Quantcast Table 8-1. Transformer Standard Base kVA Ratings.

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TM
5-811-1/AFJMAN
32-1080
effective transformer dielectric material has been
equipment will be evaluated to determine which is
selected. Costs will include all features necessary
the most cost-effective.
to provide a proper installation, including vaults,
(4) The hazardous chemical risk will be elimi-
curbs, drains, exterior screening, interior floor
nated for those facilities which are mandatory for
space, and additional wiring as applicable.
national security.
c. Nonflammable fluid. Nonflammable fluids
(5) The contract specifications will mandate
will not be used since there are no nonflammable
that the contractor (or subcontractor) is or will be
fluid transformer dielectrics that are not toxic.
certified in the current EPA regulations for han-
Tetrachloroethylene (perchloroethylene) insulating
dling, storage, transportation, disposal, clean-up,
fluid will not be used in any electrical equipment
safety and health hazards. The contract specifica-
since the Environmental Protection Agency (EPA)
tions will require that retrofitting of PCB contami-
has classified it as hazardous waste. However,
nated equipment will result in a dielectric with
tetrachloroethylene may be used as a cleaning
less than 50 ppm PCB.
agent during a retrofilling process provided the
transformer is not located in an occupied space,
8-5. Transformer Characteristics.
has a low probability of overheating, and proper
a. Capacities. Capacities will be in accordance
safety precautions are taken. Also, 1, 2, 4-trich-
with the standard sizes listed in table 8-1. Pad-
lorobenzene (TCB) will not be used as an insulat-
mounted compartmental units are not available
ing fluid since it is a possible carcinnogen. TCB
for ratings of less than 25 kVA for single-phase
may be used as a cleaning agent during the
units or 75 kVA for three-phase units. Trans-
retrofilling process.
former capacities selected will provide a rated
d. Existing dielectric hazards. The following will
capacity equivalent to not less than 90 percent of
be performed when the dielectric composition of an
the load requirement calculated in accordance
existing transformer is not listed on the trans-
with guidelines covered in chapter 2, except that
former nameplate or cannot be confirmed through
distribution transformers serving Family Housing
procurement records:
Units will be sized in accordance with the demand
(1) The dielectric will be verified either
factors given in Appendix A. ANSI loading factors
through the manufacturer's records or sample
may need to be taken into account.
testing.
b. Transformer life. Transformer life is depen-
(2) If the dielectric is determined to be listed
dent upon the thermal aging of the transformer.
by EPA or NIOSH as a hazardous chemical (PCB,
Normal life expectancy is based on operating
TCE/PCE, TCB, or Freon 113), a cost-risk analysis
transformers continuously at rated capacity to the
for each structure will be performed. The potential
limiting continuous duty temperature of the insu-
for catastrophic failure of the equipment will be
lation. Thus the transformer operating tempera-
determined for its design life using industry reli-
ture is the sum of the temperature rise (the
ability data such as IEEE Std. 500. The conse-
increase in temperature of the transformer due to
quences of failure will be analyzed and the costs to
the load) and the ambient temperature (average
mitigate the consequences will be estimated. The
temperature of the immediate air outside the
cost of replacement or retrofill will be compared to
transformer).
the cost of mitigation. Costs to replace or retrofill
(1) Load duration. Aging of a transformer is a
will include costs for changing the type of dielec-
function of time and temperature. Transformers
tric, including relocation or physical isolation of
may be operated above rated load for short periods
the equipment.
provided units are operated for much longer peri-
(3) Where the cost-risk analysis requires elim-
ods at loads below these limits, since thermal
ination of the risk, replacement and retrofilling of
Table 8-1. Transformer Standard Base kVA Ratings.8
Single-phase
Three-phase
300
3
750
25-000
75
1250
10
000
15
3
500
5
000
30 000
1667
12
500
30
100
5
750
7
500
37 500
167
2500
16
667
45
10
1000
10
000
50 000
250
3333
20
000
75
15
1500
12
000
60 000
333
5000
25
000
112-
25
2000
15
000
75 000
500
6667
33
333
150
37-
2500
20
000
100 000
833
8333
225
50
*Reprinted with permission from ANSI/IEEE Standard 141-1986, IEEE Recommended Practice for Electric Power Distribution for
Industrial Plants, copyright 1986 by IEEE. (Table 68).
8-8



 


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