHW - high moisture & heat resistance
UF - direct burial in soil but not concrete
Wire Identification
Color coded: black, red, & blue =
positive or hot wires which carry current
to appliances
White = neutral wires carry current
from appliance back to source
Green or Bare = ground all metal boxes
and appliances
Wire Identification
Wire Size: 12-2 has two strands of No.
12 wire (black & white)
12-2 w/g same, with one green or bare
12-3 has three strands of No. 12 (black,
red, white)
12-3 w/g same, with green or bare
What are Wiring Boxes?
hold cable or conduit and protect
connections
attached to building
contain all electrical connections not in
fixtures
rectangular or octagonal
various depths
some have removable sides for expansion
must be fastened securely to building
What are Wiring Boxes?
cable clamped to box
secure cable
grounded if metal
wires must be soldered or use wire nuts
no more than one wire to each screw of
receptacle
positive wires attach to yellow screws,
neutral wires to white screws, ground wires
to green screws
What are Wiring Boxes?
when a white wire is used as positive,
paint black or use black tape
four wires per standard box (not
counting ground wires)
duplex receptacle may be on two
circuits by breaking metal tab
What is a Single-Pole Switch?
it is the only switch on the circuit
it opens/closes the positive wire
Light Wiring Concepts
always begin at the SEP
Black or Hot wire comes out of SEP,
goes to the switch
Black wire leave switch, goes to light
White wire leaves light, returns to SEP
SEP
Wire Nut
SEP
SEP
SEP
Three-Way Switch
used in pairs
control from two different locations
3 terminals (screws)
current (black wire) feeds into
Common Terminal (dark screw)
current flows out of other two screws
through Traveler Wires to other 3way switch
Three-Way Switch
current flows out of other switch from
the Common Terminal
current flows to light or receptacle
SEP
W
SEP
B
W
SEP
W
R
SEP
SEP
B
W
B
SEP
B
W
B
R