A continuous-duty motor has a marked service factor of 1.15 and a nameplate full-load current of 34 amperes. Its overload device, sized at 125 percent, trips during starting even though the motor is not overloaded. To what maximum may the overload rating be increased?
- 47.6 amperes
- B42.5 amperes
Forty-two and a half amperes is 125 percent, which is where the selection started and which the problem states is already tripping.
- C44.2 amperes
One hundred thirty percent is the ceiling for motors that do not carry a service factor of 1.15 or a 40 degree Celsius rise marking. This motor qualifies for the higher ceiling.
- D51.0 amperes
One hundred fifty percent exceeds every ceiling in this section. Setting an overload there gives up meaningful thermal protection for the winding.
Why A is correct
Two numbers separate the initial selection from the maximum increase. The initial device is 125 percent of nameplate for this motor class, or 42.5 amperes. Where that proves insufficient, the maximum permitted increase is 140 percent, or 47.6 amperes. The motor class that only qualifies for 115 percent initially is capped at 130 percent on the increase.
What this question is testing
The item tests whether the candidate can distinguish the initial overload selection from the maximum permitted increase, and match the correct ceiling to the motor's nameplate markings.
On the job
Nuisance overload trips on hard-starting loads such as conveyors, compressors, and loaded fans are a daily service call. The wrong response is to jumper the overload or install the largest heater in the drawer. The Code gives a defined ceiling, and if the motor still trips at that ceiling the problem is mechanical, a supply voltage issue, or a genuinely overloaded machine, all of which need investigating rather than a larger heater.
Memory technique
Start at 125, and if it trips you may go to 140. Never past it.
Exam tip
Two ladders in one section: 125 and 115 percent for selection, 140 and 130 percent for the increase. The nameplate decides which rung.
Where to look it up
Look in the motors article, in the overload protection part, at the section on continuous-duty motors and the subsection permitting an increased selection.