Section 1:

PRELIMINARY DATA

1.1

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

distribution systems.

1.2

Cancellation. This handbook cancels and supersedes DM-4.1,

Electrical Engineering Preliminary Design Considerations, of December l979,

and change dated 1 March 1983.

1.3

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.

1.4

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.

1.5

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.

1.5.1

Demand Factor. The demand factor is the ratio of the maximum

demand on a system to the total connected load of the system or

EQUATION:

Demand factor

=

Maximum demand load

(1)

Total load connected

1.5.2

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

EQUATION:

Coincidence factor =

Maximum system demand

(2)

Sum of individual maximum demands

1

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