At-Grade Mounting. For at-grade mounting on a concrete base, there
is no kVA limit. Tamper-resistant transformers (classified as pad-mounted
compartmental-type units) should generally not be specified in ratings of over
500 kVA, but in no case larger than 750 kVA. When sheet-metal enclosures are
not tamper-resistant, provide ground-mounted units with a fenced enclosure or
even a concrete or brick structure, where adverse weather conditions make such
an installation advisable. For required clearances between buildings and
insulated transformers, refer to MIL-HDBK-1008.
Indoor installations are covered in Section
Overload Capacity. Consider the accelerated loss of equipment life
if transformers are to be overloaded. Refer to ANSI C57.91 Guide for Loading
Mineral-Oil-Overhead and Pad-Mounted Distribution Transformers Rated 500 kVA
and Less with 65 Degrees C or 55 Degrees C Average Winding, C57.92 Guide for
with 55 Degree C or 65 Degree C Winding Rise, and C57.96 Guide for Loading
Handbook for Electrical Engineers.
Transformer Noise Level. Refer to NEMA TR-1, Transformers,
Regulators and Reactors for maximum permissible noise levels for transformers.
Overhead Distribution. Use the criteria in ANSI C57.12.20,
Requirements for Overhead Type Distribution Transformers, 500 kVA and Smaller:
High-Voltage 67,000 Volts and Below; Low-Voltage 15,000 Volts and Below. Do
not use self-protected transformers having an internal secondary breaker,
internal primary fusing, and integrally mounted surge arresters. These
transformer accessories are provided for transformers generally described by
to require specialized personnel not usually available at naval facilities.
and automatic circuit reclosers for protective line considerations. Provide
switches to localize defective portions of aerial and underground circuits and
to accomplish dead-circuit work. Select from nonload-break or load-break type
Fuses. After consideration of the necessary current-carrying
capacities, interrupting duties, and time-current melting and clearing
characteristics, select fuses from the following types:
open fusible link,
boric-acid type, and