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ptc-48895541-Visitor
1-Visitor
December 19, 2012
Question

Line Ampacity Calculations

  • December 19, 2012
  • 2 replies
  • 10114 views

Hello all,

I'm an undergradute EE student who is currently working for a small engineering consultant firm and I've been asked by my boss to create two MATHCAD programs to calculate and graph the ampacity of various conductors. Two variants because one is to be done in "english" units (imperial), and the other in SI units.

So far I've been working off the IEEE Std 738-2006 standard, and have began programming various equations into MATHCAD. While doing so I've come across a few errors that I can't seem to figure out, and I am currently looking for some help in correcting them.

Please see the figure attached for a image of what my program currently consists of. The errors in the blue boxes are solely a consequence of the error in the red box.(From what I can tell) What I can't figure out is why the error in the red box is happening, and how to correct it. For the error in the red box, MATHCAD is telling me the value of Vw must be a scalar. Why is that? How do I fix this?

NOTE: I am currently using MATHCAD 14

I've also attached my most up to date version of the program thus far.

I look forward to any advice/suggestions/comments you guys have.

Thanks again!

2 replies

A
AlanStevens19-Tanzanite
19-Tanzanite
December 19, 2012

Look carefully at your definition of Qc2. You have defined Vw as a function of Ta there, when it should just be a scalar:

Qc2.PNG

Alan

R
RichardJ19-Tanzanite
19-Tanzanite
December 19, 2012

The errors in the blue boxes are solely a consequence of the error in the red box.(From what I can tell)

No. Calculations in Mathcad are top left to bottom right. All page boundaries are just to show how it would be printed, and have no effect on calculation order. So Qc is flagged red because at that point in the worksheet the function is undefined.

Two variants because one is to be done in "english" units (imperial), and the other in SI units.

In Mathcad that is not necessary. Mathcad can handle units. If the worksheet is written in the correct way you could have one sheet that will work with either imperial or metric units (or even a mixture!).

P
ptc-48895541-VisitorAuthor
1-Visitor
December 20, 2012

Hello Mr. Jackson,

Can you explain further on how I wouldn't need to create two variants of the program, since MATHCAD can handle all the changes in units?

In order to calculate the ampacity of a given conductor, my program requires that the user provide various information such as,

  1. Wind speed
  2. Conductor Diameter
  3. Conductor AC resistance
  4. Etc...

If I leave it up to the user to provide input in whichever units he pleases (for example, wind speed could have units of m/s [SI] or ft/s [English]) how can I get MATHCAD to treat things accordingly?

Also, there are other constants such as the Dynamic viscosity of air, Air density and Thermal conductivity of air that are calculated using different formulas depending on whether the units are English or SI.

I have not written any code for the SI program yet, but what I have posted above is my best crack at the English unit version. Is there a way that I can allow the user to input SI units and still obtain the approriate outputs?(Without having to write a second variant of the program of course)

If so, how?

Thank you very much for your time and comments.

Cheers!

R
RichardJ19-Tanzanite
19-Tanzanite
December 20, 2012

In most cases, making Mathcad handle units is trivial. Temperature units are a little more tricky, but not that hard. To learn about how to do this go to "help", "tutorials", "getting started primers", "Using units", and "help", "tutorials", "features in depth", "temperature and user defined units". Once you have read them, add the appropriate units for all of your input variables. When you do that some of your expressions will turn red, because of unit errors. In particular, you have some empirical formulas (weird exponents like 0.52 can only occur in an empirical formula). Making empirical formulas handle units correctly is not covered in the help or tutorials, so the easiest thing to do at that point is post the new worksheet here and I'll show you how to do it.

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