various rules. The wind and ice loadings are different from those of ANSI C2,
but the clearances illustrated are generally more stringent. Use of these
illustrations will provide a safe and economic installation. The Institute of
Electrical and Electronics Engineers (IEEE) also publishes Clapp, NESC
Handbook which was developed to aid users in understanding and correctly
applying this code.
Economic Factors. Base the number of circuits and voltage on
economic considerations. Where necessary provide life cycle cost analyses in
accordance with NAVFAC P-442, Economic Analysis Handbook.
Number of Circuits. Keep the number of circuits to a minimum
without compromising reliability, continuity of service, or any of the
technical factors stated previously and thus avoid excessive initial cost.
Voltage. Select a distribution voltage which most economically
provides for the magnitude, voltage regulation, and length of feeders (refer
to MIL-HDBK-1004/1, Preliminary Preliminary Design Considerations). Where
groups of large motors are to be served by the distribution system, the most
economical motor voltage is generally the most appropriate distribution
Transformer Losses. Most manufacturers offer a variety of designs
where decreased loss design is offset by increased cost. Both no-load (core)
and 100 percent load (coil) losses, plus transformer efficiencies at various
levels are normally available from the manufacturer.
In general, a heavily
loaded transformer has lower losses, and therefore has lower life cycle cost,
than when it is lightly loaded. Usually, transformers are manufactured with
cores made of silicon-steel materials. More recently developed transformers,
referred to as "the Amorphous Core Transformers," with cores made of amorphous
metal, are also commecially available. In comparison with transformers with
silicon steel cores the amorphous core transformers reduce core losses by
approximately 70%. The initial cost of an amorphous core transformer is about
twice that of a silicon steel core transformer, but the life cycle cost can be
significantely lower as the initial cost decreases as the demand increases. A
simplified approach to evaluating the cost of transformer losses is given in
Plants. A more detailed evaluation of distribution transformer losses is
given in the Electrical Utility Engineering Reference Book, Distribution
Systems. A method for specifying a transformer based upon minimum losses is
provided in REA 65-2, Evaluation of Large Transformer Losses.
Special Construction. Refer to MIL-HDBK-1004/4, Electrical
Utilization Systems, for criteria on the design of electrical work installed
in earthquake areas. Refer to NAVFAC DM-4.05, 400-Hertz Medium-Voltage
Conversion Distribution and Low-Voltage Utilization Systems, for criteria
applying to 400-Hz, 4,160-V distribution systems. Refer to MIL-HDBK-1012/1,
Electronic Facilities Engineering, for criteria on the design of electronic
facilities. Incoming lines to electronic facilities shall be protected
against lightning generated surges in accordance with MIL-HDBK-419, Grounding,
Bonding, and Shielding for Electronics Equipments and Facilities.