Emergency Loads. The determination of emergency electric power
requirements is based on three types of loads (refer to Section 3 for types
of loads to be included in each category):
minimum essential load,
emergency load for vital operations, and
uninterruptible (no-break) load.
When the three categories of emergency electric power requirements have been
ascertained, determine where local emergency facilities are required, where
loads may be grouped for centralized emergency facilities, and what loads are
satisfied by the reliability of the general system. Base the aforementioned
determinations on safety, reliability, and economy, in that order.
Area Loads. Area loads consist of groups of individual facility
loads served by a subdivision of the electric distribution system. The term
"area" applies to the next larger subdivision of an overall distribution
system. Demand loads for an area must be known for sizing the distribution
wiring and switching, and in a large installation will be required for the
design of substations serving the area. Table 7 gives an example of how the
coincident peak demand is calculated.
General Loads. To obtain the general load, add roadway lighting,
area lighting, obstruction lighting, and other loads not included in
individual facility loads.
Coincidence Factor. Determine the maximum expected demands, taking
into consideration whether loads within the area peak at the same or at
Relationships. Figure 1 indicates the relationship that exists
between the load factor of individual facility loads and the coincidence of
their peak demands with the peak demand of the group. This relationship was
developed by a study of the loads of selected naval shore activities and by
the application of factors developed to the formulas published by the
Institute of Electrical and Electronic Engineers (IEEE). For collateral
reading on this subject, refer to IEEE Technical Paper 45-116
Coincidence-Factor Relationship of Electric Service Load Characteristics.
Table 8 is Figure 1 in tabular form with values shown to the nearest whole
dollar, except for low load factors.
Selection. Areas with relatively insignificant residential type
loads, where the load curve indicates that most of the electric power
consumed in the area is used during the 40 normal working hours of a week,
have coincidence factors at the higher end of the range.
Electric Power Consumption. In general, areas where large amounts
of electric power are consumed outside the usual 40 working hours a week have
a coincidence factor at the lower end of the range (examples are hospitals,
areas operated on two or more shifts, or large barracks type activities).
The upper limit of the range is for a 40 hour per week operation; the lower
limit is for a 60 hour per week operation.