NCEES 510, 537 & 540

[soma]

Below are the problems I am having issues with NCEES solutions. Appreciate any comments or inputs. Thanks.

NCEES 510:

According to 432.32A(1) it is 115% but maximum as per 432C can be 130%. This question asks for maximum so the answer should be 80A not 70A.

NCEES 537:

If the diameter of conductor increases, then "C" increases and charging current increases. So the answer should be © not (B).

NCEES 540:

the problem has give 0.85 lag pf and 23% Xd'. Should we determine Z from the given PF and Xd' to calculate the fault current ?

 

[Insaf]

Regarding Ques 510: 430.32C is applicable when the settings or sizing of overload device not sufficient to start the motor or carry the load - - Thanks

 

[Insaf]

NCEES 540:

Generator transient (pu) and transformer %z (or pu) are given, so need to use that impedances to calculate short circuit current (pu). Only one thing to address, make sure both impedances are based on same MVA.

 

[Insaf]

NCEES 537:

If the diameter of conductor increases, then "C" increases and charging current increases. So the answer should be © not ( B).

Response: If rc increase then 'C' increase and eventaully Ichg (Ichg=jwCV) increase, but this increase is not directly proportional. Bottom line answer selection looks correct but answer explanation and answer wording are not.

Any better explanation will be appreciated.

Thanks

 

[soma]

Thanks for your response. I still have below concerns and would greatly appreciate your advise.

510: The question asked maximum allowed as per code, so 115% as per 430.32A(1) cant be maximum since we have 130% as per 430.32C. Am I missing something ? Does the question need to specifically state the device sized as per 430.32A(1) is not sufficient to start the motor ?

540: Power factor of 0.85 along with Xd' is given for Generator. I was not sure if we should use that to find generator Z pu and then add that to transformer by assuming same X/R for both transformer and generator. Presently we are directly adding Xd' pu with transformer Z (ofcourse on common base ), which means we are assuming X/R of transformer to be infinity.

 

[Insaf]

510 - If in question has any concern or comments about motor starting current or minimum load to serve, then have to validate setting or overload device size. If validation produce negative result, you should consider 430.32 ©, otherwise not.

Thanks,

 

[Insaf]

540: Power factor of 0.85 along with Xd' is given for Generator. I was not sure if we should use that to find generator Z pu and then add that to transformer by assuming same X/R for both transformer and generator. Presently we are directly adding Xd' pu with transformer Z (ofcourse on common base ), which means we are assuming X/R of transformer to be infinity.

-- I know that PF have a role to calculate voltage correction factor (pu), but I believe PF may not have impact on transient or subtransient impedances (Xd' depend on base KVA and KV, not PF).

Thanks,

 

[Silkworm]

Soma,

Q510:

430.32© is not applicable since the selected overload device using 430.32(a)(1) is capable of carrying the load current. The overload device will be shunted during startup, so it won't affect startup of the motor.

 

[soma]

Thanks Dean !

 

[Lielec11]

Below are the problems I am having issues with NCEES solutions. Appreciate any comments or inputs. Thanks.

NCEES 510:

According to 432.32A(1) it is 115% but maximum as per 432C can be 130%. This question asks for maximum so the answer should be 80A not 70A.

So is this mistake? I feel like it should be 80 as well?

 

[saberger_vt]

Soma, Lielec11

NCEES 510:

Silkworm is correct. You must also realize this is a "unit" issue. The problem states the maximum sized protection device (amperes) that can be applied is most nearly:

I have bolded and underlined the key words in this problem. The answer is per Table 430.250, for a 50HP, 460V three-phase motor, the full load current is 65A. You use this information with 430.32(A)(1) which is 65A*115% (65A*1.15) = 74.75A.

This is where the bolded/underlined item comes into play: Since the answer is 74.75, you would round down to find the nearest 5A integer number, which would be 70A, since 74.75 is "nearer" to 70 than to 80.

Had one of the answers been 75, then that would have been the answer, since 74.75 is "nearest" to 75.

Also, read 430.32(A)(1) carefully, it states that "This device shall be selected to trip or shall be rated at no more than the following percent of the motor nameplate full-load current rating"

Again, see the bold/underlined information. With this in mind, if they made a 74.75A overload protection device, that would be the maximum size you could use, so in this case since the answers given are 70 and 80, you HAVE to choose the one that is less than the 115% value.

 

[saberger_vt]

Soma, Lielec11

NCEES 510:

Silkworm is correct. You must also realize this is a "unit" issue. The problem states the maximum sized protection device (amperes) that can be applied is most nearly:

I have bolded and underlined the key words in this problem. The answer is per Table 430.250, for a 50HP, 460V three-phase motor, the full load current is 65A. You use this information with 430.32(A)(1) which is 65A*115% (65A*1.15) = 74.75A.

This is where the bolded/underlined item comes into play: Since the answer is 74.75, you would round down to find the nearest 5A integer number, which would be 70A, since 74.75 is "nearer" to 70 than to 80.

Had one of the answers been 75, then that would have been the answer, since 74.75 is "nearest" to 75.

Also, read 430.32(A)(1) carefully, it states that "This device shall be selected to trip or shall be rated at no more than the following percent of the motor nameplate full-load current rating"

Again, see the bold/underlined information. With this in mind, if they made a 74.75A overload protection device, that would be the maximum size you could use, so in this case since the answers given are 70 and 80, you HAVE to choose the one that is less than the 115% value.

I should clarify, even if 75A was a solution, it still would be wrong based on my last statement that you cannot have an overload device greater than the 115%.

 

[jcbabb]

Below are the problems I am having issues with NCEES solutions. Appreciate any comments or inputs. Thanks.

NCEES 510:

According to 432.32A(1) it is 115% but maximum as per 432C can be 130%. This question asks for maximum so the answer should be 80A not 70A.

NCEES 537:

If the diameter of conductor increases, then "C" increases and charging current increases. So the answer should be © not ( :B .

NCEES 540:

the problem has give 0.85 lag pf and 23% Xd'. Should we determine Z from the given PF and Xd' to calculate the fault current ?

I think the key to 510 is the wording of the question. It states "...the initial max sized protection device...". So while 430.32 © may come into play once you realize that the OCPD was not sufficient to start the motor, you still start with the 115% initially.