360
Electrical Distribution: Design Criteria
363.3
DISTRIBUTION NETWORK
The location of the substation and main switchboard in the distribution
system is a primary consideration. VA standards prefer substations to be
external, or close to the exterior of the building. But close relationship to
heavy demand functions for economy of distribution is also required.
Proximity to the motor control centers for central plant equipment, elevator
motors, pumps, etc., would suggest central locations in basements or on
the roof, but in either case, ease of access for maintenance and
replacement must be achieved.
The major horizontal and vertical feeders from the main switchboards to
the service modules would be largely permanent installations, with the
horizontal feeder distributed at the substation level. The requirements of
ease of change or relocation of load output make it necessary to use
busducts as extensively as possible for these feeders. The busducts must
be completely accessible throughout their length as required by code.
Electrical codes are based on the assumption that the maximum voltage in
a standard distribution system will be 300 volts to ground. Therefore
three-phase 480Y277 volts would be the expected maximum busduct
supply. Also, because of the economic principle of distributing at the
highest possible voltage before transforming down, this supply would be
minimum.
The optimum capacity for any busduct in the system should be considered
as 1600 amps which is generally the most economical size in terms of
amps per dollar. Above this size, busduct cost rises disproportionately to
current rating. Also, the cost of switch-gear associated with a rating above
1600 amps increases considerably. Where a stack of service modules
required a capacity of greater than 1600 amps, it would be desirable to
duplicate 1600 amp busducts than go to one with a higher rating not only
from the standpoint of economics but also selectively of switchgear.
A 1600 amp busduct could serve 20 service modules of 10,000 square
feet at the average load distribution value of 6.8 watts per square foot.
Distribution to these ten service modules would be most easily achieved
by a vertical busduct riser via a stack of service bays to avoid penetrating
fire section walls horizontally. Duplication of busducts would allow
significantly higher loads per square foot.
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