service entrance grounded conductor size as per 2008 NEC

[laurencemandin@yahoo.com]

A business has a 120/240v single phase 200 amp service. The service entrance grounded conductor shall be a minimum of ______ awg aluminum (AL). The service entrance ungrounded conductors are 3/0 awg aluminum or copper clad aluminum.

A. 2
B. 4
C. 6
D. 8

Note: My answer is B. My instructor's answer is A. Both of us used 250.24(C)(1) and Table 250.66. Which answer is correct and why?

 

[roger]

You need to be looking at 220.61

Roger

 

[charlie b]

My answer is A. Answer B would be right, if the wire were copper.

Roger, I do not understand the relationship to the service load calculation. Specifically, 220 will get you to a load value. But 250.24(C)(1) points right at 250.66 as the minimum size for this wire. So I am confused.

 

[augie47]

My answer would be "A" also based on your 3/0 AL. {250.66,230.23(C), 250.24(C) }

{My red tag would also be there based on 3/0 AL (155 ampacity) on the 200 amp service With the correct size SE conductors (4/0 or 250) the grounded would be 1/0AL}

 

[charlie b]

You forgot to look at the panel schedule, Gus. It shows the main service panel as having a MCB rated at 150 amps.

 

[roger]

Roger, I do not understand the relationship to the service load calculation. Specifically, 220 will get you to a load value. But 250.24(C)(1) points right at 250.66 as the minimum size for this wire. So I am confused.

Charlie, you have a right to be confused, I was shooting from the hip instead of openning a code book.

Roger

 

[billsnuff]

My answer is A. Answer B would be right, if the wire were copper.

:-?

ungrounded conductors are 3/0 awg aluminum or copper clad aluminum

Looking at 250.66 Largest Ungrounded AL 2/0 or 3/0 and I follow across
I see 4 AL...........must be a brain cramp I'm having..........

 

[augie47]

:-?



Looking at 250.66 Largest Ungrounded AL 2/0 or 3/0 and I follow across
I see 4 AL...........must be a brain cramp I'm having..........

Nope, I think the brain cramp was mine. I looked at 1st column, Cu conductors.
3/0 AL (undersized) would be a 4 AL ungrounded
4/0 or 250 AL would be a 2 AL

 

[newinspector1]

I see what you see billsnuff. The instructor appears to be wrong.

 

[charlie b]

Nope, I think the brain cramp was mine. I looked at 1st column, Cu conductors.

Mine too. I made the same mistake. So the correct answer, after accounting for the 150 amp MCB that makes the 3/0 wire size acceptable, is "B."

 

[Pokesomi]

In this case you are correct your instructor is not. Table 250.66 shows that if the SE is 2/0 or 3/0 AL or Copper Clad AL, you woud use 4 AL awg

 

[Bjenks]

The way ttable 250.66 is laid out has cause me many of typos when it comes to Aluminum. the tendency is to go down the column to the conductor size and then return the value right next to it, which is CU.

It is one of those brain games.

 

[Dennis Alwon]

The entire question should be void since you cannot use 3/0 alum for a 200 amp service as the op stated. If copper then #4 copper is correct, if 4/0 alum then #2 alum is correct , and if 150 amps is correct with 3/0 alum then #4 alum is okay. So which is it?

 

[don_resqcapt19]

This question cannot be answered without additional information. 250.24(C)(1) gives the minimum permitted size, but Roger was correct you also have to apply the rule in 220.61. You use the larger of the two sizes.

 

[aftershock]

Are we talking about the grounding conductor or the grounded conductor? I don't think the answers fit the question.

 

[Dennis Alwon]

Are we talking about the grounding conductor or the grounded conductor? I don't think the answers fit the question.

We are talking of the grounded conductor- neutral

 

[david luchini]

This question cannot be answered without additional information. 250.24(C)(1) gives the minimum permitted size, but Roger was correct you also have to apply the rule in 220.61. You use the larger of the two sizes.

I think there is enough information in the question, as it is asking for the minimum size, not the actual required size of the grounded conductor.

But I agree with Dennis that the question is not valid, as it lays out a scenario which would be in violation of the code anyway. Why ask a code question about 240.24(C)(1) - grounded conductor, when the rest of the question would violate 230.90(A) for the ungrounded conductors?