Rockwell Automation 284 ArmorStart User Manual - Manual (Page 433)

Appendix
E
Group Motor Installations
Application of ArmorStart®
Controllers in Group Installation
The following is a method of applying ArmorStart controllers using
group motor installation rules as defined in the National Electric Code
(NEC 2005) and Electrical Standard for Industrial Machinery (NFPA
79-2002).
1. List motors of the group in descending order of motor nameplate
full load current.
2. Select disconnect means.
a. Sum all locked rotor currents of motors that can be started
simultaneously using NEC Table 430.251.
b. Add to that value all the full load currents of any other
motors or loads that can be operating at the same time as the
motors that start simultaneously, using NEC Table 430.250.
c. Use the total current from a and b above to get an equivalent
horsepower value from Table 430.251. That value is the size
of the disconnect means in horsepower. (NEC 430.110)
3. Select fuse or circuit breaker protection: Select fuse or circuit
breaker size for the largest motor per NEC Table 430.52 and add
that ampere value to the total of the full load currents of the rest
of the motors. The final value is the fuse or circuit breaker size
required. (NEC 430.53C)
4. Select wire: Ampacity of wire feeding a group of motors is not
less than 125% of the full-load current rating of the highest rated
motor plus the sum of the full load current ratings of all the
motors in the group. (NEC 430.24)
5. The code states that any taps supplying a single motor shall have
an ampacity not less than one third the ampacity of the branch
circuit conductors. (NEC 430.53D) The branch circuit conductors
can be defined as the conductors on the load side of the fuse block
or circuit breaker. This requirement actually defines the size of
the group of motors. For example, if the wire from the fuses or
circuit breaker is AWG #8 with rated ampacity of 50 A, the
smallest wire you can use as a tap and to the motors is AWG #14
with an ampacity of 20 A. (NEC Table 310.16 for 75
°
C wire)
Note that the Bulletin 280-283 ArmorStart controllers will not
accept wire greater than #10 wires at its input terminal blocks.
The ArmorStart cabling to the motor is UL Listed for the
controller’s Hp and is supplied with the ArmorStart controller or
as an accessory when longer lengths are required.