Good engineering requires that probable future growth of loads be
considered in planning. This is usually provided for by spare capacity
in the present design of the several elements, or by provisions for possible
future additions or alterations, or both of these. Load growth is
rarely uniform throughout an area, so that growths in various parts of a
system will be different from each other and from that of the system as
a whole.
Economics
How far present capacity should provide for future load is largely
a question of economics: the cost of carrying excess capacity until it is
needed versus the cost of replacing smaller units with larger when it
becomes necessary. This is a problem of the future worth of present
expenditure, which is affected by fluctuations in rates of interest and
inflation. Standard sizes of the materials and equipment involved automatically
provide for a limited amount of spare capacity for growth,
so that any economic analysis can only be approximate. The relatively
large proportion of labor to material in the construction of a distribution
system or its parts lends itself to the installation of capacity greater than
its immediate need. Such spare capacity incidentally provides a cushion
for accommodating some of the unforeseen fluctuations in demands
described above.
Past Performance
Data from past performances, such as total system loads, substation
loads, and feeder loads, can be used as a basis for estimating such
growth. The variations from year to year, or from month to month, can
furnish a trend for such growth; separate trends can be developed for
different parts or areas. Where such data are nonexistent or patently
unreliable, estimates can include a fixed percentage growth above the
values on which planning is made.
Future Performance
To obtain some idea of what may occur in the future, it may be well
to look back a generation or two. Earlier, consumers’ appliances could
be contained in a relatively short table. To attempt to list all the electrically
operated devices, appliances, and gadgets presently to be found
in homes and commercial establishments would be an almost endless
task. To attempt to foretell what may develop in the future would be an
exercise in futility.
The advent of widespread air conditioning and space heating, together
with the almost universal use of television, not only substantially
changed consumers’ maximum demands and consumption, but also materially
affected loads, diversity, coincidence and (for larger units) power
factors, and utilization factors as well.
While the demand factor may indicate how the connected loads
are being used, the utilization factor indicates how the capacity of the
supply system is being used. Since the capacity of the supply system is
determined by its thermal capability, the increased sustained demand on
these facilities will lower their thermal capability, and hence the system
capability.
The greater use of electronically operated computers will tend to
call for narrower limits of voltage control (regulation and flicker) and a
greater degree of service reliability by stiffening the supply distribution
system, or through the installation of auxiliary equipment owned and
maintained by the consumer or rented as another service by the utility;
the choice will be determined by future developments.
