NOTE: Delete this paragraph when not required.
**************************************************************************
A [half] [fully] rated neutral bus shall be furnished continuous through
the control center. Lugs of appropriate capacity will be furnished.]
2.6.3
Combination Starters
**************************************************************************
NOTES: The minimum bus short-circuit rating for
most manufacturers is 42,000 amps rms symmetrical.
Most combination starters without current limiting
type circuit breakers or motor circuit protectors
have a short circuit rating of 25,000 amps. The
designer shall evaluate the available short circuit
current for a particular installation and place that
value in the space provided.
When short-circuit ratings above 25,000 amps are
required, the designer should consult manufacturer's
data for the availability of non-current limiting
devices at the specific rating and where needed,
show current limiting circuit breakers or motor
circuit protectors the drawings.
In accordance with NEMA ICS 2, the motor control
center short-circuit rating is the maximum available
rms symmetrical current in amperes permissible at
its line terminals which are computed as the sum of
the maximum available current of the system at the
point of connection and the short-circuit current
contribution of the motors connected to the control
center. In the absence of more precise information,
the motor short-circuit current contribution may be
assumed to equal four times the continuous current
rating of the motor control center.
Reduced voltage type starters are specified in the
following paragraph. They should be used in
specialized applications, and indicated on the
drawings. Reduced voltage starting should be
avoided where possible.
This specification does not cover reversing
starters. Where a reversing starter is required,
indicate reversing and non-reversing starters on the
drawings, and modify the specification for clarity.
NEMA sizes are based on continuous duty motors.
Where acceleration time exceeds 10 seconds, or
plugging or jogging duty are required, consult the
manufacturer.
For high efficiency motors, the designer shall
investigate time-current curve characteristics of
the circuit breaker or MCP overcurrent protection to
ensure that the increased starting current of these
motors does not exceed the NFPA 70 standard ratings.
SECTION 16403A
Page 22
Previous Page
