Rockwell Automation 294D ArmorStart LT Distributed Motor Controller DeviceNet Version - User Manual - Manual (Page 200)

200 Rockwell Automation Publication 290D-UM001A-EN-P - June 2012
Appendix A Applying More Than One ArmorStart LT Motor Controller in a Single Branch Circuit on Industrial Machinery
“7.3.1.1 Motors. Motor overload protection shall be provided in accordance with
Article 430, Part III, of NFPA 70.”
Analysis: Each ArmorStart LT motor controller incorporates an integral
overload relay. This overload function must be set in accordance with Article 430,
Part III of NFPA 70. Selecting the ampacity of the circuit conductors
appropriately (see Requirement 5) ensures the overload relays, when set
according to 7.3.1.1, will protect the conductors against overheating due
to motor overloads.
Supplementary Note: Each individual controller overload relay directly protects
the conductors connected to the input and output of that controller and the
motor that the controller supplies. The combined load conductor is protected by
the tripping of one or more of the controller overload relays, which remove(s) the
overloaded motor(s) before the combined load conductor overheats.
5. Requirement: Conductor Ampacity —The minimum ampacity
of conductors.
Text:
“12.5.3 Motor circuit conductors supplying a single motor shall have an ampacity
not less than 125 percent of the motor full-load current rating.”
“12.5.4 Combined load conductors shall have an ampacity not less than … 125
percent of the full-load current rating of the highest rated motor plus the sum of
the full-load current ratings of all other connected motors…”
Analysis: Referring to Figure 44 , (a), (b) and (c) explain the method for
calculating the minimum required conductor ampacity for each of these
conductors: input and output conductors of Bulletin 290D and 291D controllers
(a), input and output conductors of Bulletin 294D controllers (b) and combined
load conductors that supply Bulletin 290D, 291D and 294D controllers (c). The
currents I1 through I5 are the input currents to the controllers. For the Bulletin
290D and 291D controllers, these are the same as the output motor currents. For
the Bulletin 294D controllers, these currents are the rated input currents.
The example does not address conditions of use such as an ambient temperature
exceeding 30 °C or more than three current-carrying conductors in a cable or
raceway. In a particular application, these conditions of use may require derating
of the ampacity given in Table 12.5.1. This example assumes that, under the
conditions of use, both conductors have sufficient ampacity for the application.
This means the 14 AWG conductors have an ampacity of no less than
9.5 A and the 10 AWG conductors have an ampacity of no less than
27.4 A.