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REA Bulletin 60-9

Economical Design of Primary Lines for Rural Distribution Systems

REA Bulletin 60-9 Economical Design of Primary Lines for Rural Distribution Systems

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Date Published:

May 1960


Select primary conductor sizes for electric distribution lines based on the economic factors of power losses.


In designing a distribution line, an engineer is bound by threemajor considerations - voltage drop, service reliabj.lity, andpower loss in conductors. Very often voltage drop is the limitingfactor in the selection of conductor size and the number of phases;and the load to be supplied by a single feeder may be limited inthe interest of more reliable s~rvice. As the use of power continuesto grow, however, the engineer finds more instances where,having satisfied these two considerations, the selection of conductorsize and phasing must be made after an economic analysisof the power loss in conductors.

Such an analysis should be made for each conversion of an existingline to higher capacity and for construction of new multiphase lines.It should be used to determine which of two or more possible methodsof correcting excessive voltage drop on a given line is more economical.It is also usefUl in determining whether a conversion tohigher capacity can be justified by savings in power loss.

This bulletin shows two alternate methods of selecting line designson the basis of economics. The first method uses a simple comparisonof the annual costs of two or more line designs. The secondmethod is more comprehensive and gives a means of determining therange of loads .for which each type and size of line is the most economical.Either methOd is acceptable to HEA. for use in studies madefor REA borrowers.

Each method has its advantages. Generally, if only a few lines mustbe analyzed or if power costs or construction costs vary betweendifferent parts of the system, the comparison method is easier touse. For a large system with uniform power and construction coststhe loading limits method will require fewer calculations. The twomethods will give essentially the same results except where the annualload factor of the line being studied differs significantlyfrom the annual load factor of the substation. In this case thecomparison method is more accurate since it takes both l oad factorsinto account.

These methods are tools for the engineer to use in selecting conductorsize and phasing for specific sections of line. They are intendedprimarily for use in developing construction plans or annual workplans. They can also be used in determining which of two primarydistribution voltages is more economic.

This bulletin supersedes REA Bulletin 60-4, Selection of EconomicalConductor Size and Power Loss in Conductors, and all other material in conflict with its provisions .