is deteriorated, splices are incorrectly made, the
not exceed these capacities; otherwise, you risk overheating wires. To avoid over-
wiring is overloaded, or buried in thermal insula-
loading, it’s a good idea to reduce the capacity by 20%. for example, 80% of 1,800w
tion. Typically, splices that were part of the origi-
is 1,440w for a 15-amp circuit; 80% of 2,400w equals 1,920w for a 20-amp circuit.
nal installation will not be in a junction box but
must be wrapped with electrical (friction) tape
and supported by porcelain knobs on both sides
safe Circuit Capacities*
of each splice. Non-original splices must be
housed in covered boxes. Have knob-and-tube
amperes × Volts†
total CapaCItY (watts)
saFe CapaCItY†(watts)
wiring assessed or modified by an electrician
familiar with it; it’s quirky stuff. The NEC does
15 × 120
1,800
1,440
not allow knob-and-tube wiring to be buried in
insulation, although some local inspectors are
20 × 120
2,400
1,920
OK with the practice.
25 × 120
3,000
2,400
Knob-and-tube wiring lacks an equipment
ground (a separate grounding wire), so it offers
30 × 120
3,600
2,990
no protection should a faulty appliance get
plugged into a receptacle. On the other hand, a
* Safe capacity = 80 percent of total capacity.
knob-and-tube system is run completely on insu-
† Amperes multiplied by volts equals watts.
lators, a plus. The conductors of knob-and-tube
wiring were copper, coated with a thin layer of
tin (to protect the copper from sulfur in the rub-
ber wire insulation). Uninformed inspectors
often mistake the tinned copper wire for alumi-
num wire.
such as electric ranges and hot tubs. If there are
S a f e t y a l e r t
only two large wires running from the utility pole
is The sysTeM adequaTely sized?
to the house, they deliver only 120v service. A
don’t add outlets to circuits
house with two-wire service probably has a
If receptacles in your house teem with multiplugs
that have ungrounded cable;
30-amp or 60-amp main fuse or breaker, which is
wires with frayed, discolored, or
and extension cords, you probably need to add
inadequate for modern usage.
melted insulation; blackened
outlets. But there are more subtle clues: If you
metal boxes or other signs of arc-
blow fuses or trip breakers regularly, or if the
rules of thumb. The only sure way to know if
ing; or aluminum wire (a soft,
lights brown out when you plug in a toaster or an you’ve got enough capacity to add an outlet or a
dull silver) rather than copper.
electric hair dryer, you may need to add new cir-
circuit is to calculate electrical loads, as shown
don’t confuse old knob-and-tube
cuits to relieve the overload on existing circuits.
on the next page. But for the benefit of those who
wire, which is usually tinned cop-
This section will help you figure out whether
wish that an electrician would just offer an off-
per, with aluminum wire; the
your system has the capacity to do so.
hand opinion of what works most of the time,
tinned copper has a shiny silver
These days, three-wire service feeding a
here are a few rules of thumb.
coating on the outside, but at
100-amp service panel is the minimum required
Fuse box service. If you’ve got a fuse box with
the cut end, you can see the cop-
by the NEC, and many electricians install
per color of the wire.
a 30-amp or 60-amp main fuse, the best advice
150-amp or 200-amp panels if the homeowners
we can give is to upgrade your service. Don’t add
plan to enlarge the house at some point or
outlets or circuits until you replace the fuse box
acquire a lot of heavy energy-using appliances,
with a breaker panel. Fused mains are often
electrical Wiring
271
example of load Calculation for single family dwelling
CalCulatIng general lIghtIng loaD
type of load
neC reference
Calculation
total Va
lighting load
Table 220.12
2,000 sq. ft. 3 va
6,000 va
small appliance load
section 220.52
2 circuits 1500 va
3,000 va
laundry load
section 220.52
1 circuit 1500 va
1,500 va
total general lighting
10,500 Va*
A CalCulatIng DemanD For general lIghtIng loaD
type of load
Calculation
Demand Factor (DF)
total Va
general lighting
first 3000 va df
100%
3000 va
general lighting
7,500 ‡ df
35%
2,625 va
total lighting, small appliances & laundry
A 5,625 Va
B CalCulatIng DemanD For large loaD applIanCes
type of load
nameplate rating
Demand Factor (DF)
total Va
electric range
not over 12kva
use 8kva
8,000 va
Clothes dryer
6,600 va df
100%
6,600 va
Water heater
6,600 va df
100%
6,600 va
other fixed appliances
0 va df
100%
0 va
total load for large appliances
B 21,200 Va
total Va (A + B)
26,825 Va
minimum service size
total Va /240V
111.77 Va
The minimum service size is the next standard size above the total va calculated. based upon these calculations
the minimum service size is 125 amps.
*use this to calculate A
‡ Total general lighting load 10,500 va – first 3000 va = 7,500 va
usIng thIs taBle
1. square ft. for general lighting load is for the entire dwelling including habitable basements and attics.
2. neC requires a minimum of 2 small appliance loads, but it is important to add small fixed kitchen
appliances such as refrigerator, microwaves, disposals, dishwashers, large range hood, computers, etc.,
when calculating this category
3. Minimum of 1 laundry load is required for a single family dwelling.
4. The demand factor calculation is designed to take actual use into account (e.g., it is unlikely that all lights
and small appliances will be running at one time).
5. all large load appliances (high wattage) are added at 100%.
6. The final load calculation is the minimu
m. often, increasing capacity has little cost impact and is a good practice.
Some appliances require dedicated
The grounding conductor of a cable is connected to a steel All devices, including receptacles, switches, and lighting service because they are heavy energy
box with a special grounding screw, which must thread
fixtures, also must be bonded. Here, a bare ground lead
users. This is a 30amp, 125/250v
into a tapped hole.
from spliced ground wires in the box is attached to the
dryer receptacle. The breaker for this
green grounding screw of a duplex receptacle.
circuit must also be rated for 30 amps.
272 Chapter 11
abused by people trying to bypass its protection,
the service panel via an equipment grounding con-
so insurance companies often charge higher pre-
ductor (ground wire). NM cable contains a sepa-
miums on houses with fuse boxes.
rate ground wire, whereas armored cable sheath-
ing and metal conduit provide the path to ground.
adding outlets. If you have a breaker panel
with space to add an additional breaker, you can
boxes. All electrical connections must take place
most likely add a circuit to feed a new outlet or
in covered boxes. Boxes where connections are
two or more lights. If, for example, you have
made must be accessible, i.e., not buried in a wall
three-wire service and a 100-amp main, there’s
or ceiling. Based on local code requirements,
usually a lot of excess capacity.
boxes may be plastic or metal. If metal, the box
must also be connected to the equipment ground-
adding a circuit for general use. If there is an ing conductor (bonded). If NM cable is used, the
unused space in the panel, have an electrician deter-
ground wire must be connected to a metal box
mine whether the panel can handle another circuit.
with either a ground screw or a ground clip. If
adding a kitchen or bath circuit. If you
AC cable or metal conduit is used, it must be
want to add a bath fan or some new light fix-
properly attached to the box to ensure effective
An AfCI breaker can detect current
tures, and there’s space in the panel, have an elec- bonding. If the box is plastic, it does not need to
fluctuations associated with arcing.
trician see if you can add a circuit. Adding a
It then shuts off power to protect you
be (and cannot be) grounded; run a ground wire
20-amp, small-appliance circuit to reduce the
from house fires. note: Arcfault
to the device or fixture only.
breakers are designed to detect arcing
load on an existing circuit is smart.
patterns of current—many amps
remodeling a kitchen. Kitchens are compli-
general-use
(40 or more) for very short time
cated and often full of big energy users. Use the
CirCuiT requireMenTs
intervals.
chart at left to help you add up the loads. If there General-use circuits are intended primarily for
aren’t many open spaces for breakers, you may
lighting, but small loads that are connected via a
need to upgrade to a larger panel.
cord and plug, such as televisions, fans, and vac-
adding dedicated circuits. If you need to
uums, are allowed—as long as the power they
add dedicated circuits for heavy-use items such
draw doesn’t exceed the capacity of the circuit.
as an electric range (50 amps) or a hot tub
lighting and small loads. Although 14AWG
(60 amps), get out the calculator and do the math. wire is sufficient for lighting, electricians often
run 12AWG wire on general-use circuits to
general Wiring guidelines
accommodate future uses. Calculate lighting
Electricians follow the NEC, as adopted (or
loads at 3w per square foot, or roughly one
amended) by local jurisdictions,which was com-
15-amp circuit for every 500 sq. ft. of floor space.
piled to promote safe practices and prevent house When laying out the lighting circuits, do not put
fires. Consider these requirements before you start all the lights on a floor on one circuit. Otherwise,
drawing plans, but be sure to consult local electri-
should a breaker trip, the lights on the entire
cal code—it is the final authority in your area.
floor would be affected.
The guidelines given here apply to all circuits
receptacles. There must be a receptacle within
in the house, whether general lighting or heavy-
6 ft. of each doorway, and receptacles should
use appliance circuits. Local codes rarely require be spaced at least every 12 ft. along a wall. (This is
you to change existing circuits—as long as they
also stated as, “No space on a wall should be
are safe—but new electrical work should reflect
more than 6 ft. from a receptacle.”) Any wall at
current electrical code.
least 2 ft. wide must have a receptacle, and a
Circuit wiring. Wire gauge must be large enough receptacle is required in hallway walls 10 ft.
to carry the circuit load and be protected by a
or longer. Finally, any foyer of more than 60 sq. ft.
comparably sized breaker or fuse at the panel.
must have a receptacle on any wall 3 ft. or longer.
General-use and lighting circuits are typically
outlets. The NEC does not specify a maximum
14AWG wire, protected by 15-amp breakers;
number of lighting and receptacle outlets on a
kitchen, bath, and workshop circuits usually have residential lighting or appliance circuit, although
12AWG wire, protected by 20-amp breakers.
local jurisdictions may. Figure roughly nine out-
acceptable cable. Most circuits are wired with
lets per 15-amp circuit and 10 outlets per 20-amp
NM cable because the cable is protected behind a circuit.
finished surface such as drywall or plaster. When light switches. There must be at least one wall
circuit wiring is to be left unprotected and
switch that controls lighting in each habitable
exposed, it must be armored cable or in conduit.
rooms, in the garage (if wired for electricity), and
Grounding. All receptacles, appliances, and elec-
in storage areas (including attics and basements).
trical equipment must be connected (bonded) to
There must be a switch controlling an outside
electrical Wiring
273
along a kitchen countertop should be more than
2 ft. from an outlet—in other words, space coun-
kitchen lIGhtInG basICs tertop receptacles at least every 4 ft. Every coun-
ter at least 12 in. wide must have a receptacle.
Kitchen lighting should be designed to utilize natural light during the day and achieve
Kitchen lighting. Adequate lighting is particu-
a balance of general light and task lighting at night. Do not be afraid of energy-
larly important in kitchens so people can work
efficient lighting such as fluorescent. today’s energy-efficient lighting is instant,
safely a
nd efficiently. Lay out a good balance of
dimmable, and available in colors that match incandescent light. Kitchen lighting is
general and task lighting. Be aware that many
often highly regulated for energy efficiency. Check with local building officials before
jurisdictions have energy-efficiency requirements
you begin your design.
for lighting in kitchens, so check with your local
general lighting
building authority first.
general lighting is meant to illuminate the space generally and can come from
bathroom lighting. It is important to illuminate
recessed cans, surface-mounted fixtures, track lighting, or cove uplighting. Consider
the face evenly in mirrors. Common practice is to
cabinetry and appliances when laying out new light fixtures. a general rule is 2w
place good-quality light sources either above the
incandescent or 1w fluorescent per sq. ft. of kitchen area, but even illumination is
vanity mirror or on either side of it. Be careful
the goal.
when using recessed cans over the vanity because
they can leave shadows across the face. Many
task lighting
jurisdictions also have energy-efficiency require-
task lighting is meant to provide a higher level of illumination at work areas (sinks,
ments for lighting in bathrooms.
countertops, and islands) and can be achieved with recessed cans, pendants, or
dedicated circuits. All critical-use and fixed
undercabinet fixtures. If cabinets are over countertops, undercabinet fixtures
appliances should, and in most cases must, have
(t5 fluorescent or leD strips) are by far the best choice and should be spaced for even
their own dedicated (separate) circuits. These
illumination of the counter surface. For islands and sinks, choose a recessed can with
fixed appliances include the water pump, freezer,
a slightly higher wattage and narrower lamp beam spread, or install pendants with
refrigerator, oven, cooktop, microwave, furnace
similar attributes.
and/or whole-house air-conditioning unit, win-
dow air conditioners, and water heater. A bath-
room heater requires a dedicated circuit, whether
it is a separate unit or part of a light/fan. Laundry
light near each outdoor entrance. Three-way
S a f e t y a l e r t
room receptacles must be on a dedicated circuit;
switches are required at each end of corridors
so must an electric clothes dryer.
and at the top and bottom of stairs with six steps
all bathrooms and kitchens
or more. When possible, put switches near the
must be gfCi protected. gfCi
Renovation 4th Edition Page 64