Electrical Wiring
Instructional Materials Service
Texas A&M University
Introduction
• Well-planned wiring system must be safe,
economical, adequate, & expandable.
• Install according to the National Electric
Code (NEC) which insures safety
• An economical system is safe, adequate,
and efficient and can be acquired at a
minimum cost.
• NEC requirements state that electrical
systems should be of sufficient capacity to
operate all appliance, lighting, and heating
circuits with a voltage drop not to exceed 2%.
• An adequate number of outlets and switches
of proper type and rating should be included
in the system
• Anticipated future electrical needs should be
considered & allowances made for their
installation.
Wiring Materials
• Wire Size:
– Size of conductor is VERY important
– Larger wire will safely carry more amperage
– Inefficient operation of electrical devices caused by
attempting to force too much current through a small wire
• Increases resistance
• Decreases available voltage
– Conductors are numbered to indicate size (gauge)
according to the American Wire Gauge (AWG) Standards
• Small number = large wires
• Large number = small wires
• Kinds of Wire:
– Solid or stranded
– Stranded:
• Made up of several small wires wrapped together
• Provides more flexibility, particularly the larger sizes
• Indoor wire larger than a No. 8 is generally stranded.
• No. 6 stranded conductor is made up of 7 smaller wires with a total
cross-sectional area the same as that of a No. 6 solid conductor
• Diameter of electrical conductors measured in thousands of circular
mils (kcmil)
• Circular mil = area of a circle with a diameter of one onethousandth of an inch
– Copper (CU) or Aluminum (AL)
– Copper:
• Most common conductor used in residential
& commercial wiring
– Aluminum:
• Lighter in weight
• Requires approximately one size larger
than copper to carry the same load in
amperes
• Aluminum conductors generally used in
overhead applications
• Types of Wires:
– Outdoor:
• Used for wiring between buildings
• Usually covered with neoprene or polyethylene
• Common three-conductor self-supporting service drop
– Two insulated wires wrapped around a bare stranded wire used as the
ground & supporting wire
• Underground conductors (USE) have increased significantly over
the years
– Eliminates ice & wind damage
– Adds to attractiveness of farm, home, or business
– Generally required in new subdivisions for electric & communications
service
Outdoor Wire
Type Designation
UF
USE
SE/SEU
ASE
Meaning
Use
Underground feeder
Underground service –
direct burial in soil, not
concrete
Underground
entrance
Mechanical protection
required where wires
enter and leave ground,
moisture-resistant
service
Service
entrance/unarmored
To bring service into
building – flameretardant & moistureresistant
Armored
entrance
To bring service into
building
service
– Indoor:
• Designed for use inside buildings
• Some types can be used both inside and
outside
• Special coatings have been developed for
wires used inside buildings having
excessive moisture
• Most of the insulators used on inside
conductors are made of fire & moisture
resistant materials.
Indoor Wire
Type Designation
Meaning
Use
Thermoplastic
Indoor use – dry locations
Moisture resistant
thermoplastic
Dry or wet locations
THHN
Heat-resistant thermoplastic
(nylon jacket)
Dry locations with high
temperature
THW/THWN
Moisture- and heat-resistant
thermoplastic (nylon jacket)
Wet locations with high
temperature
Rubber, heat resistant
Carries higher voltage than
T or TW – dry and damp
locations
Rubber, heat & moisture
resistant
Service entrance or larger
appliance- dry and wet
locations
Moisture-resistant and flameretardant thermoplastic
High moisture and high
temperature
T
TW
RH
RHW
XHHW
– Most residential wiring is non-metallic
(NM) sheathed cable, commonly called
RomexTM.
• Plastic sheath that encloses a white
conductor, a black conductor & a
bare conductor
• Bare conductor is wrapped in paper
• Sheath is lined with paper
– Manufacturer, type of covering, size &
number of conductors, presence or
absence of a ground wire, and max
voltage rating are stamped on sheath
– In accordance with the NEC:
• Black conductor = current carrier
• White conductor = neutral carrier
• Bare conductor = ground
– Bare ground conductor may not always be
the same gauge as the current carriers
• Conduit:
– Wiring requiring mechanical protection
necessitates the use of conduit
– Other advantages:
• Greater choice of wiring colors
• Use of additional conductors
• Added safety if the conduit is properly
grounded
– Available as:
• Flexible or nonflexible
• Metallic or non-metallic
• Nonflexible Metallic:
– Can be rigid or thin wall (EMT)
– Rigid:
» threaded ends
» approx. 1/8” thick
» bent with a special type of bender
– EMT:
» Thinner
» Type most often used for agricultural & residential
applications
» Easily bent
» Available in 10-ft. lengths
» Can be mounted to steel boxes
» May be cut with a 32-tooth hacksaw
• Rigid non-metallic tubing (ENT):
–
–
–
–
–
–
–
–
–
Schedule 40 or Schedule 80 PVC pipe
Either gray or white in color
Somewhat flexible
Pre-manufactured bends used for 45o & 90o bends
Cut & assembled in same manner as other PVC pipe
products
Schedule 80 PVC has a smaller inside diameter (ID) than
Schedule 40 & can carry fewer conductors
NEC handbook addresses the number & rating of conductors
that can be placed in various sizes of ENT
Outside diameter (OD) of PVC is same for all schedules
PVC conduit identified as DZYR in NEC Handbook
– Conduit is supported & kept in place with
metal conduit straps
– EMT is fastened to the boxes with metal
connectors & screw locks
– DO NOT damage or reduce the internal
diameter when bending conduit
– Bends that reduce the internal crosssectional area of conduit are not allowed in
the NEC
Outlet, Junction, &
Switch Boxes
• Installed at every switch, light, splice, or
junction point where wires are attached or
spliced, with the exception of some outside
wiring
• Outlet Boxes:
– Usually octagonal or square
– Octagon & square boxes are about the same size
& measure 4” in diameter & 1½” deep
• Switch Boxes:
– Made for flush or surface mounting
– Flush Mounting:
• Used in walls where wire is concealed
• Most commonly used box has beveled
corners with clamps for nonmetallic cable
• Sides are removable in order that boxes
can be ganged together to provide for a
series of switches or receptacles at one
location
– Surface Mounting:
• Made to fasten to an exposed surface
• Widely used in barns & outbuildings
• Corners are round
• Do not have clamps for nonmetallic cable
• Can be used for conduit or cable wiring
• If NM cable is used, they should have a bare
ground.
– To prevent possible damage to the wires, the NEC
specifies the number of wires of various sizes
permitted in boxes.
MAXIMUM NUMBER OF CONDUCTORS PERMITTED
IN OUTLET, SWITCH, AND JUNCTION BOXES1
Trade Size of Box
Maximum Number of Conductors
#14
#12
#10
#8
4” octagon box, 1½” deep
7
6
6
5
4” square box, 1½” deep
10
9
8
7
Switch or handy box, 2” deep
5
4
4
3
Switch or handy box, 2½” deep
6
6
5
4
Switch or handy box, 3” deep
7
7
6
1These
numbers of wires apply if no cable clamps, fixture studs, switches, or receptacles are
contained in the box. If the box contains cable clamps or fixture studs, the number of wires shall
be reduced by one, with a further reduction of one wire if flush switches or receptacles are
contained in the box. Each wire running through the box without splice is counted as one. Each
wire terminating in the box is counted as one.
Receptacles
Switches
• Three common types:
– Single-pole:
• Have two terminal screws
• Control lights from one location
– Three-way:
• Three terminals
• Control lights from two locations
– Four-way:
• Four terminals
• Used in combination with three-way switches to
control lights from three or more locations
Blank Covers
• Used to cover surface mounted boxes that
serve as junction boxes for splices or
future receptacle installations
Wall Plates
• Available in bakelite, metal, glass & other
materials
Connectors
• Connect cables to boxes that do not have built-in
cable clamps
• Available to fit different sizes of knockouts in boxes
• Pressure-type, split bolt connector used to connect
outside wires to entrance wires
• When connecting aluminum to copper wires, a spacer
should be placed between the two to prevent
corrosion.
• Solderless connectors: join wires inside junction
boxes
Grounding Screws,
Clips, & Clamps
• Screws & Clips:
– May be used to ground conductors to boxes
– Boxes having tapped holes use the screw and pigtail
– Boxes without tapped holes may use the grounding clip
• Clamps:
– Two types:
• One is for use with a ground rod
• The other is for grounding the system to deeply buried
water pipes
– Grounding rods should be at least 8’ long
Grounding
Clip
Grounding
Wire
Grounding
Clamp for Rod
Grounding Clamp
Insulators & Entrance Elbows
• Insulators:
– Support outdoor wires
– With screw knobs: placed in areas having a moderate
amount of tension
– Wrap-around clamps: used in metal structures
– Racks of insulators are used for long spans of multiple
conductors
• Entrance Elbows:
– Use where conduit or SE cable enters the building
– Removable cover on elbow aids in installing the wires
Entrance Cap or Weatherhead
• Protects the entrance cable connection
Common Splices & Connections
• Western Union:
– Used when strength is require
– Remove enough insulation from wires to
allow 5-6 turns of the wire
– If using a knife, cut insulation at a slant
– Combination wire cutters/strippers
• Adjust to correct wire gauge
– To begin splice:
• Bend each wire into a 90o angle about 1” from the insulation
• Cross the wires and wrap each wire around the other wire
• Pliers can be used to smooth sharp edges
– To tape a splice:
• Extend electrical tape about ¾ of an inch over insulation on one end
• Extend each lap ½ the distance of the previous wrap
• Keep tape tight
• Wrap about ¾ of an inch on opposite end
• Two complete wrappings results in four thicknesses of tape sufficient
for proper insulation
• Center Tap:
– Used to branch off a wire & go in another direction
– Has very little strength
– Procedure:
• Remove about 2½” of insulation
• Bend tap wire into a 90o about 1” from insulation
• Place tap wire around center of main wire & make
7-8 wraps
• Tape
– Split bolt connectors used with center tap splices on
larger conductors
• Rat Tail:
–
–
–
–
Use to clamp conductors inside junction boxes
Has very little strength
Use electricians pliers to twist this splice tight
Insulate with a solderless connector
• Other connections:
– When making connections to terminals on
switches, outlets, etc., attach the wire loop to the
terminal in the direction in which the screw
tightens
Types of Switches
& Circuit Diagrams
• Simple Switches:
– Single pole-single throw (SPST)
– Controls a circuit from one location
– Types:
• Toggle
• Mercury (thermostat & humidistat)
• Silent
• Rocker button
• Push button
– NEVER install switches in a neutral line
• Three-Way Switches:
– Single pole-double throw (SPDT)
– Use to control a circuit from two locations
– Have three terminals
• 1 common terminal & 2 traveler terminals
• Four-way Switches:
– Used between three-way switches to provide additional
points of control
– Have four terminals
Installing Lighting
& Outlet Circuits
•
•
•
•
•
•
•
Use No. 14 wire on demonstration board
In actual wiring the NEC requires No. 12 wire
Outlet & lighting circuit: No. 14-2 WG
Three-way switches: No. 14-3 WG
Use black tape to mark white wires used as hot
Use solderless connectors to make splices
Use romex clamps to secure cables to boxes
• Outlet Circuit:
– Strip romex and remove insulation from wires
– Place romex in breaker box
– Connect black to breaker terminal
– Connect white & bare to neutral bar
– Place other end of romex through 1st outlet
box, strip & remove insulation
– Connect black wire to brass screw (small slot
side of outlet)
– Connect white wire to silver screw (large
slot side of outlet)
– Connect bare wire to safety-ground (green)
screws
– Use same color code & connect 1st outlet
to 2nd outlet with another length of romex
– Jumper wires may be used from
receptacles to main wires in order to
maintain continuous neutral & ground
conductors
– Install outlets & place covers on outlet
boxes
• Lighting Circuit:
– Place No. 14-2 WG into breaker box
– Strip romex and remove insulation
– Connect black wire to breaker terminal
– Connect white & ground wires to neutral bar
– Push other end of romex into first junction box, then strip &
remove insulation
– Place one end of another length of No. 14-2 WG into
junction box and the other end into 1st switch box
– Strip & remove insulation
– Install another No. 14-2 WG from 1st junction box to 2nd
junction box
– Strip & remove proper amount of insulation
– Now make these connections
• 1st Junction box:
– Black wire from breaker box to white (marked
with black tape) from 1st switch box and to black
going to 2nd junction box
– Black from switch box and black lead of light
– White from breaker box to white lead of light and
to white wire going to 2nd junction box
– Safety ground from breaker box, switch box & 2nd
junction box should be attached to a clip
– Place porcelain light fixture on junction box &
install
• 1st Switch Box:
– Connect black to one switch terminal
– Connect white (marked with black tape) to other terminal
– Attach safety ground to switch box using a grounding clip
– Attach bare safety ground to each switch box using grounding
clips (No need for clips if there is a grounding terminal)
• 3-Way Switch Boxes:
– Connect black to common terminal
– Connect red & white to traveler terminals of the 3-way
switches
– Use clips to attach ground wire to each switch box
• 2nd Junction Box Splice:
– Black from 1st junction box and black from
one three-way switch
– Black lead of light & black from 2nd 3-way
switch
– Red traveler wires
– White traveler (marked with black tape)
– White from 1st junction box & white lead of
light
– Ground from 1st junction box & from each 3way switch should be attached to a ground
clip
– Mount porcelain light fixture on junction
box & install three-way switches & cover
plates
– Energize fuse box & check circuits and
switches to determine if functioning
properly
– Circuit should be polarized when energized
– Polarizing: neutral wire from power
source is connected to neutral wire of
demonstration board
Planning the Wiring System
• The Wiring System
– Includes both exterior & interior wiring
– Extends from the point of supply to various outlets
located inside & outside of buildings
• Well-planned Wiring System
– Can pay dividends by reducing:
• Size of wire
• Size of electrical equipment needed
• Amount of wire used
• Installation labor
• Amount of electricity used
• Suggested Planning Procedure:
– Prepare a plan with building & layout plans to scale
– Determine number of circuits needed in each building and locate the outlets
• General Purpose Outlets: at least one outlet for every 12 ft of wall space
(in a residence)
– Provides a circuit for any appliance having a cord 6 ft. long
– Max of eight convenience or light outlets on any one circuit (less in
kitchen)
– Number & size of light fixtures determined by amount of floor space in a
given area & candle-power of fixtures
– At least two general-purpose circuits in kitchen for high-wattage
appliances
– No. 12 wire is used for general purpose circuits & should be protected
with 15 or 20 ampere fuses, circuit breakers, or ground fault circuit
interrupters
• Special Purpose Circuits:
– Needed for the range, water heater, air
conditioner(s), food freezer, water system &
clothes dryer
– Wire size and circuit protection based on
safe ampere capacity & permissible voltage
drop
– Place electrical symbols on building floor
plans to indicate outlets
• Service Entrance and Distribution Panel:
– Requirements can be determined from
plans and by consulting with the power
supplier
– Service entrance cables & service
equipment are capacity rated and should
have a rating adequate to handle the
electrical load safely
– Modern home may require equipment with a
capacity to handle a minimum of 200
amperes
– Distribution panel with 36-37 branch circuit
positions recommended for the home
Calculating Voltage Drop
• Voltage drop causes problems such as less
efficient heaters, dimmer lights, and inefficient
electric motors.
• Voltage at load or outlet may be significantly
lower than transformer voltage due to distance
or use of improperly-sized conductors
• Running equipment or appliances on low
voltage can cause damage
• Use a multi-meter to determine voltage at a
wall outlet
• Allowing for a 3% voltage drop, the lowest
acceptable reading on a multi-meter should be
at least 116 volts
• 120 volts x 3% voltage drop = 3.6
120 – 3.6
= 116.4
Round down to 116
OR
120 volts x 97%
voltage = 116.4
Round down to 116
• When working with voltage drop calculations,
always round down to next lowest whole
number
– Provides extra safety factor in calculations
• EXAMPLE:
– Nameplate on the motor of a 4’ floor fan:
• “1/3 hp, 1725 rpm, 115V, 6.3 A”
– Fan operates on a 10’ cord from a wall
receptacle
– Multi-meter reads 110 volts at receptacle
– Question: Will the floor fan be operating
within an allowable voltage drop of 4%?
• Ed = I x R
– Used when trouble-shooting electrical
circuits with a multi-meter
– Ed: voltage drop
– I: circuit amperage
– R: resistance of the conductors in ohms
– Based on Ohm’s Law
• EXAMPLE:
– Voltage in a 240V motor circuit was checked at the
load with a multi-meter and had a reading of 232 volts
– Conductors rated resistance: 0.16 ohms
– Motor rated at 50 amperes
– Using the formula:
• Ed = 50 x 0.16
• Ed = 8 volts
– Predicted voltage drop = Ed = 8 volts
– Difference between 240V & 232V is also 8V
– Calculation agrees with test reading
– 240 volts x 5% max voltage drop = 12 volts
– 8 volts reading is less than 12 volts predicted
with a 5% voltage drop
– Motor circuit will operate efficiently within
voltage drop limitation
– If measured voltage drop is greater than 5%,
conductor size must be increased for motor
to operate efficiently
Acknowledgements
Developed by Kristie Weller, Undergraduate Technician,
Instructional Materials Service, developed this
PowerPoint presentation.
Kirk Edney, Curriculum Specialist,
Instructional Materials Service,
reviewed this PowerPoint presentation.
Vickie Marriott, Office Software Associate,
Instructional Materials Service, assisted in the
development of this PowerPoint presentation.
ALL RIGHTS RESERVED
Reproduction or redistribution of all, or part, of this
presentation without written permission is
prohibited.
Instructional Materials Service
Texas A&M University
2588 TAMU
College Station, Texas 77843-2588
http://www-ims.tamu.edu
2006