Helping a student with a problem.....I need help.

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kc_hubbs

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Cumberland, KY. USA.
Someone brought this question to me, and Im a little unsure on the answer.

Here is goes:
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The following question is regarding the electric motor specified below:
150 HP, Three Phase, 60HZ
444T Frame, 1750 RPM, Code F
460 Volts, 180 FL Amperes.
Continuous Duty, 40C Ambient.

Q: What is the maximum allowable motor running overcurrent protection trip setting without using exceptions?

A.) 207 Amps
B.) 189 Amps
C.) 198 Amps
D.) 225 Amps

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When working this problem I sized the overloads based on full load current times 125% due to the motor meeting the 40C Ambient temp. Giving me an answer of 225 Amps. The workbook shows the answer as 207Amps. They get that answer from sizing overloads using 115%. Why do they use this number though ? Im a little confused.
 
ActionDave said:
Someone is confusing overcurrent protection with overload protection, either the question writer or the question reader.
Click to expand...

That seems to be the issue. If the word were changed it would make more sense but either way 115% is the most obvious answer.
 
Its just a book but together by someone giving an electrical class for Mine electricians.

He said he believes a lot of the book was copied from this website:
https://laborcommission.utah.gov/media/pdfs/boilerelevatormine/pubs/U Electric.pdf


If you look at that document, the same exact type of question is asked on page 143, questions 26 and 28. They both use 115% to determine overloads for some reason.

The question i posted about is pn Page 141.
 
I would also point out that the question could say that the ambient temperature rise is 40° C and it would still be 115%. It would need to have the motor physically marked with the temperature rise to use the 125% factor from 430.32(A)(1).

Motors with a marked temperature rise 40°C or less
Click to expand...
 
K8MHZ said:
Ambient temperature has nothing to do with the motor, it's the temperature of the air around it.
Temp rise I think is self explanatory.
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I understand the difference in terms, my problem is that I have assumed that the 40C marked on motor nameplates was always the rise. Never paid real close attention.

It means there are a few motors out there with slightly too big OLs. Whoops.

You are allowed to increase the size if needed for start up so I guess it is not my worst mistake.

Still sucks though.
 
jumper said:
I understand the difference in terms, my problem is that I have assumed that the 40C marked on motor nameplates was always the rise. Never paid real close attention.

It means there are a few motors out there with slightly too big OLs. Whoops.

You are allowed to increase the size if needed for start up so I guess it is not my worst mistake.

Still sucks though.
Click to expand...

Are we supposed to interpret "running overcurrent protection" as meaning overload protection?
If so there is no additional allowance for motor starting. To that extent the question is poorly written, using terms that the NEC does not use or define.

Overcurrent protection (short circuit and ground fault protection) is where you can definitely increase the nominal value to allow starting.
 
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