Service conductors are attached to a meter
base and then to the service panel. Straddling the
two sets of terminals on its base, the meter mea-
sures the wattage of electricity as it is consumed.
The service panel also routes power to various
You never know what you’ll find inside a
circuits throughout the house.
service panel, so stay out. This panel is so
The utility company will install the drop
crowded that it will be tough to replace
wires to the building and will install the meter.
the cover.
a service entrance and Panel
Weatherhead
zzzzzz
Service drop
A typical threewire service assembly
has two insulated hot conductors
wrapped around a bare messenger
Mast
cable, which also serves as the
neutral.
Meter base
Conduit
90° LB condulet
The service panel distributes power to circuits throughout
the house. Breakers interrupt power if the circuits become
overloaded.
Service panel
(entrance panel)
262 Chapter 11
Wires,
inside the service Panel
Cables,
and ConduCtors
zzzzzz
the terms wires, cables, and conductors are often
Hot feeder lines
(incoming power)
used interchangeably, but they are not exactly the
same. a conductor is anything that carries or con-
Incoming neutral
ducts electricity. a wire is an individual conduc-
Main breaker
tor, and cable is usually an assembly of two or
Main bonding jumper
more wires, protected by plastic or metal sheath-
Neutral buses
Incoming ground
ing (also called a jacket). a cable derives its name
Single-pole breaker
from the size of the wires within and the type of
Hot buses
Hot wire
sheathing it is assembled with. For example,
12-gauge cable contains 12aWg (american Wire
Ground buses
Circuit cable
gauge) wire. more specifically, 12/2 with ground
Neutral wire
denotes cable with two 12aWg wires plus a
Ground wire
ground wire. nm designates nonmetallic sheath-
Grounding
ing, mC for metal-clad sheathing and so on.
electrode
Knockouts for
conductor
circuit cables
on all sides
of panel
The homeowner is responsible for everything
beyond that, including the meter base and break-
er panel, which a licensed electrician should
install.
serviCe Panels
At the main service panel, the two hot cables
from the meter base attach to lugs or terminals
on the main breaker. The incoming neutral cable
Incoming ground
attaches to the main lug of the neutral/ground
bus. In the main panel, neutral/ground buses
hot feeder lines (incoming power)
must be connected together, usually by a wire or
Incoming neutral
metal bar called the main bonding jumper. In
subpanels and all other locations downstream
main breaker
from the main service panel, ground and neutral
hot bus bar (behind breakers)
components must be electrically isolated from
each other.
neutral bus bar
In a main fuse box, the hot conductors from
the meter attach to the main power lugs, and the
ground bus bar
neutral cable to the main neutral lug. Whether
the panel has breakers or fuses, metal buses run
from the main breaker/main fuse. Running down
the middle of the panel, hot buses distribute
power to the various branch circuits either
through fuses or through breakers. The neutral/
ground buses are long aluminum bars containing
many terminal screws, to which ground and neu-
tral wires are attached.
grounding electrode conductor
Each fuse or breaker is rated at a specific
number of amps (15- and 20-amp breakers or
fuses take care of most household circuits).
When a circuit becomes overloaded or a short
The main panel (service panel) houses incoming cables from
circuit occurs, the breaker trips or a fuse strip
the meter as well as the breakers and wires that distribute
electricity to individual circuits. At the service panel,
melts, thereby cutting voltage to the hot wire. All
neutral conductors (white wires), equipmentgrounding
current produces some heat, and as current
conductors (bare copper or green insulated wires), the metal
increases, the heat generated increases. If there
service panel, and the grounding electrode system
were no breakers or fuses, and if too much cur-
(grounding rods) must be bonded together.
electrical Wiring
263
Stay away from the area around the main breaker. The terminals above the
A service entrance panel being used as a metermain.
breaker remain hot even when the main is off, presenting a serious hazard if
Placed outside the house, it provides easy access for
the cover is off.
service or emergencies.
rent continued to flow, the wires would overheat
Edison-style fuses are more common. They have
and could start a fire. Amperage ratings of break-
little windows that let you see a filament. When
ers and fuses are matched to the size (cross-
the circuit has been overloaded and the fuse is
sectional area, measured as “gauge”) of the
blown, the filament will be separated. A black-
circuit wires.
ened (from heat) interior could mean a short cir-
cuit—a potentially dangerous situation calling
the main breaker. All electricity entering a
for the intervention of a licensed electrician. The
house goes through the main breaker, which is
less common cartridge fuses are used to control
usually located at the top of a main panel. In an
240v circuits and are usually part of the main dis-
emergency, throw the main breaker to “off” to
connect switch, or serve heavy-duty circuits for
turn off all power to the house. The main breaker an electric range or a clothes dryer.
is also the primary overcurrent protection for the
electrical system and is rated accordingly. (The
grounding basiCs
rating is stamped on the breaker handle.) If the
main breaker for a 200-amp panel senses current Because electricity moves in a circuit, it will
that exceeds its load rating, the breaker will auto-
return to its source unless the path is interrupted.
matically trip and shut off all power.
The return path is through the white neutral
wires that bring current back to the main panel.
Meter–mains. Increasingly common are meter–
Ground w
ires provide the current with an alter-
mains, which house a meter base and a main
native low-resistance path.
breaker service in a single box. Meter–mains
Why is having a grounding path important?
allow a homeowner to put the main breaker out-
Before equipment-grounding conductors (popu-
side the house, where it can be accessed in an
larly called ground wires) were widespread,
emergency—if firefighters need to cut the power
people could be electrocuted when they came in
to the house before they go inside, for example.
contact with voltage that, due to a fault like a
When meter–mains are used, electricians often
loose wire, unintentionally energized the metal
locate a panel with the branch circuit breakers
casing of a tool or an electrical appliance.
(called a subpanel) in the garage or another cen-
Ground wires bond all electrical devices and
tralized location inside that is easy to access,
potentially current-carrying metal surfaces. This
such as a laundry room.
bonding creates a path with such low impedance
A fuse box subpanel.
Fuse boxes. Many older homes still have fuse
(resistance) that fault currents flow along it,
boxes. Fuses are the earliest overcurrent protec-
quickly tripping breakers or fuses and interrupt-
tion devices, and they come as either Edison-type ing power. Contrary to popular misconceptions,
(screw-in) fuses or cartridge (slide-in) fuses.
the human body usually has a relatively high
264 Chapter 11
impedance (compared with copper wire); if elec-
tricity is offered a path with very low resistance—
Making sense of grounding
the equipment grounding conductor—it will take
the low resistance path back to the panel, trip the
grounding confuses a lot of people, including some electricians. Part of the
breaker, and cut off the power.
problem is that the word ground has been used imprecisely for more than a century
Ground wires (equipment grounding conduc-
to describe electrical activity or components. The term ground wire, for example,
tors) connect to every part of the electrical sys-
may refer to one of three different things:
tem that could possibly become energized—
The large, usually bare-copper wire clamped to a ground rod driven into the
metal boxes, receptacles, switches, fixtures—and,
earth, or to rebar in a concrete footing. because the rod or the rebar is technically a
through three-pronged plugs, the metallic covers
grounding electrode, this “ground wire” is correctly called a grounding electrode
and frames of tools and appliances. The conduc-
tors, usually bare copper or green insulated wire,
conductor.
create an effective path back to the main service
short conductors that connect one piece of electrical equipment to another to
panel in case the equipment becomes energized.
eliminate the possibility of a voltage difference between the two. These wires
That allows fault current to flow, tripping the
should be called bonding conductors.
breaker.
The bare copper or green insulated wires in all modern circuits, which ultimately
the neutral/ground bus. In the service panel,
connect electrical equipment, such as receptacles and fixtures, to the service panel
the ground wires attach to a neutral/ground bus
neutral/ground bar. These connections create a low-impedance fault path back to the
bar, which is bonded to the metal panel housing
service panel. Why? in case the equipment becomes energized by a hot wire touching
via a main bonding jumper. If there’s a ground
a metal cover or other part, this conductor allows current to flow safely and trip the
fault in the house, the main bonding jumper
breaker. in this book, the term ground wire or grounding conductor refers to this con-
ensures the current can be safely directed to the
ductor. These wires are properly called equipment-grounding conductors.
ground—away from the house and the people
inside. It is probably the single most important
connection in the entire electrical system.
Ground Faults Can kill
Current flowing unintentionally
to ground (“earth”) is called a
avoiding electrical shocks
ground fault. the ground wire is
One hand
intended to be a low-impedance
zzzzzz
touches defective
appliance with
path to ground, to safely carry
fault.
the current of a ground fault
until the circuit breaker trips.
You, however, can also be a path
to ground should you come in
contact with an energized con-
Current flows
through body.
STAY OUT
ductor. In this case, you would
OF THIS
become part of the circuit, with
LOOP!
current flowing through you. so
Other hand touches
grounded water pipe
be careful: only a little current
(faucet).
flowing through your heart can
kill you.
electrical Wiring
265
Major grounding elements
The equipment-grounding system acts as an expressway for stray current. by
bonding conductors or potential conductors, the system provides a low-impedance
path for fault currents. in a ground fault, the abnormally high amperage (current
flow) that results trips a breaker or blows a fuse, disconnecting power to the circuit.
it’s required by the neC to ground metal water piping and metal gas piping in
case it becomes energized. an underground metal water pipe can’t serve as the only
grounding electrode. otherwise, someone could disconnect the pipe or install a sec-
tion of nonconductive pipe such as PvC, thus interrupting the grounding continuity
and jeopardizing your safety. in new installations, code requires that underground
metal water piping be connected to the electrode system and supplemented with
another electrode.
At the service panel, the smaller
copper wire at the top runs to a
Major grounding elements
ground rod; the thicker copper wire
below and the neutral wire feed a
Metal water
zzzzzz
subpanel within the house.
pipes
Bonding
Clamp
Neutral/ground
jumper
bus
Service
panel
Bonding
Main bonding
conductor
jumper
Water
heater
Grounding
electrode conductor
The main ground wire from the
service panel clamps to an 8ft.
grounding electrode (or ground rod)
Grounding
electrode
driven into the earth. It diverts
(ground rod)
outside voltage, such as lightning
str
ikes.
Bonding
Clamp
jumper
Service
panel
Main bonding
jumper
Grounding
electrode
conductor
Water
heater
Connections to coldwater pipes and
gas pipes prevent shocks should the
pipes become inadvertently
energized.
Grounding
electrode
Cold-water
(ground rod)
jumper
266 Chapter 11
Also attached to the neutral/ground bus in the hot tubs, and the like. Wiring a GFCI receptacle
service panel is a large, usually bare, copper
is covered on pp. 302–303.
ground wire—the grounding electrode conductor
(GEC)—that clamps to a grounding electrode
CuTTing PoWer aT The Panel
usually either a ground rod driven into the earth,
always shut off the power to an outlet before
or a “Ufer grounding electrode”, a 20-ft. length of working on it—and then test with an electri-
steel rebar or heavy copper wire in the footing of
cal tester to be sure there’s no voltage present.
the foundation. The grounding electrode’s prima-
In rare instances, a circuit may be mistakenly fed
ry function is to divert lightning and other out-
by more than one breaker! Because individual
side high voltages to the earth before they can
devices such as receptacles, switches, and fix-
damage the building’s electrical system. Although tures can give false readings if they are defective
the grounding electrode system (GES) is con-
or incorrectly wired, the only safe way to shut off
nected to the equipment grounding system at the the electricity is by flipping a breaker in the ser-
service panel, the GES has virtually nothing to do vice panel or subpanel.
with reducing hazards from the problems in the
Turning off the power at a breaker panel is
wiring inside the house. That’s the role of the
usually straightforward. After identifying the
equipment grounding conductors.
breaker controlling the circuit, push the breaker’s
The National Electrical Code (NEC) requires
handle to the off position. The breaker handle
grounding electrode conductor size to be based
should click into position; if it doesn’t, flip it
on the sizes and types of conductors in the ser-
again until you hear a click. (A breaker that won’t
vice. Typically, residential GECs are size 6
GfCI receptacles detect miniscule
snap into position may be worn out or defective
Renovation 4th Edition Page 62
