motor overload protection?

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journeyman0217

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philadelphia,pa
Hi everyone,
I am currently studying to take the phila. electrical exam. I came across a practice question that I got wrong and don't really understand why.
The question was: "determine the minimum size overload protection required for a 480 volt, three phase, continuous duty AC motor when given the following conditions: design B, temperature rise-40 degree C , service factor - 1.12, actual nameplate current rating- 18 amps" the answers to choose from were a) 20.7amps b) 18amps c) 22.5amps d) 23.4amps. before I tell you guys what I answered I would like to see what your thoughts were on the answer. I know that to size overload protection you use the actual nameplate rating, apparently I messed up when I used article 430.32 (a) (1) to adjust the amp rating. what do you guys think the answer would be??
 
I choose (c) 22.5 based on 430.32 and a "temp rise 40°C or less"
 
augie47 said:
I choose (c) 22.5 based on 430.32 and a "temp rise 40°C or less"
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that is what I answered based on 40 degree c. but the answer key is showing that they are multiplying the 18amps by 115% which would be categorized as "all other motors" in 430.32. I was wondering if because the question is giving you both the service factor and the temperature rise and the service factor is not 1.15 or greater would you use the "all other motors" 115%?
 
journeyman0217 said:
that is what I answered based on 40 degree c. but the answer key is showing that they are multiplying the 18amps by 115% which would be categorized as "all other motors" in 430.32. I was wondering if because the question is giving you both the service factor and the temperature rise and the service factor is not 1.15 or greater would you use the "all other motors" 115%?
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Did not read it, but from what I recall that sounds right.
 
kwired said:
Did not read it, but from what I recall that sounds right.
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The last thread that discussed this came to the consensus that if either condition is satisfied you used the corresponding adjustment.
The plain English of the code section does NOT require both conditions to be met.
 
Actually, you can't answer that question with the choices given, because it depends on the instructions provided by the OL relay manufacturer. Some manufacturers have YOU do the necessary calculations before hand, some do them for you already, i.e. the selection YOU make is the motor FLA (in your case, 18A), because they have ALREADY factored in the 115% pick-up point. So if you have one of those OL relays, and you add 115%, you are now at 115% of 115% (132%) of the motor FLA before the relay BEGINS to take action and you have caused undue thermal stress to the motor insulation. This is a VERY VERY common mistake that people make, and in fact is so common that it has made it into tests! But nonetheless, it is INCORRECT because it assumes facts not in evidence.

The only correct answer would be "RTFM".
 
Jraef said:
Actually, you can't answer that question with the choices given, because it depends on the instructions provided by the OL relay manufacturer. Some manufacturers have YOU do the necessary calculations before hand, some do them for you already, i.e. the selection YOU make is the motor FLA (in your case, 18A), because they have ALREADY factored in the 115% pick-up point. So if you have one of those OL relays, and you add 115%, you are now at 115% of 115% (132%) of the motor FLA before the relay BEGINS to take action and you have caused undue thermal stress to the motor insulation. This is a VERY VERY common mistake that people make, and in fact is so common that it has made it into tests! But nonetheless, it is INCORRECT because it assumes facts not in evidence.

The only correct answer would be "RTFM".
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I agree with your statement and concern about that "everyday" problem but, the way the question is worded it talks about OL protection in general and not specifically overload relays, etc. In Code general terms is it not possible one of the answers is not correct (apparently not the one I chose) assuming such scenariosas using fuses for overload protection.
 
augie47 said:
I agree with your statement and concern about that "everyday" problem but, the way the question is worded it talks about OL protection in general and not specifically overload relays, etc. In Code general terms is it not possible one of the answers is not correct (apparently not the one I chose) assuming such scenariosas using fuses for overload protection.
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Gus,
You are absolutely right! My bad, I was caught not reading ONLY what it SAYS.
...determine the minimum size overload protection required ...
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It didn't say overload relay or heater...
 
Jraef said:
Gus,
You are absolutely right! My bad, I was caught not reading ONLY what it SAYS.

It didn't say overload relay or heater...
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But you were correct in that if using an overload relay of most any kind you need to follow instructions for that device, as they already did the calculating for you and you just need to select correct element from a table or set a dial according to instructions. About the only time you follow the NEC procedure is if providing overload protection with fuses, which is sort of a rare thing to do.

Where I see people going wrong when they are used to just setting a dial or selecting elements from a chart is not paying attention to service factor of the motor. If not 1.15 or 1.25 you usually have to make adjustments to the usual setting/selection.
 
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