Need help solving a test question

[antenna2001]

In the master electrician exam, there was a question that I was not sure how to answer:

What is the minimum size service to supply a dwelling installation consisting of seven 2-wire branch circuits?

I took and passed the exam yesterday but this question is bothering me. I don't remember seen a specific section in the NEC that describes, explicitly, how to arrive at the answer.

I only had about two minutes to answer so I quickly assumed that all the circuits were 20A for a total of 140A.

The multiple answer choices were
1. 100A
2. 125A
3. 150A
4. 200A

I went for the third answer, 150A.

I can not properly justify the answer. I am now reading through Art. 210 and I hope to come up with the correct section numbers to back up the answer. Could someone help me find the right procedure?

Thanks

 

[jumper]

I say 100 amps.

230.42 and 230.79(C)

 

[augie47]

IMO, without additional info, you would base your answer on 230.79(C), 100 amp.
Keep in mind the load would be the same regardless of the number of circuits.

 

[infinity]

Is the size of the service disconnecting the size of the service? I may be wrong but IMO it's not unless this falls under specific installations.

230.42 Minimum Size and Rating.
(A) General. The ampacity of the service-entrance conductors before the application of any adjustment or correction factors shall not be less than either (A)(1) or (A)(2). Loads shall be determined in accordance with Part III, IV, or V of Article 220, as applicable. Ampacity shall be determined from 310.15. The maximum allowable current of busways shall be that value for which the busway has been listed or labeled.
(1) The sum of the noncontinuous loads plus 125 percent of continuous loads
(2) The sum of the noncontinuous load plus the continuous load if the service-entrance conductors terminate in an overcurrent device where both the overcurrent device and its assembly are listed for operation at 100 percent of their rating
(B) Specific Installations. In addition to the requirements of 230.42(A), the minimum ampacity for ungrounded conductors for specific installations shall not be less than the rating of the service disconnecting means specified in 230.79(A) through (D).

 

[augie47]

Rob, I agree but with the info given, I figured that was the most probable answer and, not seeing the actual questio, thought it might have included the word "disconnect".

 

[Little Bill]

In the master electrician exam, there was a question that I was not sure how to answer:

What is the minimum size service to supply a dwelling installation consisting of seven 2-wire branch circuits?

I took and passed the exam yesterday but this question is bothering me. I don't remember seen a specific section in the NEC that describes, explicitly, how to arrive at the answer.

I only had about two minutes to answer so I quickly assumed that all the circuits were 20A for a total of 140A.

The multiple answer choices were
1. 100A
2. 125A
3. 150A
4. 200A

I went for the third answer, 150A.

I can not properly justify the answer. I am now reading through Art. 210 and I hope to come up with the correct section numbers to back up the answer. Could someone help me find the right procedure?

Thanks

I just took the test a month or so ago, and this is one of their "trick" or make you think questions. They ask the question and add other information not relevant to what the answer is. The question ask for the minimum size service, so the answer is 100A because that is the minimum allowed for a dwelling (220.82). How many circuits has nothing to do with the answer they want.

 

[Cavie]

I just took the test a month or so ago, and this is one of their "trick" or make you think questions. They ask the question and add other information not relevant to what the answer is. The question ask for the minimum size service, so the answer is 100A because that is the minimum allowed for a dwelling (220.82). How many circuits has nothing to do with the answer they want.

AND THE ANSWER IS................Just as Little Bill stated. Word for word.

 

[infinity]

I just took the test a month or so ago, and this is one of their "trick" or make you think questions. They ask the question and add other information not relevant to what the answer is. The question ask for the minimum size service, so the answer is 100A because that is the minimum allowed for a dwelling (220.82). How many circuits has nothing to do with the answer they want.

AND THE ANSWER IS................Just as Little Bill stated. Word for word.

Could either of you explain how 220.82 provided the answer of 100 amps?

 

[Dennis Alwon]

What about 230.42(B)

230.42 Minimum Size and Rating.
(A) General. The ampacity of the service-entrance conductors before the application of any adjustment or correction factors shall not be less than either (A)(1) or (A)(2). Loads shall be determined in accordance with Part III, IV, or V of Article 220, as applicable. Ampacity shall be determined from 310.15. The maximum allowable current of busways shall be that value for which the busway has been listed or labeled.
(1) The sum of the noncontinuous loads plus 125 percent of continuous loads
(2) The sum of the noncontinuous load plus the continuous load if the service-entrance conductors terminate in an overcurrent device where both the overcurrent device and its assembly are listed for operation at 100 percent of their rating
(B) Specific Installations. In addition to the requirements of 230.42(A), the minimum ampacity for ungrounded conductors for specific installations shall not be less than the rating of the service disconnecting means specified in 230.79(A) through (D).

 

[infinity]

What about 230.42(B)

Yes, that was my guess when I threw that one out there in post #4. IMO, 220.82 alone does not satisfy the question but other posters feel differently. Just wanted to here some opinions as to how 220.82 applies.

 

[Little Bill]

Could either of you explain how 220.82 provided the answer of 100 amps?

220.82 says Feeder and Service Load.....with ampacity of 100A or greater.

What about 230.42(B)

Like Dennis said , 230.79 says to look at Article 220. 230.79(C) says minimum 100A.
Do you not agree that 100A is the minimum?

 

[infinity]

220.82 says Feeder and Service Load.....with ampacity of 100A or greater.



Like Dennis said , 230.79 says to look at Article 220. 230.79(C) says minimum 100A.
Do you not agree that 100A is the minimum?

Yes, i agree that the service disconnecting means must be 100 amps or more but does the disconnecting means size automatically determine the size of the service? Couldn't I have a 100 amp service disconnect switch with 60 amps fuses in it and still satisfy 230.79(C)?

IMO the real determining factor in concert with 230.79(C) is 230.42(B). So what exactly are specific installations?

 

[Dennis Alwon]

Y
IMO the real determining factor in concert with 230.79(C) is 230.42(B). So what exactly are specific installations?

I assume the specific installations are those mentioned in 230.79

 

[infinity]

I assume the specific installations are those mentioned in 230.79

You might be right.

 

[dana1028]

I just took the test a month or so ago, and this is one of their "trick" or make you think questions. They ask the question and add other information not relevant to what the answer is. The question ask for the minimum size service, so the answer is 100A because that is the minimum allowed for a dwelling (220.82). How many circuits has nothing to do with the answer they want.

"How many circuits has nothing to do with the answer they want."

I disagree with you - I'm thinking the intent of that language is to differentiate this installation from that in 230.79(B) - 'two-circuit installations.'

I agree the 7 circuits prob. has nothing to do with a load, but to insure you do not use the 2-circuit option.

 

[Little Bill]

"How many circuits has nothing to do with the answer they want."

I disagree with you - I'm thinking the intent of that language is to differentiate this installation from that in 230.79(B) - 'two-circuit installations.'

I agree the 7 circuits prob. has nothing to do with a load, but to insure you do not use the 2-circuit option.

There are a lot of variables in determining a service load, I'm just saying the other info they put in, ie: (7) 2-wire branch circuits, has nothing to do with the answer they are looking for, which is 100A. That is the minimum for a dwelling unit.

 

[antenna2001]

Thanks

Thanks

Thanks everyone,
After looking at your answers and after reading 210 word-by-word, it is clear to me that I did not miss anything that woud obviate the answer. I now tend to agree with the 100A minimum required by 230.79(C). It would have taken me too long to try to solve this on my own. Thanks again for the help.