masters exam stumper
- Thread starter jsouth25
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ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: masters exam stumper
I'm coming up with 600KCMIL.
475 amps (90 degrees) times .8 for derating =380 amps times 2 for parallel =760 amps, allowed by 240.4to round up.
Hope this helps.
Anyone else?
I'm coming up with 600KCMIL.
475 amps (90 degrees) times .8 for derating =380 amps times 2 for parallel =760 amps, allowed by 240.4to round up.
Hope this helps.
Anyone else?
- Location
- Massachusetts
Re: masters exam stumper
We can only use 240.4(B) if the conductors can cover the load.
If the load is 800 amps we will need 800 amps of conductor.
We can only use 240.4(B) if the conductors can cover the load.
If the load is 800 amps we will need 800 amps of conductor.
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: masters exam stumper
Bob, the next size up is 700KCMIL, which was not an option. I would have to assume that the answer is 600KCMIL. Again this is an assumption, and you know what that can mean....
[ December 11, 2003, 08:54 PM: Message edited by: ryan_618 ]
Bob, the next size up is 700KCMIL, which was not an option. I would have to assume that the answer is 600KCMIL. Again this is an assumption, and you know what that can mean....
[ December 11, 2003, 08:54 PM: Message edited by: ryan_618 ]
websparky
Senior Member
- Location
- Cleveland, Ohio
Re: masters exam stumper
Hi Ryan,
The question does not say that the raceway is longer than 24".
Based on the information provided, 500kcmil is plenty!
Hi Ryan,
The question does not say that the raceway is longer than 24".
Based on the information provided, 500kcmil is plenty!
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: masters exam stumper
Hi Dave: I've never seen an overhead service with less than a two foot mast
Hi Dave: I've never seen an overhead service with less than a two foot mast
- Location
- Massachusetts
Re: masters exam stumper
Dave that's good
Dave that's good
- Location
- Massachusetts
Re: masters exam stumper
Dave I do have to ask how 2 sets of 500s (760 amps) is plenty.
This would only be true if the calculated load is less than 760 amps.
761 amp and above will need 2 sets of 600s (forgetting about derating)
Bob
Dave I do have to ask how 2 sets of 500s (760 amps) is plenty.
This would only be true if the calculated load is less than 760 amps.
761 amp and above will need 2 sets of 600s (forgetting about derating)
Bob
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: masters exam stumper
LOL
See, now I'm right again
LOLwebsparky
Senior Member
- Location
- Cleveland, Ohio
Re: masters exam stumper
At 90 degrees I get 860A.
Is there a reason I could not use 90?
Are not the lugs on the breaker or bus rated at 90?
[ December 11, 2003, 09:56 PM: Message edited by: websparky ]
At 90 degrees I get 860A.
Is there a reason I could not use 90?
Are not the lugs on the breaker or bus rated at 90?
[ December 11, 2003, 09:56 PM: Message edited by: websparky ]
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: masters exam stumper
If you are talking about 500KCMIL again, you could not use 90 degree ratings because the terminals will not be rated for 90 degrees. I used 90 degrees for the derationg only. Or am I not understanding your question?
websparky
Senior Member
- Location
- Cleveland, Ohio
Re: masters exam stumper
Hi Ryan,
You are understanding just fine!
Actually THWN is only rated at 75* not 90*.
500kcmil THWN is rated at 380A.
Two of them will get you to 760A.
Now, here is the fun part;
The question asks for the size based on the service supplied not the calculated load.
That means you may go to the next size breaker.
Hi Ryan,
You are understanding just fine!
Actually THWN is only rated at 75* not 90*.
500kcmil THWN is rated at 380A.
Two of them will get you to 760A.
Now, here is the fun part;
The question asks for the size based on the service supplied not the calculated load.
That means you may go to the next size breaker.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: masters exam stumper
Was ?900 MCM? one of the available choices?
I would work the problem backwards. Absent a clear statement that the load is under the 800 amps of the selected service size, I would have to presume it is not. I conclude that the service conductors will have to carry 800 amps. So it?s 400 amps each. Absent a clear statement that the conduit is less than 24? long, I would have to presume it is not. Thus the conductors would be derated by 80%. A conductor?s ampacity has to start off at 500 amps (i.e., 400/0.8), in order for its ampacity to be 400 amps after the derating. You have to use the 75C table, because THWN is not rated for 90C. The smallest conductor that is rated for 500 amps or more is a 900 MCM.
Was ?900 MCM? one of the available choices?
I would work the problem backwards. Absent a clear statement that the load is under the 800 amps of the selected service size, I would have to presume it is not. I conclude that the service conductors will have to carry 800 amps. So it?s 400 amps each. Absent a clear statement that the conduit is less than 24? long, I would have to presume it is not. Thus the conductors would be derated by 80%. A conductor?s ampacity has to start off at 500 amps (i.e., 400/0.8), in order for its ampacity to be 400 amps after the derating. You have to use the 75C table, because THWN is not rated for 90C. The smallest conductor that is rated for 500 amps or more is a 900 MCM.
websparky
Senior Member
- Location
- Cleveland, Ohio
Re: masters exam stumper
Hi Charlie,
Good point!
I was trying to have some fun with these incomplete questions that are often asked at all of the forums.
Seems like it is a result of the resent full moon.
Maybe the next time we get one of these "I have a question but I'm only going to give you part of the information so you can play the 20 questions game" types, you could retitle it "20 Questions and Speculations"!
The best I can remember the last overhead 800A. service, we ran 3 sets of 300kcmil in separate conduits. I can't imagine a real world situation where 900kcmil would be used in one 5" conduit.
Hi Charlie,
Good point!
I was trying to have some fun with these incomplete questions that are often asked at all of the forums.
Seems like it is a result of the resent full moon.
Maybe the next time we get one of these "I have a question but I'm only going to give you part of the information so you can play the 20 questions game" types, you could retitle it "20 Questions and Speculations"!
The best I can remember the last overhead 800A. service, we ran 3 sets of 300kcmil in separate conduits. I can't imagine a real world situation where 900kcmil would be used in one 5" conduit.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Re: masters exam stumper
The sad part is that there are many people out there writing questions for exams that impact the livelihood of electrical professionals, but who do not have the special skills to write such questions. To do, and to write questions about doing, are far, far different tasks. They don?t seem to realize that they are playing the ?20 questions game? in an exam that should not be a game.
By the way, the question did not say that this was a real life situation, nor that the conductors were going into a f inch conduit. The best way to beat the ?20 questions game? is not to read into the question any fact that is not there. Here?s the rule:
The question does not say what you think it says. And even if you wrote the question, it does not say what you wanted it to say. It says what it says: nothing else, and nothing more.
Back to ?jsouth25?: You posted the original issue, and I think it?s time for you to chime back in. What was the ?correct? answer?
The sad part is that there are many people out there writing questions for exams that impact the livelihood of electrical professionals, but who do not have the special skills to write such questions. To do, and to write questions about doing, are far, far different tasks. They don?t seem to realize that they are playing the ?20 questions game? in an exam that should not be a game.
By the way, the question did not say that this was a real life situation, nor that the conductors were going into a f inch conduit. The best way to beat the ?20 questions game? is not to read into the question any fact that is not there. Here?s the rule:
The question does not say what you think it says. And even if you wrote the question, it does not say what you wanted it to say. It says what it says: nothing else, and nothing more.
Back to ?jsouth25?: You posted the original issue, and I think it?s time for you to chime back in. What was the ?correct? answer?
websparky
Senior Member
- Location
- Cleveland, Ohio
Re: masters exam stumper
Hi Charlie,
You are soooooo right!
Wording statements and questions is way more difficult than what meets the eye.
As all of us know from past experience, when we sit down to take a test, one of the first things we think about is trying to not read into the question. Some questions seem so hard because they are incomplete thoughts in the form of a question.
Thanks,
Hi Charlie,
You are soooooo right!
Wording statements and questions is way more difficult than what meets the eye.
As all of us know from past experience, when we sit down to take a test, one of the first things we think about is trying to not read into the question. Some questions seem so hard because they are incomplete thoughts in the form of a question.
Thanks,
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