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.