Rockwell Automation 284D ArmorStart - Safety Version - User Manual - Manual (Page 285)

Group Motor Installations D-5
If the Group motor design were carried out with the intent to use an
inverse-time circuit breaker from NEC Table 430.52 to the rules of
NEC 430.53C, we start with the largest motor, 10 Hp, and calculate
14A x 250% = 35 A to this we add the FLC of the 5 Hp motor, 7.6 A,
plus the other calculated drive currents for the motors controlled by
the VFD-ArmorStarts. The calculated drive currents are given in the
following table:
Therefore for the standard inverse-time circuit breaker available not
exceeding 56.94 A we need to use a 50 A inverse-time circuit breaker.
This design will also allow the use of 8 AWG for the motor branch
circuit. Continuing than and applying NEC 430.28 the individual
motor tap conductors can be sized down to 1/3 the ampacity of the
trunk and following the restrictions in NFPA 79, 7.2.10.4 and Table
7.2.10.4 for this case where we have used a 50 A inverse-time circuit
breaker. NFPA 79, Table 7.2.10.4 indicates that the smallest tap
conductor can now be 14 AWG. See the above figure for this Group
motor circuit design.
The above method instructs one on applying ArmorStart controllers
using group motor installation rules. Because of the ArmorStart’s
capability, rating and Listing this method provides the minimum
branch circuit wire and SCPD protection size that can be used. The
Armor Start has been evaluated and tested for group motor
installations when being feed by a power source having 65,000 Amps
available fault current. The ArmorStart is not a listed combination
motor controller, however, but is Listed as Industrial Control
Equipment per UL 508 for group motor installations per NFPA 79.
Under this Listing the NEC and actually NFPA 79 puts an upper
bound on the SCPD to be used. That upper bound is dictated by the
maximum ratings in Table 7.2.10.4.
Motor Hp Motor FLC (A) Inverse-Time CB Current (A)
10 14 35 A
5 7.6 7.6 A
2 3.4 4.89 A
1 2.1 3.15 A
1 2.1 3.15 A
1 2.1 3.15 A
Total Fuse Current 56.94 A