voltage drop

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dereckbc

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Re: voltage drop

The specific resistance (K) of a material is the resistance offered by a wire of this material which is 1 foot long with a diameter of 1 mil. Copper = 10.8 to 13.1, common value of 12.9

[ August 19, 2003, 07:54 PM: Message edited by: dereckbc ]
 

justin

Senior Member
Re: voltage drop

The constant K represents the resistance for a 1000-circular mils conductor that is 1000 ft long at an operating temp of 75 deg C. Constant K is 12.9 for Copper and 21.2 for Al. (not 1 mil at 1 ft, that would be an outragous amount of resistance)
 

roger

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Fl
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Re: voltage drop

I'm all ears too Justin. (actually eyes in net world :D )

Roger
 
G

Guest

Guest
Re: voltage drop

OK, I'm in:
Wouldn't 1000 at a 1000
be the same as
1 at 1?

1000/1000=1

../Wayne

PS: What did I miss here?
 

dereckbc

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Staff member
Location
Plano, TX
Re: voltage drop

Originally posted by hurk27:
I'm wondering if this is what hottwire
is looking for (The specific resistance ) or what the leter K means on the end of a number?
Since he referenced voltage drop you would assume specific resistance.

Justin take a look in the NEC chapter 9 table 8. Find the column for “uncoated copper ohms per Kft”. Then go down to 1000 KCMIL cable and see what the resistance is. You will see the resistance is .0129 ohms per 1000 feet. You were close, only missed by a factor of 1000 :)

Next do a search on Google for "Specific Resistance of Materials" and see what you come up with.

[ September 09, 2003, 09:13 AM: Message edited by: dereckbc ]
 

charlie b

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Location
Lockport, IL
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Retired Electrical Engineer
Re: voltage drop

Let?s take this back to basic principles. The resistance of any isotropic material (fancy word for ?properties are the same in any direction? ? we can include our common wires in this category) is found from

R = (rho) times ( l / A ),

where

?rho? is resistivity of the material (Not the same rho that shows up in Table B.310.7. It has units of resistance times length, e.g., ohms-ft),
?l? is the length of the material (i.e., wire), and
?A? is the cross sectional area of the material (i.e., wire).
(Reference: My 30+ year old High School physics book that I still keep at my desk.)

If you change the area from 1 cm to 1000 cm, you cut the resistance by a factor of 1000. If you change the length from 1 foot to 1000 feet, you increase the resistance by a factor of 1000. If you do both, you do not change the total resistance. In this respect, awwt was correct. In light of this truth, the formulas given by dereckbc and justin were both right. Therefore, justin was incorrect, when he said that dereckbc?s formula was incorrect.

As to the use of ?K? (versus ?k?): Within the scientific, mathematical, and engineering communities, we have made use of all 26 English letters (in both upper and lower cases), all 24 Greek letters (in both upper and lower cases), and a host of letters whose origins elude me. We?ve run out of letters, and we have to keep re-using them (witness the two uses of the Greek letter rho, as discussed above). The letter ?K? and its lower case version have both been used to denote ?kilo,? a prefix meaning 1000. In the context of the original question, the letter ?K? was probably chosen for this specific constant for no better reason that that it is one of the more popular letters used to denote constants.

As to dereckbc?s reference to the values shown in Table 8, the column head clearly shows the units of ohms/1000 ft. But that Table does not address the ?K? issue. If you know the formula, you can calculate an ?exact K? from the tabulated resistance values. Or you can just use the ?approximate K? values given by several contributors above.
 

c-h

Member
Re: voltage drop

Charlie,
The resistance of any isotropic material (fancy word for ?properties are the same in any direction? ? we can include our common wires in this category)
If we leave the real world for a moment and enter the world of audio and hi-fi, wires are no longer isotropic. :D

Supra cables
 
G

Guest

Guest
Re: voltage drop

That sounds like a theory manufactured to meet the desired end result. Very interesting. Thanks for that!

../Wayne C.
 
G

Guest

Guest
Re: voltage drop

Originally posted by websparky:
<snip>no bi-directional flow in the conductor.
After skimming the link my understanding is they are trying to use the best orientation of the cabling. Even if it's traveling in one direction they want the cable oriented to maximize the noise rejection. I believe they send the signal against the best electron flow. It's like radial tires on vehicles have a "direction" and once you install them you have to keep them rolling the same way when you rotate the tires. In the old days we cross rotated tires, but now they do them front-to-back and back-to-front to maintain the "grain". It's not the perfect analogy, as the wire grain is set from the factory, whereas tire grain is set by usage.

I'm still wrestling with this one. To put it mildly right now it still looks a bit fruity. I'm keeping an open mind, and I'll keep probing, but I have my doubts.

../Wayne C.

PS: I knew a man that sold solar panels that were hooked up to a compressor to generate hot water or electricity (I don't remember the details). But I do remember he told customers that it worked really well on sunny days. Some customers wanted to know about rainy days. His answer, "oh, it works even better then because of the friction of the rainwater on the panels resulting in extra heat generated to the compressor". The compressors started blowing up after he sold a few. I also heard somebody say that the bigger a tree is that the more stable it is because there is more gravity to hold it up. I'm more open minded about this wire polarity thing for audio as data and audio react differently than just furnishing amperage.

[ September 09, 2003, 05:22 PM: Message edited by: awwt ]
 

justin

Senior Member
Re: voltage drop

to all, my very embarassing mistake! after doing the research that i should have done, i realised i was exactly wrong. it's as easy as Table 8, chapter 9. lesson llearned and i appreciate the schooling, i obviously needed it!!
 

dereckbc

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Location
Plano, TX
Re: voltage drop

Justin you really did not make a mistake, you just didn't take the expression down to the lowest common denominator. Your expression was correct mathmatically as was mine, you just didn't realize it.

Good luck.

Dereck
 
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