Motor Circuit (Table 430.52)

[kevinware]

I would like the opinion of the all and powerful form

I had a question on a test that was worded as follows:

"What is the Maximum size Dual Element (time-Delay) fuse that can be used to protect a three-phase 40hp motor at 460V?"

A)175 amp
B)100 amp
C)125 amp
D)150 amp

The answer I chose was "C" and I got there this way:
T430.250 Full load current = 52 amps
looking at table 430.52 I should multiply 52 x 175% but the question ask for the "Maximum size" so I look at 430.52 Exception #2(b) and it allows me to multiply 52 x 225% and come up with 117 amps and 240.6 allows me to upsize to 125 amp fuse. I was told I was wrong.....

The answer was suppose to be "B" and the math worked out like this:

T430.250 Full load amps = 52 amps
52 x 175%(T430.52) = 91 amps
240.6 upsize to 100 amp fuse

If I use a 100 amp fuse and the motor does not start would I not have to use a larger fuse up to a 225%??? How would you have answered this question??

Thanks,
Kevin

 

[bphgravity]

Re: Motor Circuit (Table 430.52)

I wouldn't consider exceptions unless the question indicates this is needed. So, really, this isn't a code issue, it is a performance issue. The maximum permitted is per the table, if the performance of that table is ot adequate, the exception can be used.

I can see how this could be a tricky question.

 

[engy]

Re: Motor Circuit (Table 430.52)

"...in no case exceed 225 percent..."

 

[friebel]

Re: Motor Circuit (Table 430.52)

As an instructor at a local community college, I can certainly see that this is a very tricky question. The person taking the exam, and using the Example was not Wrong with his answer, but the Answer that they wanted to see was not using the Exception.
Mike Holt has always said in his video tapes, that this is what we need to be concerned with. I do agree with the one answer given of not using the Exception. Hopefully, all questions that are asked would be consistent, and that is the key. Be consistent with your questions.
I know that this is a subject that I will be discussing with my NEC Class on Thursday evening, because I do have several students that will be taking the Masters Exam in November in the State of Delaware.

 

[jwelectric]

Re: Motor Circuit (Table 430.52)

Engy has the correct answer:

(b) The rating of a time-delay (dual-element) fuse shall be permitted to be increased but shall in no case exceed 225 percent of the full-load current.

The exception says in no case exceed 225 percent therefore when it hits between two sizes we must drop back instead of 240.4 (B)

 

[pierre]

Re: Motor Circuit (Table 430.52)

engy has said this very succinctly - maybe too succinctly.

430.52(C)(2)(b) is the proper location to look for the percentage for multiplying. The wording here is key.

...but shall in NO CASE EXCEED 225 percent...

Your math was correct, but instead of rounding up, you need to round down.

I have my guys mark the exception and write, ROUND DOWN next to this in the code book so they do not forget to round down for this type of question.

The key wording in this question by the way is: MAXIMUM.

 

[dillon3c]

Re: Motor Circuit (Table 430.52)

Originally posted by kevinware:
I would like the opinion of the all and powerful form

I had a question on a test that was worded as follows:

"What is the Maximum size Dual Element (time-Delay) fuse that can be used to protect a three-phase 40hp motor at 460V?"

A)175 amp
B)100 amp
C)125 amp
D)150 amp

Question is put forth as Maximum size.

Table 450.52 Maximum Rating or Setting-

Question doesn't imply Exception of:- is not sufficiant for the starting current of the motor., but maximum size.

[ October 05, 2005, 09:41 AM: Message edited by: dillon3c ]