shall be provided for alignment of plug-in units. Plug-in units and doors
shall be accurately constructed and aligned to prevent binding. Control
centers shall be arranged for front mounting of equipment and control units
shall be plug-in type with wiring accessible from the front. Plug-in units
shall be draw-out or tilt-out type with provisions for padlocking in the
disconnected position. Disconnecting stabs shall be pressure type of
silverplated high-strength copper alloy. Control center disconnecting
devices shall be provided with external operating handles arranged for
padlocking in the "off" position and the compartment doors shall be
interlocked to permit opening only when the disconnect device is in the
"off" position. Control center enclosure shall be NEMA 1 and wiring shall
be NEMA Class II, Type B. Control center main horizontal buses shall be
rated as required for the connected loads, but not less than 600 ampers,
and vertical buses for each section shall be rated as required for
connected load, but not less than 300 amperes, both at a continuous rating
at 50 degrees C rise. Buses shall withstand stresses of short-circuit
currents of 42,000 amperes rms. A ground bus shall be provided at the
bottom for the full length of the motor control centers. Furnish each
vertical section with continuously energized space heaters as required to
prevent condensation during construction. Space heaters shall be applied
at one-half rated voltage. Space heaters in a shipping section shall be
wired together and connected to a terminal block that is accessible before
the motor control center is uncrated. Control center motor starters shall
be circuit-breaker combination type and shall be rated at 460 volts,
3-phase, and shall be the NEMA size classifications required for motors
furnished. Motor starter shall be magnetic, across-the-line, single speed,
two speed or reversing as required. Each starter shall be provided with a
control transformer rated 480/120 volts having a volt-ampere capacity to
suit the control load including control devices, but not less than 100 VA.
A fuse shall be provided in the 120-volt control circuit for each starter.
Control connections shall be as required. Each starter shall be equipped
with three thermal overload relays. Short-circuit rating of combination
starter shall be 42,000 amperes rms symmetrical. Starters shall be wired
to the terminal block for a remote auxiliary contact to be provided by the
Contractor. The auxiliary contact will be wired in series with the stop
push button, selector switch or control device to open the control circuit
to the starter contractor when the auxiliary contact is opened. Each motor
control center shall be equipped with two NEMA Size 1, two NEMA Size 2, and
one NEMA Size C complete, spare, full-voltage, nonreversing motor starters.
Control center circuit breakers shall be rated 600 volts ac, manually
operated, trip free from the handle, and with a molded case. Breakers
shall be 3 pole unless otherwise specified. Breakers used in combination
motor starters shall be adjustable instantaneous only, coordinated to
provide short-circuit protection for both the motor circuit and the thermal
overload relays in the magnetic starter. Breakers shall contain
inverse-time thermal overload protection and instantaneous magnetic
short-circuit protection. Combination motor starter circuit breakers shall
have a symmetrical interrupting capacity at 480 volts not less than the
motor control center short-circuit ratings. If current-limiting fuses are
used to achieve the specified short-circuit rating, a positive acting
anti-single-phase trip mechanism must be furnished. Each motor control
center shall be equipped with two 100 A, 3-pole, and one 225A, 3-pole,
spare feeder circuit breakers. Include external handle which clearly
indicates when breaker is "on," "off," or "tripped." Handle shall be
lockable in the "off" position. Control centers shall be ambient
compensated to 50 degrees C. Control centers that utilize frame sizes 225
ampere and larger shall be furnished with interchangeable trip units.
Circuit breakers shall conform to NEMA AB 1 and shall meet the appropriate
classification of FS W-C-375.
SECTION 15864N
Page 68
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