UNIT 31 HOME ELECTRICAL SYSTEM. NEC NATIONAL ELECTRICAL CODE The National electrical code is a...
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Transcript of UNIT 31 HOME ELECTRICAL SYSTEM. NEC NATIONAL ELECTRICAL CODE The National electrical code is a...
UNIT 31HOME ELECTRICAL
SYSTEM
NECNATIONAL ELECTRICAL
CODE• The National electrical code is a publication
sponsored by the National Fire Protection Association.
The NEC book includes recommendations and guidelines on wiring materials and methods.
• The NEC is updated every three years.
Electric Service Entrance
• Is the part of the system that extends from electric company’s last transformer, to the breaker box of a house.
• If the service runs above ground it is called a service drop.
• If the service runs underground it is called a service lateral.
Transformer on a pole
S.E. Cable Used for the Service Drop
HLP’s Step down transformer
This transformer is used to supply a service lateral
An electric service generally contains
• A service entrance cable (SE cable)• A watthour meter • And a breaker box. (Load center)
Service entrance cable in conduit
service lateral
Watthour Meter
Breaker Box
This conduit contains the wire the ties the Neutral Bus to the Ground Rod.
THREE-WIRE SERVICE• A three wire service is used in order to
supply two different voltages (120-240).• The SE cable has two current carrying
conductors called ( Hot ) wires, and one noncurrent carrying conductor called ( Neutral ).
• Hot wires are insulated, the neutral may be bare.
Neutral Bus Bar
The White Neutral wire and the Bare Ground wire are usually tied to the Neutral Bus Bar
Hot wires
Neutral wire
Why supply two different voltages???
• $$$$$$$$$$$$• It is cheaper to operate large
amperage loads on 240 volts than 120 volts
Example: An electric water heater uses 5500W heating
elements.
• Remembering I = P/E • 5500/120 = 45.833 A• 5500/240 = 22.91 A
The lower current value is important for two reasons
• The cost of installation, (smaller wire) is cheaper.
• Power loss is directly proportional to the square of the current!!!
The Watthour Meter
• Records the amount energy used.
The Load Center• Usually called a Breaker Box• Could be a fuse box• Contains the main disconnect, and
usually several breakers or fuses.• Distributes the electricity to the
many branch circuits throughout the building.
The Service Ground• Is there to reduce the chance of shock
and to protect against lightning.
• The neutral wire of the SE cable is connected to a eight foot copper rod, driven into the ground eight foot deep.
• The ground rod is connected to the neutral bus bar in the breaker box.
BRANCH CIRCUITS
• Branch circuits distribute the electricity from the breaker box to the different rooms of the building.
Individual Branch Circuits 240VLarger Amperage
Small Appliance Branch Circuits 20 Amp
General Purpose Branch Circuits15 or 20 Amp
There are three types of branch circuits.
• General-Purpose• Small-Appliance
• Individual
GENERAL-PURPOSE CIRCUITS
• Are used for lighting and outlets.• Things like radios, TV’s, and small
appliances (not kitchen appliances).
SMALL-APPLIANCE CIRCUITS
• Are dedicated for the kitchen only.• These outlets are for refrigerators,
toasters, coffee makers, and any other of the 10,000 appliances that are found in our kitchens.
INDIVIDUAL CICRUITS
• Are used for only one piece of equipment (dryer, water heater, AC, and heater)
• These loads are generally 240 volt loads.• A clothes washer and dish washer are only
120 volt loads but are require to be on an individual circuit.
Wiring Branch circuits is usually done with these
materials…
• Nonmetallic Sheathed Cable• Armored Cable• Electrical Metallic Tubing
Nonmetallic Sheathed cable ( ROMEX )
• NM cable is used for all kinds of indoor wiring.
• Can not be exposed to sunlight, buried underground, in cement,or plaster.
• Most common sizes are 14 And 12 AWG
Romex• A style of wire that has two or three
insulated conductors inside an outer cover of insulation called a sheath
• It may or my not have a ground wire, which is usually bare( no insulation )
• 12/2, would indicate that there are 2 # 12 AWG wires in the cable.
• 14/3wg, would indicate the there are 3 # 14 AWG wires in the cable as well as a ground wire.
Neutral wireHot wire
Ground wire
Sheathing
Armored Cable
• In armored cable, the insulated wires are in a flexible metal sheath.
Metal sheath
Ground wire
Hot wire
Neutral wire
Weather-tight Flex conduit
Safety Disconnect
PVC conduit
Electrical Metallic Tubing
• Known as EMT• Is a thin wall metal pipe in which
wires are pulled into, for protection while running through a building or outside the building.
WIRING DEVICES
• Switch boxes• Junction boxes• Connectors• Wire nuts
Wiring devices are used to provide the safe, convenient, and reliable use
and control of electricity.
• All switches, receptacles, and splicing must be done or mounted in boxes.
Switch box
Junction box
Receptacle
Switch
Locknut
Cable connector
Conduit connector
GFCI receptacle
A GFCI work by detecting the amount of current passing through the HOT wire and comparing the current to coming back on the Neutral wire. If as little as 5mA-7mA does not return on the neutral the GFCI will trip and must be reset.
The minimum length of 6 inches individual wire is exposed
A little slack in the Romex cable is a good thing….
Tape covering the splice in the Hot wire.
Always curl the hook of a wire to be terminated under a screw head in the same direction as the screw tightens.
The pig tail splice of the ground wires are tightly twisted and has 4 to 5 twist in the first inch.
This little trick (at the end of the White wire) will help keep the braided wire bundled tightly and not spread out from underneath the screw head.
This is how we terminate 2 different wires under 1 screw head. This is commonly called a pigtail splice
Natural Buss BarTermination block for incoming voltage