Table 430.52(C)(1) and Allen Bradley 140MT MPCBs

[jakeparsons03]

Looking at properly sizing MPCBs. According to 430.52(C)(1) the magnetic trip should be 250% of the FLA. Allen Bradley 140MTs are have a fixed magnetic trip that is 14x the operational current. I am having a hard time understanding how that makes sense. I would think it should be 2.5 to be compliant with NEC. What am I missing?

 

[augie47]

Look at 430.52(C)(7)... listed factory combination motor starters can use MCPs which can be 13 X as opposed go an Inverse Time breaker limit of 2.5

 

[jakeparsons03]

Very interesting. It calls out 1700 percent for Design B energy efficient and Design B premium efficiency motors but that's it. It also calls them out as fused in the informational note. I wonder if CBs get lumped in there or if they would call out CBs directly. Maybe these circuit breakers are tailor made for those motors?

 

[jakeparsons03]

I think I am on the verge of something here but not quite there. 430.52(C)(3)(b)(2)(a) states that "motors other than deign b energy efficient and design b premium efficiency motors shall be permitted to be increased but shall in no case exceed 1300 percent of the motor full load current."
Granted, this calls out the instantaneous portion of the breaker needs to be adjustable.

I feel like this has to be related to that in some way because the numbers are so close (14x being very random). If the fixed magnetic portion was adjustable and that was the max then I could see it, but its not.