Sizes of
conductors of primary circuits are also based on acceptable voltage drop and
losses in the conductor and the cost of the facilities; the mechanical
requirement may be the decisive factor. The principles and methods given for
secondary circuits also apply here.

As indicated
earlier for secondary systems, the most economical size of conductor for a
proposed load (present, future, and contingency) may be determined by an
analysis of the annual carrying charges for the system considered and the
annual cost of energy losses in the conductor.

Branches of
the primary circuit may supply from one to a great many transformers. Where
only one transformer is involved, voltage drops and losses may be calculated as
a concentrated load at the end of the line.

Where the
branch is relatively long and serves a few transformers widely spaced, these
values may be derived from a circuit considered to have a distributed load.
Where the length is short, or where a larger, more closely situated number of
transformers exist, the circuit may be considered as supplying a uniformly
distributed load; the total loads of these transformers can be assumed to be
concentrated at a point half the length of the branch (from the tap-off at the
main to the last transformer) in calculating the maximum voltage drop, and at
one-third the distance (from the tap-off at the main) for calculating losses in
the entire length of the branch.

For
single-phase circuits, the characteristics of the neutral conductor should also
be considered. For polyphase branches, each phase and the transformers
connected to it may be considered separately; the loads on the separate phases may
be considered balanced and the neutral ignored.

Voltage and
loss calculations for the three-phase main portion of the feeder may be
considered to be concentrated at the tap-off point of the main; these, together
with the transformers connected to the main, can be considered as a uniformly
distributed load on the main.

In some instances,
the main may proceed from the substation for a certain length before serving
any branches or transformers. In this case, the main can be considered in two
parts.

The portion
to which branches and transformers are connected may be considered to have
uniformly distributed load, with voltage and losses calculated accordingly. The
untapped portion of the main (from the substation to the first load connected
to it) may be considered to be a line with the entire load (the uniformly distributed
load mentioned earlier) concentrated at its end (where the first load is
connected).

The loads
may be assumed to be balanced and the neutral neglected. The total voltage drop
is the sum of the drops in the two portions of the main; the total losses in
the feeder main are also the sum of those in the two portions.

In
considering the total annual cost of the primary line for comparison with the
annual cost of the losses in it, in addition to the cost of the conductors in
place, the cost of poles, insulators, switches, etc., must also be included as
well as the annual costs of operation and maintenance.

Voltage
drops and energy losses are reduced substantially as the applied voltage values
increase. For primary circuits, particularly those operating at higher
voltages, these values are considerably less than for comparable secondary
quantities.

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