BMEH584040C SCHNEIDER

General

Overload relays are intended to protect motors, 

controllers, and branch-circuit conductors against 

excessive heating due to prolonged motor 

overcurrents up to and including locked rotor 

currents. Protection of the motor and the other 

branch-circuit components from higher currents, 

due to short circuits or grounds, is a function of the 

branch-circuit fuses, circuit breakers, or motor 

short-circuit protectors.

Nameplate Versus NEC Full-load Current

The 1996 NEC in Tables 430-147, 430-148, 430-

149 and 430-150 lists full-load currents according 

to motor horsepower and voltage. According to 

Article 430-6, these full-load currents should be 

used, rather than the motor nameplate full-load 

current, to determine the ampacity of conductors, 

ampere ratings of switches, or branch-circuit 

overcurrent devices, etc. It is specifically stated in 

Article 430-6, however, that “separate motorrunning overcurrent (overload) protection shall be 

based on the motor nameplate current rating.”

Service Factor

NEMA standards for motors list service factors of 

1.15 to 1.25 for general-purpose ac motors from 

1/2 to 200 horsepower. Other motors, such as 

totally-enclosed, fan-cooled, and motors over 200 

horsepower have a standard 1.0 service factor. 

Because of the way in which the standards are 

established, there is no simple way of determining 

the service factor without looking at the motor 

nameplate. For this reason all of the thermal unit 

selection tables in this catalog are designed for 

1.15 to 1.25 service factor motors, except the 

tables for Class 8198 High Voltage Starters. 

These tables are designed for 1.0 service factor. It 

is important to know when a motor has a service 

factor of 1.0, because failure to recognize this fact 

will result in an over-sized thermal unit selection 

and reduced motor protection.