TM
5-811/1AFJMAN
32-1080
CHAPTER 8
TRANSFORMER INSTALLATIONS
8-1. Definitions.
dangerous, generally more difficult to maintain,
requires more space, and there is rarely a signifi-
Transformers are classified in various ways accord-
cant cost saving since fencing is required. Pad-
ing to their construction and application. In order
mounted compartmental transformers may only be
to clarify usage in this manual, transformer termi-
used outdoors even when designed for both indoor
nology is defined in the glossary. Additional trans-
and outdoor installation. Unit substation trans-
former definitions and terminology can be found in
formers may be used indoors or outdoors.
IEEE Std C57.12.80.
(2) Pad-mounted compartmental liquid-insu-
8-2. Installation of Distribution-to-Utilization
lated transformers. Three-phase pad-mounted com-
Voltage Transformers.
partmental transformers may be applied in ANSI
standard sizes through 2500 kVA, except that they
a. Aerial mounting.
will not be used where the primary voltage ex-
(1) Unit capacities, mountings, and types.
ceeds 15 kV or where fault currents are so large
Aerial transformer installations may utilize a
that standard equipment does not provide the
three-phase unit or banked single-phase units.
required primary interrupting duty. The choice
Transformers, either singly or in banks, having an
between pad-mounted, compartmental transform-
individual unit or combined capacity greater than
ers and unit substations in conjunction with inte-
300 kVA will not be mounted on single wood
gral or non-integral load-center transformers will
poles. Special structural considerations will be
be based on: the application; potential for expan-
given for single-pole installations greater than 100
sion; short circuit and protective device coordina-
kVA. Pole-platform mounting (two-pole structure)
tion; sound engineering judgment; accepted indus-
will not be used, except where other mounting
try practice; and the operating, maintenance and
methods are not satisfactory. Cluster mounting for
reliability considerations listed below.
transformer banks is preferred over crossarm
(a) If instruments and switches are rou-
mounting as less visually objectionable. Similarly,
tinely used by operators, choose unit substations
the cluster or three-phase bracket mountings will
and integral or non-integral load-center transform-
be permitted for mounting of surge arresters and
ers. For pad-mounted compartmental transformers,
cutouts if acceptable to the Using Agency responsi-
instruments and operating devices are inside a
ble for the operation and maintenance of trans-
locked compartment and are not readily accessible.
former installations. Figures 8-1 and 8-2 indicate
(b) Pad-mounted compartmental transform-
transform bank installations. Self-protected trans-
ers require use of a switch stick for switch mecha-
formers have internal primary fuses that must be
nism operation and cable-disconnection (due to the
replaced by experienced personnel. Therefore, self-
dead-front requirement). The typical high-voltage
protected transformers will not be specified.
switch used with unit substations does not require
(2) Location. Aerially mounted installations
use of a special tool and is therefore easier to
may supply several buildings. When that is the
operate.
case, transformers will be installed at the pole
(c) Fan cooling and 12 percent additional
location closest to the building with the greatest
capacity for 55/65 degree C temperature rise is
load. Secondary wiring should drop directly to the
available for unit substations and integral load-
buildings served, if the span does not exceed 125
center transformers, but is not available for pad-
feet; otherwise, intermediate poles are required.
mounted compartmental transformers which are
b. Ground level mounting.
self-cooled; fans cannot be added to increase the
(1) Types and capacities. Ground-level mount-
cooling capacity.
ing may be either the pad-mounted compartmental
(3) Residential service. Pad-mounted trans-
type or the unit substation type. Figures 8-3 and
formers will be routinely used in residential and
8-4 show a typical pad-mounted compartmental
light commercial service.
transformer installation. Figure 8-5 shows a typi-
(4) Industrial service. Pad-mounted transform-
cal secondary unit substation transformer installa-
ers may be used for industrial, commercial, or
tion. The use of conventional-type (pole-mounted)
industrial application, provided that only one
transformers, with connections to separate pri-
building is served; metering and secondary switch-
mary and secondary protective devices, is not
gear can be located in the building; and short
permitted since this type of installation is more
8-1
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