After mapping
out your home's wiring circuits, the
next step is to determine your
present usage, or electrical load.
This would be a time-consuming task
if you had to go around the house
and add up all the wattages of the
lights and appliances; however, the
National Electrical Code has
established certain values that
represent typical electrical usage.
Three watts per
square foot of existing living space
and space for future use is used to
figure electrical load for
general-purpose circuits (general
lighting and receptacles). A nominal
value of 1,500 watts is used for
each 20-amp small-appliance circuit
(circuits that power receptacles in
the kitchen, dining room, family
room, breakfast room, and pantry)
and for a laundry circuit.
By applying
these values to your own home and
using the actual nameplate values
affixed to major appliances, you can
use one of several formulas to
calculate your electrical load.
100 amps or more: One formula for
homes with 120/240-volt service and
a rating of 100 amps or more is
presented as a worksheet below.
To understand
how the formula works, consider the
example of a house with 1,800 square
feet (based on outside dimensions)
of finished living space and space
adaptable for future use. The house
has the usual two small-appliance
circuits (3,000 watts), a laundry
circuit (1,500 watts), a hot water
heater (5,500 watts), a clothes
dryer (5,600 watts), a dishwasher
(1,500 watts), a garbage disposal
(600 watts), a range (15,000 watts),
and a central air conditioner (5,000
watts).
The first step
is to multiply 1,800 square feet by
3 watts per square foot. The total
is 5,400 watts for lighting and
general-purpose circuits. Add 3,000
watts for the two small-appliance
circuits and 1,500 watts for the
laundry circuit for a total of 9,900
watts. Next, add the values of all
the major appliances, except the air
conditioner, for a total of 38,100
watts.
The next step
is to figure 40 percent of the
amount over 10,000 watts (0.40 x
28,100 = 11,240 watts). Adding the
10,000 watts to the 11,240 watts
gives a subtotal of 21,240 watts.
Then add the 5,000 watts of the air
conditioner for a grand total of
26,240 watts. This is your estimated
load in watts.
To figure the
current needed to carry that load,
divide 26,240 watts by 240 volts.
The total comes to 109.33 amps;
therefore, of the standard service
ratings (60, 100, 125, 150, and
200), the rating for the sample
house should be 125 amps or higher.
Now you can try
it yourself: Enter the values
appropriate to your home on the
worksheet, then compare the total
load in amps with your present
service rating. If the two values
are close together, your present
service cannot handle the addition
of many new loads.
Less than 100
amps: If your service rating is less
than 100 amps, you can't use the
formula given on the worksheet to
calculate your load. You can,
however, use a different formula
that incorporates the same NEC
values for typical electrical usage.
Therefore, the general-purpose
circuits, small-appliance circuits,
and laundry circuits are computed
exactly as they are in the first
three entries of the table.
Once you've
figured the general-purpose circuit
load (3 watts x number of square
feet of living area), add 1,500
watts for each 20-amp
small-appliance circuit and laundry
circuit. Using this total, add 100
percent of the first 3,000 watts and
35 percent of the balance over 3,000
watts [3,000 + 0.35 (total -
3,000)].
Add to this
value the nameplate ratings of all
major appliances (space heater,
garbage disposal, dishwasher, etc.).
This gives you your estimated load
in watts. You can find the current
by dividing the total wattage by
your voltage--120 volts for two-wire
service or 240 volts for three-wire
service.