AS-B875-002 SCHNEIDER

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.