UFC 3 -520-01
June 10, 2002
4-5.2.4 Install dry-type transformers rated for over 35,000 volts i n a vault as shown in
Figure 4-4. Installation Requirements for Transformers Rated Over 35,000 Volts
Less-Flammable, Liquid -Insulated, and Oil-Insulated Transformers .
Use dry-type transformers wherever possible for interior applications .
4-5.3.2 The use of liquid-insulated and oil-insulated transformers must be justified for
interior applications. If such use is justified for the particular application, refer to MIL -
HDBK-1008C for guidance regarding installation criteria.
Fire Protection Criteria . Provide fire protection for transformer installations
as specified in MIL -HDBK-1008C.
Size transformers to have adequate kVA capacity. Size the associated
transformer conductors to accommodate the rated kVA capacity in accordance with
NEC criteria. The transformer conductor sizing establishes the overcurrent protection
requirements for the system. Appendix B provides a transformer and associated
conductor sizing example.
The transformer capabili ty is directly related to its temperature rise during
operation. Users in some industries size transformers assuming that they will apply the
transformer up to its overload rating. Do not follow this practice for interior installations;
instead, size tra nsformers assuming 100 percent of rated capacity and the overload
capacity will provide future margin, if necessary.
Do not size transformers to have significant spare capacity unless load
growth is the system is expected. Most transformers in service are lightly loaded.