Motor Test Question

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bphgravity

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Suppose you have a 100-ampere inverse time circuit breaker already installed at a panelboard. What would be the maximum permitted 3?, 208V Induction-Type Squirrel Cage Motor that could be installed using this 100-ampere SC/GF device?

A. 7.5 hp
b. 10 hp
c. 15 hp
d. 5 hp
 
Re: Motor Test Question

Are you sure?

The 15hp motor has an FLC of 46.2 amperes. A 100-ampere SC/GF device would be approximately 215% of that. Would that not be permitted considering Table 430.52 is a maximum permitted rating of the device.

The 10hp has a FLC of 30.8 amperes which protected by the 100-ampere device would be well in excess of the 250% max permitted, save the allowance in the exception.

What do you think?
 
Re: Motor Test Question

The code does not specify an minimum size for the branch circuit or feeder short circuit and ground fault protective device setting. The code only specifies a maximum size for this device. Any size at the maximum or below is permitted. Note that if it is too small the motor may not run, but that is not a code issue, that is a design issue.
Don
 
Re: Motor Test Question

There is not a minimum size other than 115% of NEC FLA per 2002 NEC 430.108 & 430.110.

Obviously it must be rated high enough for the motor to start & run.
 
Re: Motor Test Question

Section 430.52(C) references table 430.52. The maximum setting for an inverse time breaker for motor protection is 250 percent of full load amps of Table 430.250 values. 100A/2.5 = 40A. Referring to Table 430.250, the closest value without exceeding 40A is 10 HP. From table 430.250, 10 HP @ 208V = 30.8A. From table 430.52, 250% of 30.8A = 77A, the next standard size is 80 amps. So the 100A inverse time breaker is TOO BIG for a 10HP motor.

The 100 amp inverse time breaker is suitable for the 15 HP motor. From Table 430.25, 15 HP = 6.2A. The maximum breaker allowed is 2.5 x 46.2= 115.2A The MAXIMUM next std size allowed is 125A, but a 100A breaker is permitted, based on 430.109(A)2).
According to UL, breakers are suitable to handle up to 6 times the rating of the breaker and suitable as the disconnecting means according to 430.109(A)(2). In my opinion the correct answer according to the Code is "C" 15HP.
 
Re: Motor Test Question

We need to be careful with the terms ?minimum? and ?maximum? here, because the question was worded ?backwards.? The code does give us a maximum value for overcurrent protection (given the motor rating), as Don pointed out. But the question was worded in the opposite sense. Phrasing the question another way, ?What is the largest motor for which the code would allow the use of a 100 amp OCPD??

I like John?s approach and his answer.
 
Re: Motor Test Question

For motors and inverse time CB's I like to use the Sq D Motor Data Calculator.
It lists a 90 A CB for a 15 hp 208 volt motor.
If I was using time delay fuses they would be about 60 amps
 
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