Scope. This handbook provides the criteria necessary for the
proper selection of electric power sources and distribution systems. It
covers preliminary load estimating factors, electrical power sources, and
Cancellation. This handbook cancels and supersedes DM-4.1,
Electrical Engineering Preliminary Design Considerations, of December l979,
and change dated 1 March 1983.
Load Data. Before specific electric power sources and distribution
systems can be considered, realistic preliminary load data must be compiled.
The expected electric power demand on intermediate substations, and on the
main electric power supply, shall be calculated from the connected load
layout by applying appropriate factors. Determine these factors by load
analysis and by combining loads progressively. To combine the loads, start
at the ends of the smallest feeders and work back to the electric power
source. Because all loads must be on a common kilowatt (kW) or kilovolt-
ampere (kVA) basis, it is necessary to convert motor horsepower ratings to
input kilowatts or kilovolt-amperes before combining them with other loads
already expressed in those terms. Preliminary electric power load estimates
can be made by using the approximate value of one kilovolt-ampere of input
per horsepower (hp) at full load. Preliminary estimates of lighting loads
may be made by assuming watts per ft2 (m2) of building area.
Load Analysis. To determine appropriate load estimating factors,
using the tables and factors in this manual as guides analyze the
characteristics of each load. Consider items such as environmental
conditions of weather, geographical location, and working hours, as the
situation dictates. Notice that when the load densities in w/ft2 (m2) are
used only in preliminary estimates, the demand and load factors will be used
in the final designs.
Terminology. Five terms are essential to the analysis of load
characteristics: demand factor, coincidence factor, diversity factor, and
maximum demand. These terms are defined in paras. 1.5.1 through 1.5.4.
Demand Factor. The demand factor is the ratio of the maximum
demand on a system to the total connected load of the system or
Maximum demand load
Total load connected
Coincidence Factor. The coincidence factor is the ratio of the
maximum demand of a system, or part under consideration, to the sum of the
individual maximum demands of the subdivisions or
Coincidence factor =
Maximum system demand
Sum of individual maximum demands