TM5-811-1/AFJMAN
32-1080
CHAPTER 3
VOLTAGE SELECTION
3-1. General.
b. Voltage ranges. Voltage ranges are classified
as low-voltage (1 kV or less); medium-voltage
The design of electric supply and distribution
(above 1 kV to 99.9 kV); and high-voltage (above
systems can proceed only after a distribution
99.9 kV).
voltage level has been determined. The electrical
c. Voltage rating of equipment. Voltage rating of
impact of the installation or facility as well as its
equipment is based on nominal voltage classes
location will influence the selection. A new service
which, in conjunction with the maximum voltage
may be necessary or extension of an existing
rating for that class, provides a simple method for
service may be acceptable. Before discussing selec-
rating equipment. Table 3-1 indicates the nominal
tion of the system voltage, system voltage termi-
voltage class designation (also known as the insu-
nology and preferred voltage ratings need to be
lation class) used in this manual, along with the
defined. Refer to the glossary for definitions of
maximum voltage rating that may be handled by
standard voltage terms.
the equipment, and the normal basic insulation
3-2. System Voltage Classifications.
level (BIL) applying, and relates these characteris-
tics to system use and voltage range.
Voltage systems are classified either by the system
d. Nominal system voltage. The nominal system
use or the voltage range. More specific methods
voltage is the nominal value assigned to designate
include using the voltage rating of equipment, the
a system of a given voltage class. Nominal system
nominal voltage class, or the nominal system
voltages are classified by IEEE Std 141 as stan-
voltage.
dard and nonstandard voltages. Table 3-2 lists
a. System use. The requirement for electric
standard and nonstandard nominal system volt-
power transfer will cause certain voltage levels to
ages.
be more economical than others. A transmission
system transfers energy in bulk between the
3-3. Selection of Primary Distribution Voltage
source of supply (the utility) and the center for
for New Installations.
local distribution (the main electric supply sta-
tion). A primary distribution system delivers en-
A preferred nominal system voltage such as 12
ergy from a main electric supply station to utiliza-
kV, 12.5 kV, 13.2 kV or 13.8 kV, will be selected
tion transformers. A secondary distribution system
for the primary distribution system. On sizable
delivers energy from a utilization transformer to
installations where distances to loads are consider-
points of utilization.
able or loads are large, the use of 34.5 kV or 24.9
Table 3-1. System Use and Voltage Range Relationship to Equipment Rating,
Equipment
rating
Nominal
Rated maximum
System
Voltage
voltage class
voltage
BIL
use
range
kV
kV, rms
kV
230
242
900
High
161
169
750
Transmission
138
145
650
115
121
550
69
72.5
350
46
48.3
250
35
38.0
200
Medium
25.8
25
150
Primary
15.5
15
110
distribution
7.5
8.25
95
5
5.08
95
3-1
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