NCEES 510

[LMAO]

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?

 

[Flyer_PE]

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........

 

[cableguy]

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.

 

[Flyer_PE]

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.

 

[LMAO]

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........

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.

 

[cableguy]

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

 

[Flyer_PE]

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.

 

[Flyer_PE]

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.

 

[LMAO]

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.

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.

 

[cableguy]

{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.

 

[Flyer_PE]

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.

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.

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.

 

[knight1fox3]

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.

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.

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

 

[Flyer_PE]

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.

 

[cruzy]

{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!?

 

[LMAO]

{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!?

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.