Master Electrician Practice Exam

For a motor with a temperature rise of 50 degrees C, what should the overload device be set to trip at, based on the motor's full-load ampere rating?

1. 100 percent

2. 110 percent

3. 115 percent

4. 120 percent

The correct answer is: 115 percent

The recommended setting for an overload device in relation to a motor's full-load ampere rating, when considering a temperature rise, is determined by the need to ensure the motor operates safely while providing adequate protection. For a motor with a specified temperature rise, such as 50 degrees Celsius, setting the overload device to trip at 115 percent of the motor's full-load ampere rating is standard practice. This setting allows for a margin that accommodates the normal variations in load and operational conditions without causing unnecessary tripping of the overload device. A 115 percent setting is a common industry norm because it strikes a balance between protecting the motor from overheating due to overload and allowing for brief surges or increases in demand that the motor is capable of handling. A setting at 100 percent would provide no leeway for normal operational variations and might cause frequent nuisance tripping. Choosing a setting of 110 percent is slightly more generous but does not account for enough of a buffer, particularly for longer duration overloads. Meanwhile, a 120 percent setting, while seemingly protective, may not sufficiently protect against overheating under sustained overload conditions, as it allows for a greater chance of damage to the motor before tripping occurs. Thus, a setting of 115 percent is considered optimal