they are to carry safely and the permissible voltage variations; the final
design, however, cannot be divorced from mechanical, economic,
and other considerations. Several different designs may serve the same
electrical requirements adequately; each, in turn, may be modified by
mechanical considerations.
The design ultimately selected must reflect
economic considerations: specifically, the design that results in the least
annual expense in supplying the load or loads in question. This necessarily
involves the evaluation of losses, as well as capital, maintenance,
and operation expenses. Often, other considerations must also be taken
into account including government regulations (at all levels), national
and local industry construction and safety codes, taxes, public relations,
and some other, intangible requirements.
SERVICES
Rather than design a separate service for each consumer, it is more
practical and economical to determine the capacity and construction
requirements on a group basis for different types of consumers. The
maximum demand for a consumer group is determined by the connected
load, to which a demand factor may be applied. The factor can
be an estimate based on observation or a logical analysis of the operations
of the several devices comprising the load, or it may be obtained
from previous experience. Further, service entrance equipment specified
by national and local codes (and often installed by the consumer) has
minimum ratings based on the number and kinds of circuits installed
within the consumer’s premises.
Each group represents consumers whose maximum demands fall
within certain ranges, expressed in volt-amperes or kVA, and for which
certain conductor sizes are specified, listed in Table 4-1. These values
are based on a single-phase three-wire 120/240 V supply via three-conductor
self-supporting cable for a 100-ft length, carrying the maximum
amperes listed and producing a 1 percent voltage drop. Losses at the
maximum loads are less than 2 percent. For longer service drops (sometimes
in rural areas), these values may be exceeded.
Three-conductor self-supporting service cables are almost always
specified because of their appearance and ease of installation as compared
to older (now almost obsolete) open-wire-type services; voltage
drop for the same load and length is slightly less than for the open-wire
type. The sizes of conductors specified are more than ample to support
the mechanical stresses imposed on them, even in severe weather conditions.
Local conditions, including varying costs of both labor and materials,
rates of growth, and other factors, may substantially change the
values shown in Table 4-1. Services for the relatively fewer larger commercial
and industrial consumers served at secondary voltages are usually
determined individually.
