PE exam - NEC related questions

[GreenNGold]

I came across a couple practice NEC questions and had some questions:


1) An air conditioner (compressor and fan) with a FLA of 55 Amps will be installed on the rooftop. What is the maximum rating of the short circuit protection?


A) 50A
B) 100A
C) 200A
D) 250A


Answer is B, which makes the most sense but according to sec. 440.22, the max rating should not exceed 175% of the FLA, correct? I don't see any exceptions? So if 90 and 100 were options, would 90 be the correct answer?


2) What is the maximum overload setting for a 100HP, 3ph, 460V motor?


A) 50A
B) 150A
C) 200A
D) 250A


Answer is B.


To gather overload settings, we would need to use NP value right? If that is not given, we then use the tables in article 430?


And again, the max overload setting should not exceed 115% of the FLA but 150A is the best standard value choice?


3) The continuous load in a commercial building is 645A. What is the size of the aluminum grounding electrode conductor required to effectively ground the service?


A) #1AWG
B) #1/0AWG
C) #2/0AWG
D) #3/0AWG


Answer is D.


When they say "continuos load" in this problem, does that imply the 1.25 factor has already been taken into consideration? And are they asking for grounding conductor (250.66) or equipment ground (250.122)? My assumption was grounding conductor but that wouldn't get us the right answer.


4) Determine the conduit size for a secondary feeder of a 225kVA, 480-208/120V, dry-type transformer.


How do you know to assume non-continuous load? They sized the cable for 624A (225kVA/(1.73*208)).


Thank you.

 

[infinity]

I'll take a few guesses:


1-They didn't tell you the type of short circuit protection, there are different percentages based on the type in T430.52. You have assumed time delay fuse protection.

2-According to 430.32(C) you could go as high as 130%.

3-I would guess that they want you to add 125% to the ampacity given and find the minimum conductor size. Then you would use that size with T250.66 to find the size of the grounding electrode conductor (GEC) that is mentioned in the question.

4-Your calcualtion gives the maximum load that the transformer can supply within it's rating. You would use that to determine the minimum conductor size and then the raceway size.

IMO these questions are poorly written especially #1 and #4.

 

[david luchini]

I agree with Rob, these questions are poorly written.

1- You can't answer this one. You need to know the rated-load current of the compressor, and the full load current of the fan to properly answer this (440.22(A)(1) or (A)(2).)

2- A "thermal protector" can be up to 140% of the motor full-load currents in the tables, rather than the motor nameplate. This would be 124*1.4=173.6A, so 150A would be correct.

3- With a 645A continuous load, you need a conductor that is at least 645A*1.25=806.25A. The GEC would depend on how you ran the conductors. One set of 1750mcm (assume Cu) would require a 250MCM GEC...two sets of 600mcm would require a 250MCM GEC, three sets of 300mcm would require a 4/0 GEC, four sets of 4/0awg would require a 4/0 GEC...NONE of the answers appear correct.

4- The size of the secondary feeder must not have an ampacity smaller than the secondary OCPD. Without knowing the secondary OCPD size, you cannot answer the question.

 

[iceworm]

... 3- With a 645A continuous load, you need a conductor that is at least 645A*1.25=806.25A. The GEC would depend on how you ran the conductors. One set of 1750mcm (assume Cu) would require a 250MCM GEC...two sets of 600mcm would require a 250MCM GEC, three sets of 300mcm would require a 4/0 GEC, four sets of 4/0awg would require a 4/0 GEC...NONE of the answers appear correct. ...

I would look at T250.66 and note 2. Max size GEC is 3/0 CU.

ice

 

[infinity]

I would look at T250.66 and note 2. Max size GEC is 3/0 CU.

ice

True, but the question was specifically about an aluminum GEC:

3) The continuous load in a commercial building is 645A. What is the size of the aluminum grounding electrode conductor required to effectively ground the service?

 

[iceworm]

True, but the question was specifically about an aluminum GEC:

Well, you may quote me exactly: AARRRGGGGG:ashamed1:

Excellent answer David

Thank you

ice

 

[david luchini]

I would look at T250.66 and note 2. Max size GEC is 3/0 CU.

ice

Just out of curiosity, where were you going with Note 2? ("Where there are no service-entrance conductors...")

 

[iceworm]

Just out of curiosity, where were you going with Note 2? ("Where there are no service-entrance conductors...")

First none of this matters for the OP - you were correct.

So, where was I going? I think the same place you were without mentioning note 2.

As all already know, T250.66 picks the GEC by conductor size. When there is no SEC, one picks the "largest service entrance conductor required for the load served. I currently believe your method is correct. Continuous load x 1.25, pick a conductor size from 310.15 that meets load requirements, select GEC from T250.66.

So why not use paralleled conductors instead of a single conductor? Amps/circular mil goes down with decreasing diameter. Paralleled SEC will yeild a smaller total cross sectional area - thus a smaller GEC. Infact one might always go to paralleled 1/0 and potentially get the smallest GEC.

My curent thinking is one should use the largest single conductor up to the limit of 310.15 (2000kcmil) - even though no one in their right mind would likely use anything over 750; 500 or 350 more common.

Why would this matter. For me, it comes up with a GEC for generation. The Gen is an SDS with really small conductors from the alternator to the first OCP. How does one pick out the GEC? Certainly not by the gen disconnect fuse/CB rating. I have been using your method: GenFLA x 1.25, pick the equivalent SEC from T310.15.B.16. Essentially, about 325kw and up, one is choosing 3/0 CU - instead of saying, Oh, we are using 4-1/0 and the GEC is 1/0.

ice

 

[david luchini]

2- A "thermal protector" can be up to 140% of the motor full-load currents in the tables, rather than the motor nameplate. This would be 124*1.4=173.6A, so 150A would be correct.

:ashamed1: Too long of a week...there is no standard size for "overload protection." None of the answers in question 2 are correct either.

 

[david luchini]

First none of this matters for the OP - you were correct.

So, where was I going? I think the same place you were without mentioning note 2.

As all already know, T250.66 picks the GEC by conductor size. When there is no SEC, one picks the "largest service entrance conductor required for the load served. I currently believe your method is correct. Continuous load x 1.25, pick a conductor size from 310.15 that meets load requirements, select GEC from T250.66.

So why not use paralleled conductors instead of a single conductor? Amps/circular mil goes down with decreasing diameter. Paralleled SEC will yeild a smaller total cross sectional area - thus a smaller GEC. Infact one might always go to paralleled 1/0 and potentially get the smallest GEC.

My curent thinking is one should use the largest single conductor up to the limit of 310.15 (2000kcmil) - even though no one in their right mind would likely use anything over 750; 500 or 350 more common.

Why would this matter. For me, it comes up with a GEC for generation. The Gen is an SDS with really small conductors from the alternator to the first OCP. How does one pick out the GEC? Certainly not by the gen disconnect fuse/CB rating. I have been using your method: GenFLA x 1.25, pick the equivalent SEC from T310.15.B.16. Essentially, about 325kw and up, one is choosing 3/0 CU - instead of saying, Oh, we are using 4-1/0 and the GEC is 1/0.

ice

Thanks, I just don't think I've ever referenced Note 2 in Table 250.66.

If I'm following you, for a service, you can base the GEC on the actual size of the conductors (based on how you paralleled them) but for a non-service, you'd have to base the GEC on the largest single conductor size that would carry the load. That is for a non-service, you lose the benefit of the lower GEC size you may get by using more parallel sets.

 

[iceworm]

Thanks, I just don't think I've ever referenced Note 2 in Table 250.66.

If I'm following you, for a service, you can base the GEC on the actual size of the conductors (based on how you paralleled them) but for a non-service, you'd have to base the GEC on the largest single conductor size that would carry the load. That is for a non-service, you lose the benefit of the lower GEC size you may get by using more parallel sets.

It's pretty hard to have a service (connection to a utility) without conductors. I've never seen one. I guess it could be busbars from the utility transformer to the service disconnect - I haven't seen that. The issue comes up for me cause a lot of my work is off-grid self generated - the gen is an SDS. T250.66 applies for the GEC for an SDS. See note a. And Note 2 comes up occasionally for gens and xfm.

For gens, I pick the fat GEC. The issue is not about saving $1.43 in CU costs. For my applications, the GEC is often part of the path to trip the OCPD. The GEC generally connects to building steel or ground mat - not a useless ground rod. Generators are notoriously low in available SCC. Low impedance GEC is good.

Now consider transformers, grounded Y systems. It only matters for FLA x 1.25 around 600A - say 350kva 480V, or 225kva 208V. Often the N-G bond is made at the transformer secondary and the feed to the first OCPD is 5wire. GEC connects at the xfm. In my not-so-infinite-wisdom I use the conductor area of the 5wire feed - not the largest it would take if we were not using parallel conductors.

Why the difference between xfms and the gens?
With the xfms I have a conductor size to point at. In the gens I don't. And the xfms are generally way lower impedance than the gens.

Could I make an argument to use a smaller GEC on occasion - yes, but why?

ice

 

[iceworm]

:ashamed1: Too long of a week...there is no standard size for "overload protection." None of the answers in question 2 are correct either.

Yes, adding to dl's response
Following OP and using T430.250, 124A, cause we don't have a nameplate, I get:
143A
155A
161A
174A

So I would agree

ice

 

[GreenNGold]

Thanks for the responses guys. I definitely agree that these questions are horribly written so I am not going to stress over them too much anymore.