# NCEES 510



## LMAO (Oct 22, 2010)

I bumped into this one a long time ago but I totally forgot about it. Per NEC, to size a motor overload we should use the motor nameplate current rating and not the NEC table; so why is answer using the table?


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## Flyer_PE (Oct 22, 2010)

LMAO said:


> I bumped into this one a long time ago but I totally forgot about it. Per NEC, to size a motor overload we should use the motor nameplate current rating and not the NEC table; so why is answer using the table?


I think you have it backwards there. Per NEC 430.6(A)(1)



> Other than for motors built for low speeds (less than 1200 RPM) or high torques, and for multispeed motors, the values given in Table 430.247, Table 430.248, Table 430.249, and Table 430.250 *shall* be used to determine the ampacity of conductors or ampere ratings of switches, branch-circuit short-circuit and ground-fault protection, *instead* of the actual current rating marked on the motor nameplate........


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## cableguy (Oct 22, 2010)

I think LMAO is right, look (I think it's right above that, could be below though, I don't have my NEC with me).

You use the tables for conductors and short circuit / ground protection, but for overload, you use the nameplate. I bumped in to that the other day also.


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## Flyer_PE (Oct 22, 2010)

I stand corrected. Turning the page to 430.6(2):



> Separate motor overload protection shall be based on the motor nameplate rating.


One more reason I'm glad I don't have to jack with the code much.


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## LMAO (Oct 22, 2010)

Flyer_PE said:


> LMAO said:
> 
> 
> > I bumped into this one a long time ago but I totally forgot about it. Per NEC, to size a motor overload we should use the motor nameplate current rating and not the NEC table; so why is answer using the table?
> ...


No, according to 430.6(A)(1),tables 430.247 to 430.250 are used for breakers, fuses and conductors only, and not for overloads. Per 430.32(A)(1), overloads are sized based on manufacturer nameplate and not the NEC table.

I am pretty much sure NCEES 510 is botched.


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## cableguy (Oct 22, 2010)

One week and one hour from now I intend for it to be forgotten.


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## Flyer_PE (Oct 22, 2010)

LMAO said:


> No, according to 430.6(A)(1),tables 430.247 to 430.250 are used for breakers, fuses and conductors only, and not for overloads. Per 430.32(A)(1), overloads are sized based on manufacturer nameplate and not the NEC table.I am pretty much sure NCEES 510 is botched.



IF they had given a motor nameplate ampere rating, I would agree. If all the rated HP is all you have, it's what you use.


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## Flyer_PE (Oct 22, 2010)

cableguy said:


> One week and one hour from now I intend for it to be forgotten.


It will live on in your dreams (or nightmares) until you get the results.


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## LMAO (Oct 22, 2010)

Flyer_PE said:


> LMAO said:
> 
> 
> > No, according to 430.6(A)(1),tables 430.247 to 430.250 are used for breakers, fuses and conductors only, and not for overloads. Per 430.32(A)(1), overloads are sized based on manufacturer nameplate and not the NEC table.I am pretty much sure NCEES 510 is botched.
> ...


sorry but that's not how it works; the bottom line is that the problem is flawed and has no correct answer. You can't just make up exceptions to the code.


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## cableguy (Oct 22, 2010)

{Now that I can look at the problem...} I have a feeling that they will say "If you don't have the nameplate, then use the table". We are not given the nameplate - though it _should_ have been given to us. I agree, that's a flaw in this problem.


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## Flyer_PE (Oct 22, 2010)

LMAO said:


> Flyer_PE said:
> 
> 
> > LMAO said:
> ...


How many times do you actually have the motor nameplate available? The numbers in the table are a conservative assumption. A lot of the time, that's all you have, especially if you're dealing with small motors.


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## knight1fox3 (Oct 22, 2010)

Flyer_PE said:


> How many times do you actually have the motor nameplate available? The numbers in the table are a conservative assumption. A lot of the time, that's all you have, especially if you're dealing with small motors.


A very subjective discussion. As I mentioned in a few other of my posts, I work in the material handling industry and work frequently with 3-phase vector-duty induction motors and frequency drives. During the design phase, you have to initially use the NEC motor tables as a starting point in the calculations to size your drives, breakers and conductors. At that point, the motor has only been quoted by a motor vendor. You won't have the nameplate rated values of that _specific_ motor until the motor is built and the test data has been collected. As Flyer mentioned, NEC is already quite conservative. Depending on the HP and speed of the motor, often times the nameplate FLA is lower than what is listed in the tables so your sizing calculations end up being just that much more conservative. I think problem 510 is open to interpretation. The nameplate data may have not been available so you use what you have, the HP. If nameplate FLA had been given, then that would have been used.



cableguy said:


> One week and one hour from now I intend for it to be forgotten.





Flyer_PE said:


> It will live on in your dreams (or nightmares) until you get the results.


LOL! So true. It was a L-O-N-G 12 week plus wait. :brickwall:


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## Flyer_PE (Oct 22, 2010)

knight1fox3 said:


> A very subjective discussion. As I mentioned in a few other of my posts, I work in the material handling industry and work frequently with 3-phase vector-duty induction motors and frequency drives. During the design phase, you have to initially use the NEC motor tables as a starting point in the calculations to size your drives, breakers and conductors. At that point, the motor has only been quoted by a motor vendor. You won't have the nameplate rated values of that _specific_ motor until the motor is built and the test data has been collected. As Flyer mentioned, NEC is already quite conservative. Depending on the HP and speed of the motor, often times the nameplate FLA is lower than what is listed in the tables so your sizing calculations end up being just that much more conservative. I think problem 510 is open to interpretation. The nameplate data may have not been available so you use what you have, the HP. If nameplate FLA had been given, then that would have been used.


The other end of that is working with really old equipment. Nameplates can range from illegible to non-existent.


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## cruzy (Oct 23, 2010)

cableguy said:


> {Now that I can look at the problem...} I have a feeling that they will say "If you don't have the nameplate, then use the table". We are not given the nameplate - though it _should_ have been given to us. I agree, that's a flaw in this problem.


That's what I think...the nameplate wasn't given so we use the tables per rated HP. I understand they want us to show we know how to use the NEC, but damn can you give us some less tricky questions!?


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## LMAO (Oct 23, 2010)

cruzy said:


> cableguy said:
> 
> 
> > {Now that I can look at the problem...} I have a feeling that they will say "If you don't have the nameplate, then use the table". We are not given the nameplate - though it _should_ have been given to us. I agree, that's a flaw in this problem.
> ...


this is not a "tricky" question; the question is flawed and has no correct answer. NEC tables are not meant to be used to size overloads.


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