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