of the electrical devices that constitute his or her “connected load” at the
same time, or to their full capacity. It would evidently be unnecessary
to provide facilities to serve such a total possible load, and much more
economical to provide only for a probable load, the load creating the
demand on the distribution facilities.
Maximum Demand
The actual load in use by a consumer creates a demand for electric
energy that varies from hour to hour over a period of time but reaches
its greatest value at some point. This may be called the consumer’s
instantaneous maximum demand; in practice, however, the maximum
demand is taken as that which is sustained over a more definite period
of time, usually 15, 30, or 60 min. These are referred to as 15-, 30-, or 60-
min integrated demands, respectively.
Demand Factor
The ratio of the maximum demand to the total connected load is
called the demand factor. It is a convenient form for expressing the relationship
between connected load and demand. For example, a consumer
may have ten 10-hp motors installed; at any one time, some will not be
in use and others will not be fully loaded, so that the actual demand may
be only 50 hp; the demand factor is 50 divided by 100, or 50 percent.
The demand factor differs for different types of loads, and by averaging
a large number of loads of each type, typical demand factors can
be obtained. These values are important in determining the size of facilities
to be installed for a particular service; they are extremely useful in
making estimates in planning new distribution systems or in expanding
existing ones.
Load Factor
The load factor is a characteristic related to the demand factor,
expressing the ratio of the average load or demand for a period of time
(say a day) to the maximum demand (say 60 min) during that period.
For example, a consumer household may have a maximum demand of 2
kW during the evening when many of its lights, the TV, the dishwasher,
and other appliances are in use. During the 24-h period, the energy
consumed may be 12 kWh; thus the average demand or load is 12 kWh
divided by 24 h, or 0.5 kW, and the load factor in this case is 0.5 kW
divided by 2 kW, or 25 percent. This provides a means of estimating
particular consumers’ maximum demand if both their consumption and
a typical load factor for their kind of load are known.
Diversity
Consumer load diversity describes the variation in the time of use,
or of maximum use, of two or more connected loads. Load diversity
is the difference between the sum of the maximum demands of two
or more individual consumers’ loads and the maximum demand of
the combined loads (also called the maximum diversified demand or
maximum coincident demand). For example, one consumer’s maximum
demand may occur in the morning, while another’s may occur in the
afternoon, and still another’s in the early morning hours.
Diversity Factor
The diversity factor is the ratio of the sum of maximum demands
of each of the component loads to the maximum demand of the load
as a whole (or the coincident maximum demand). For example, each of
the loads mentioned above may have a maximum demand of 100 kW,
while the coincident maximum demand on the system supplying the
three may be only 150 kW. The diversity factor is then 300 (100 + 100 +
100) divided by 150, or 2, or 200 percent. Such diversity exists between
consumers, between transformers, and between feeders, substations, etc.
Note that the demand factor is denned so that it is always less than 1 or
100 percent, while the diversity factor is the reciprocal of the demand factor
and is always greater than 1 or 100 percent. This is a most important
factor in the economical planning and design of distribution facilities.
Coincidence Factor
The coincidence factor is the ratio of the maximum coincident total
demand of a group of consumers to the sum of the maximum demands
of each of the consumers.
Utilization Factor
The ratio of the maximum demand of a system to the rated capacity
of the system is known as the utilization factor. Both the maximum demand
and the rated capacity are expressed in the same units. The factor
indicates the degree to which a system is being loaded during the load
peak with respect to its capacity. The rated capacity of a system is usually
determined by its thermal capacity, but may also be determined by voltage
drop limitations, the smaller of the two determining the capacity.
Power Factor
The ratio of power (in watts) to the product of the voltage and
current (in volt-amperes) is called the power factor. It is a measure of
the relation between current and voltage out of phase with each other
brought about by reactance in the circuit (including the device served).
Since facilities must be designed to carry the current and provide for
losses which vary as the square of the current, and for voltage drops
which are approximately proportional to the current, it is necessary that
current values be known. The power factor enables loads and losses designated
in watts to be converted to amperes. Transformer sizes, wire and
cable sizes, fuses, switch ratings, etc., are all based on values of current
they must carry safely and economically.
