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Appendix A
Applying More Than One ArmorStart LT Motor Controller in a Single Branch Circuit on Industrial Machinery
188
maximum rating of the fuse protecting the branch circuit must be reduced to the
lower value so that all components are applied according to their ratings. For
example, as shown in Requirement Three, a lower value may be necessary to
protect the motor controller within its ratings because its specified maximum
protection is less than the rating that
conductor. Another reason to use a lower rating of fuse is to provide more
conservative conductor and controller protection. However, in all cases it is
important to ensure the ampere rating is sufficient to start and operate the motors
without nuisance opening of the fuse(s).
Supplementary Note 2: The note at (b) points to the conductor on the output
of the ½ Hp Bulletin 294E controller in order to emphasize that the smallest
conductor in the circuit includes the conductors between each controller and
motor. This includes the output of the variable-frequency AC drive-based
Bulletin 294E controllers; even though these drives have electronic short-circuit
protection. According to NFPA 79, the fuse, and not the drive's electronic short-
circuit protection, provides the short-circuit fault and ground-fault protection
for these output conductors.
Supplementary Note 3: Generally, connecting a smaller conductor to a larger
conductor requires the installation of fuses at the connection. This connection
may be made without this fuse, in some cases, through the use of a tap rule that
indirectly protects the smaller conductor by limiting two things: the ratio of the
ampacity of the larger conductor to the ampacity of the smaller conductor and
the maximum length of the smaller conductor (see, for example, 7.2.8.2). When
applying 7.2.10.4(2), such a tap rule is neither applicable nor necessary. In
Figure
61, the smaller 14 AWG conductors may be connected to combined load
conductors of any size because 7.2.10.4 does not indirectly protect the smaller
conductor by limiting the ratio of the larger to smaller conductor ampacities and
the conductor length. Instead,
by specifying the maximum rating of fuse that may protect a branch circuit that
contains a conductor of that size.
3. Requirement Three: Controller Short-Circuit and Ground-Fault
Protection — Each motor controller must be protected according to its own
ratings, that is, applied in accordance with its listing.
Text: "(1) Each motor controller and overload device is... listed for group
installation with specified maximum branch-circuit protection..."
Analysis: See (d) in
Figure
protect the conductors (see Requirement 2) must now be compared to those in
the controller's ratings. To comply with the listing of each motor controller and
overload relay, the fuse(s) must comply with the maximum branch-circuit
protection specified in the controller markings. Therefore, the fuse(s) must be
of a class marked on all of the controllers and the rating of the fuse(s) must not
exceed the rating marked on any of the controllers. The markings of each
controller specify that a fuse having a maximum rating of 45 A may protect the
motor controller. When connecting to an electrical supply having an available
fault current of 5000 amperes or less, the class of the fuse is not specified and may
be any class. When connecting to an electrical supply having an available fault
Rockwell Automation Publication 290E-UM001B-EN-P - June 2012
Table 30
permits for the smallest circuit
Table 30
protects the smallest conductor directly
61. The characteristics of the fuse(s) permitted to
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