Motor Branch Circuit Design
Minimum safety provisions for the control of
motors are set forth in the National Electrical
Code. Although these minimum provisions must
be met, they are no substitutes for an intelligent
selection of protective devices made on the basis
of the motor circuit being designed. The code
recognizes this fact in Article 430, Section D,
“Where maximum branch-circuit protective device
ratings are shown in the manufacturer’s overload
relay table for use with a motor controller or are
otherwise marked on the equipment, they shall
not be exceeded even if higher values are allowed
(by the code).”
Square D furnishes an instruction sheet with
every starter. Each instruction sheet includes
thermal unit selections and either fuse or circuit
breaker selections, or both as applicable.
The capability of industrial systems to deliver high
short circuit currents has been increasing steadily
over the years. This fact has caused much
concern about the capability of motor controllers
to withstand high current faults, without creating
hazards for personnel and destruction of
equipment. NEMA standards require that
contactors be able to interrupt currents up to 10
times full load current. Therefore, it is acceptable
for the overload relay to respond before the shortcircuit protector up to this level. At currents above
10 times motor full load current the short circuit
protective device must respond first to minimize
equipment damage. A fully coordinated system is
achieved when the overload relays operate in
response to motor overloads before the fuses or
circuit breaker, and the fuses or circuit breaker
open the circuit before the overload relays trip or
burn out on short-circuit currents.
Proper coordination requires a thorough
knowledge of the time versus current limits of all
of the branch circuit components as well as the
time versus current trip characteristics of the
overload relay and short circuit protective device.
