LOAD
COMPUTATIONS
Load computations involve computing the demand load for a building system or a
distribution system extending from a panelboard. This load includes the total of all
general lighting, appliance, and equipment loads in the building. The demand load
allowed by the code takes into account that all of the electrical connections will not be
in use at one time. As a result, the demand load is not a total of all connected loads,
but rather a fraction of the connected loads. Code prescribes methods of computing
the demand load, which trims down the total connected load to a safe, more
reasonable level.
General considerations
in demand load
computations
1. Compute the general lighting load. This is calculated for all
types of occupancies based on the unit load given in the table
(in watts) times the square footage of the building. (For this
exercise it is based upon Code specified 3 VA/ft2 for
residences). The floor area is determined using the outside
dimensions of the building involved and the number of stories.
For dwellings, do not include any open porches, garages, or
carports. Any unfinished or unused spaces do not have to be
included in the square footage unless they are adaptable for
future use.
For the (hypothetical) residential system example, the
outside dimensions (excluding garage) of the garage are:
61 ft 6 in x 36 ft, there are 2214
𝟐
𝐟𝐭
of floor area
The minimum general lighting load, based on the
specified 3 VA/ft2 for residences:
3 VA/ft2 x 2214
2
ft
= 6642 VA
2. Compute the appliance and laundry circuit load.
Code requires at least two 20 A appliance branch
circuits for the kitchen. The load is based on 1500 VA
(from Code) for each appliance branch circuit in the
kitchen. In addition, one 20 A circuit is required for
laundry room appliances.
For the residential system example, this results in a
total of three 20 A branch circuits for appliances:
Appliance and laundry load = 3 circuits x 1500 VA
= 4500 VA
3. Subtotal the general lighting, appliance, and laundry
branch circuit loads. For the residential system example:
General lighting
Appliance and laundry circuits
Subtotal
6642 VA
4500 VA
11 142 VA
4. The demand load allowed by the Code takes into
account that all of the electrical connections will not
be in use at one time. Although there are limits to this
reduction for certain types of occupancies, in a
dwelling the first 3000 VA are taken as 100%, and
from 3000 to 120 000 VA, only 35% of the load is
calculated.
For the residential system example, the load subtotal is
11 142 VA, so:
First 3000 VA at 100%
Remaining 8142 VA at 35%
Total demand load
3000 VA
2850 VA
5850 VA
The loads of all other appliances and equipment
(motors) must be added to this demand load to
determine the total service load on the system.
5.) To determine the appliance and equipment load, all
appliances and equipment that will not be on the lines
discussed above must be listed along with their
electrical requirements. Although typical ratings are
given in Tables 19.12 through 19.16, nameplate
ratings from manufacturers’ data should be used in the
design.
For the residential system example, the following is a
list of fixed appliances and equipment and their ratings
from manufacturer’s data:
Water heater
Clothes dryer
Dishwasher
Range
Waste disposal
Air conditioner
Garage door opener
3800 VA
4400 VA
1000 VA
11 700 VA
1000 VA
6300 VA
1000 VA
❑ The demand load for an electric range, consisting of an
oven and a cooktop unit, is taken from manufacturer’s
data.
For the residential system example, an electric range with a
rating of 11.7 kW, the demand load would be 8 kW (or 8000
VA):
Electric range demand load = 8000 VA
❑ The demand load for the clothes dryer is the total
amount of power required according to the
manufacturer’s data.
For the residential system example, the full 4400 VA
must be used in the calculation:
Clothes dryer demand load = 4400 VA
❑ The demand for fixed appliances (other than the range,
clothes dryer, and air conditioning and space heating
equipment) is taken as 100% of the total amount
required, except that when there are four or more of
these fixed appliances (other than those omitted), the
demand load can be taken as 75% of the fixed appliance
load.
For the residential system example, there are three fixed
appliances: the water heater at 3800 VA, the dishwasher at
1000 VA, and the waste disposal at 1000 VA. The total of
the ratings is 4800 VA, also the demand load.
Fixed appliances demand load = 5800 VA
❑ Motors, such as those used in central air
conditioners, have their demand loads calculated as
125% of the motor rating. For the residential system
example, the air conditioner is rated at 6300 VA and
the garage door opener is at 1000 VA. The total
demand load will be 7300 VA 125% 9125 VA.
Motor (air conditioner/opener) demand load
= 9125 VA
The total demand load for all of the lighting and appliances is then
tabulated. For the residential system example:
General lighting, appliances, and laundry 5850 VA Electric range
8000 VA
Clothes dryer
4400 VA
Fixed appliances
5800 VA
Motor (air conditioner/opener)
9125 VA
Total demand load
33 175 VA
CIRCUIT DESIGN
Circuit design involves ascertaining the number and rating of
circuits needed in the panelboard. It generally involves the following
steps:
1. Determine the minimum number of lighting circuits by dividing
the general lighting load by the voltage, finding the amperage
required and dividing the amperage into circuits.
For the residential system example, the general lighting
load was calculated above as 6642 VA and the voltage used
for the lighting is 120 V. Therefore:
6642 VA/120 V = 56 A
Because the general purpose branch circuit size is limited
to 80% of the rating, four 20 A branch circuits for a total of
64 A, or five 15 A branches for a total of 60 A, are needed.
Because the general purpose branch circuit size is limited to 80% of the rating,
four 20 A branch circuits for a total of 64 A, or five 15 A branches for a total of 60
A, are needed.
For 20 A circuits: 56 A>(20 A 80%) = 3.5
= 4 circuits
For 15 A circuits: 56 A>(15 A 80%) = 4.7
= 5 circuits
This is the minimum number
of branch circuits required to
serve the residence. In laying
out the circuits, almost all
designs
will
have
more
circuits than the minimum
number required. This is
because most designers will
limit each circuit to fewer
receptacles, lights, or a
combination of receptacles
and lights
2. Lay out and number all branch circuits on the drawing. Bear in
mind that, in most cases, all of these general use receptacles and all
lighting will use 120 V service. In large commercial applications, 277 V
may be used for interior lighting. Note that each circuit is numbered.
For the residential system example, there are a total of 14 circuits.