6.7 Secondary
and panelboard bussing, bus duct, and primary and
secondary feeders, branch feeders, and branch circuits.
Branch Power Distribution
Note: The use of aluminum may be permitted via a
deduct alternate listing the specific items of equipment
and material in which the use of aluminum would be
Feeder Assignments (Bus Ducts vs. Cable-In-Conduit).
permitted. The alternate shall list and address the
The secondary main branch power distribution system is
comparative increase in conductor and conduit sizes.
designed to convey power to the various load groups
The Contracting Officer shall decide whether or not the
distributed throughout the building. The decision as to
alternate is accepted.
whether this power is conveyed to the various loads in
copper cables-in-conduit, or copper bus duct, will affect
Motor Control Centers. Grouped motor controls shall
the following:
be used where eight (8) or more starters are required in
an equipment room. Motor control center (MCC)
Design of the main switchgears
construction shall be NEMA Class I, Type B, copper, with
Coordination in the lower levels
magnetic (or solid state if appropriate) starters and either
Design of the electric rooms
molded case circuit breakers or fused switches. Minimum
Flexibility
starter size in motor control centers shall be Size 1.
Control circuit voltage shall be 120V connected ahead of
each starter via fused control transformer. Reduced
Cost
voltage starters may be used for larger motors to reduce
At the early stages of a project, alternate designs comparing
starting KVA.
the factors listed above would determine the scheme to
Time delay relays shall be incorporated in the starters or
be recommended. The size of the floor plate (i.e. gross
programmed in the BAS system to reduce inrush currents
area of the floor) and the height of the building are also
on the electrical system.
important factors in the determination of the
recommendation.
Where variable frequency drives are used on a project,
it should be economically evaluated to specify VFDs
Bus Duct. Bus ducts shall be copper, fully rated, 3-phase,
incorporated into the MCCs. If determined not appropriate,
3-wire or 3-phase, 4-wire with 100% neutral and an integral
then VFDs should be powered for distribution panels
ground bus, sized at 50 percent of the phase bus. Provide
installed for that purpose.
calculations supporting the specified short circuit rating.
Elevator Power. It is recommended that the load of the
Conductors. The base bid specifications shall require that
elevator feeders be divided among the secondary
copper be utilized throughout, including motor windings,
switchgears, provided that there are 2 or more
transformer windings, switchgear bussing, switchboard
switchgears, and that alternate elevator machines shall
be fed from different switchgears.
186 F A C I L I T I E S S T A N D A R D S
FOR THE
PUBLIC
BUILDINGS
SERVICE
6.7 Secondary Branch Power Distribution
Revised March 2005 PBS-P100
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