Exam Question by NEC questions

[whofrankw]

A three phase 4 wire Y system has 270 amps on each leg. If all loads are linear loads, what is the size of the neutral in amps? |

A. 270
B.249
C.810
D.567

I thought it was 270. book says its 249 and i have no idea how to come up with that. any help

 

[petersonra]

A three phase 4 wire Y system has 270 amps on each leg. If all loads are linear loads, what is the size of the neutral in amps? |

A. 270
B.249
C.810
D.567

I thought it was 270. book says its 249 and i have no idea how to come up with that. any help

i don't know where they came up with 249 either. perhaps there is more information that you did not impart to us.

That answer would make sense if all the loads were L-N except for a 21 A 3 phase load and they were asking for the required neutral ampacity.

 

[whofrankw]

that was word for word out of the book. as long as you dont know either i feel better. i think they just messed up

 

[infinity]

Why not 0, do you need to assume that there are 120 volt and 3Ø loads?

 

[charlie b]

If all loads are linear loads, what is the size of the neutral in amps?

We don't size conductors in units of amps. The size of the neutral would be expressed using AWG sizes. If they are asking for the minimum required ampacity of the neutral, in amps, then the question is badly worded. I would not spend any time trying to figure out how they got their answer. Their question is meaningless, and their answer more so.

 

[LarryFine]

The correct answer as the question was asked is zero.

 

[infinity]

that was word for word out of the book. as long as you dont know either i feel better. i think they just messed up

Curious how did you come up with 270?

 

[don_resqcapt19]

Curious how did you come up with 270?

If you assume they are asking about 220.61(A) and assume line to neutral loads, you come up with 270.

 

[infinity]

If you assume they are asking about 220.61(A) and assume line to neutral loads, you come up with 270.

That's is a good assumption but not the answer that they said was correct.

 

[whofrankw]

they gave four answers. i did not come up with the question or answers so i have to choose what fits best. the question came out of Electrical and fire alarm exam questions published by nec questions

 

[roger]

they gave four answers. i did not come up with the question or answers so i have to choose what fits best. the question came out of Electrical and fire alarm exam questions published by nec questions

You say by "nec questions" who or what is the actual publisher of the exam? Can you provide a link?

Roger

 

[whofrankw]

You say by "nec questions" who or what is the actual publisher of the exam? Can you provide a link?

Roger

http://necquestions.com/

 

[david luchini]

A three phase 4 wire Y system has 270 amps on each leg. If all loads are linear loads, what is the size of the neutral in amps? |

A. 270
B.249
C.810
D.567

I thought it was 270. book says its 249 and i have no idea how to come up with that. any help

See 220.61(B)(2)

 

[480sparky]

See 220.61(B)(2)

We can't assume this is a service/feeder.

 

[david luchini]

We can't assume this is a service/feeder.

Yes, we can.

 

[infinity]

We can't assume this is a service/feeder.

Yes, we can.

Funny in reading the question I would assume that the full 270 amps is from all 3Ø loads.

 

[charlie b]

See 220.61(B)(2)

That at least matches the "book answer." Take 70% of the amount of 270 that exceeds 200, and add that to 200, and you get 249. So the mystery is solved. But I still say it is a poor question.

 

[steve66]

Funny in reading the question I would assume that the full 270 amps is from all 3Ø loads.

Could be. So I agree there isn't enough info. to answer the question.

We don't know if its a service or feeder or branch circuit. And we don't know if the loads are single phase unbalanced, or 3 phase balanced.

Assuming they are all unbalanced, and its a service or feeder, then I agree with David.

But a good test question would have clarified both points.