Old c-10 test question...

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Bob Anchorite

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Ok, this was on the C-10 electrical state test.

"Usually the current-carrying capacity of a conductor is in direct proportion to the diameter of the conductor, all other conditions remaining the same."
True/False

The answers is "false" according to those in the "know".... what's the trick of this question.

Bob
 
Maybe the trick is the word direct.
To me this would imply that if you double the diameter, say 250 kcmil to 500 kcmil, you would have double the ampacity. That's not true.

Size and ampacity are in proportion but not direct proportion.
 
Bob Anchorite said:
Ok, this was on the C-10 electrical state test.

"Usually the current-carrying capacity of a conductor is in direct proportion to the diameter of the conductor, all other conditions remaining the same."
True/False

The answers is "false" according to those in the "know".... what's the trick of this question.

Bob
Click to expand...

appropriate would be "Usually the current-carrying capacity of a conductor is in direct proportion to the area of the conductor, all other conditions remaining the same."
 
Believe it or not, test writers are told not to write TRICK questions. Some do slip through but as a rule it isn't necessary to trick a test taker. Tests are designed to measure knowledge, not paranoia.
 
GeorgeB said:
appropriate would be "Usually the current-carrying capacity of a conductor is in direct proportion to the area of the conductor, all other conditions remaining the same."
Click to expand...

This sounds false too.
 
Bob Anchorite said:
Ok, this was on the C-10 electrical state test.

"Usually the current-carrying capacity of a conductor is in direct proportion to the diameter of the conductor, all other conditions remaining the same."
True/False

The answers is "false" according to those in the "know".... what's the trick of this question.
.
Bob
Click to expand...

was this a multiple choice test??

If diameter increases or decreases all other can not remain the same
resistance is inversley proportional. if CMA increases Resistance decreases

ampacity is inverse to overall resistance.

maybe this will help
 
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Dave58er:

You are almost right, but 250KCM is not a diameter, it is an area.

GeorgeB:

Due to the skin effect, the ampacity of a conductor is not in direct proportion to the area of a conductor either. Notice that the ampacity of 500KCM is only 380/255 = 1.5 times the ampacity of 250KCM.

Take the 250KCM diameter (from Chapter 9, Table 8 ) of .575 inches, and the 500KCM diameter of .813. If the diameter was directly proportional to ampacity, the ampacity of 500 would be .813/.575 * 255 = 360 amps. But it is 380 amps.

So although ampacity does go up with area and with diameter, it is not "directly" proportional to either.
 
Bob Anchorite said:
Ok, this was on the C-10 electrical state test.

"Usually the current-carrying capacity of a conductor is in direct proportion to the diameter of the conductor, all other conditions remaining the same."
True/False

The answers is "false" according to those in the "know".... what's the trick of this question.

Bob
Click to expand...
USUALLY???? I don't think so!, that's ALWAYS the case, in "direct proportion" means that if the diameter (of the conductor) increases, so does its capacity to carry current; considering the other factors or conditions (temperature, voltage, etc.) remain constant.
 
vector1:

What you are describing would be "in proportion". If A goes up, then B goes up.

The question said "in DIRECT proportion". That means if A doubles, B also doubles.

So it's almost NEVER the case. (Although I'll bet you can find one case where it does double).

Steve
 
Hi Steve, actually what you are describing is a Linear Direct Proportionality, where the constant of proportionality is a constant multiple(non-zero). But, if we accept your definition, how would you call a proportion when one of the variables decreases in relation to the other?
The fact is that, mathematically, two variables that behave in the previously stated maner (or having their absolute values increase) are said to be directly proportional to each other. In the case when one of the variables changes inversely (or directly to the multiplicative inverse) of the other, are said to be inversely proportional to each other.
We can not derive the relationship based on the tables' figures, we need to know the formulas used to get the values of the tables, and that is why infering a proportion out of the published values can be erroneous.
 
Dave58er said:
Parallel conductors?

Come on Steve, don't leave us hanging.:)
Click to expand...

No, I hadn't even thought about parallel conductors. I'm just thinking that since the ampacity of 500KCM is pretty close to twice the ampacity of 250KCM, so we can probably find some odd sizes of wire where this might work. I think that's why the question had the word "usually" in it.

For example, it just might be that the ampacity of 510 KCM wire would be twice the ampacity of 255 KCM wire (with a little rounding). I only used those sizes as an example, I'm not up for all the math to find out what size this might work for.
 
vector1 said:
Hi Steve, actually what you are describing is a Linear Direct Proportionality, where the constant of proportionality is a constant multiple(non-zero).
Click to expand...


Not according to Wolfram:

http://mathworld.wolfram.com/DirectlyProportional.html

vector1 said:
But, if we accept your definition, how would you call a proportion when one of the variables decreases in relation to the other?
Click to expand...

That might or might not be "Inversely proportional".

vector1 said:
The fact is that, mathematically, two variables that behave in the previously stated maner (or having their absolute values increase) are said to be directly proportional to each other.
Click to expand...

So your are saying for Y=X^3, Y is directly proportional to X? Sorry, but that's not right.

Steve
 
bkludecke said:
Believe it or not, test writers are told not to write TRICK questions. Some do slip through but as a rule it isn't necessary to trick a test taker. Tests are designed to measure knowledge, not paranoia.
Click to expand...

This is not a trick question though. it is pretty straightforward and is making sure the test taker understands an important principle.

<added>let us go back to some high school math. the cross sectional area (e.g.- kcm) is NOT proportional to the diameter. it is proportional to the square of the diameter. ampacity is more directly related to cross sectional area, although skin effect and heating issues make it non-linear.
 
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