Resistivity

[cm304eex]

Hi all this question is doing my poor head in headbang so any help would be fab...

Find the cross-tional area of a copper cable 42m long which carries current of 36A with a voltage drop of 2.69V.

If you could explain it to me in simple terms I would be gratefull

 

[septiclecky]

Hi all this question is doing my poor head in headbang so any help would be fab...Find the cross-tional area of a copper cable 42m long which carries current of 36A with a voltage drop of 2.69V.

If you could explain it to me in simple terms I would be gratefull

Just need to transpose the volt drop formula.

 

[cm304eex]

thanks septic trouble I can not rember how to transpose it has been a long time since I was last at school.

 

[tom79]

does this help you?

http://www.tlc-direct.co.uk/Book/4.3.11.htm

 

[cm304eex]

no use i still can not get it off to bed wit a headache:_|

 

[SPECIAL LOCATION]

Hi all this question is doing my poor head in headbang so any help would be fab...Find the cross-tional area of a copper cable 42m long which carries current of 36A with a voltage drop of 2.69V.

If you could explain it to me in simple terms I would be gratefull

Assuming we are looking for one of the "standard" cable sizes not an obscure bit of customised gauge cable....

Then I think one of your values may be slightly out... But anyway... here goes... :|

ALL cable has a resistance value which consumes or "drops" some of the voltage that travels down it and the amount of voltage dropped also increases with current.

There are tables in BS7671 for Volt drop per amp per meter for all of the standard cable sizes..

See page 133 one site guide table 6E2.

because the voltage dropped is very very small the table lists it as:-

"millivolts per amp per meter"

so you need to find out how much of your 2.69volts is dropped per meter & per amp..

Cable is 42m long.

2.69volts over 42 meters is 2.69/42 = 0.06404v per meter.

but this is with 36 amps current..

so 0.6404volts over 36amps is 0.06404v/36A = 0.001778v per meter per amp

convert it to milivolts....

0.001778x1000 = 1.78millivolts per amp per meter

look down table 6E2... g 133 O.S.G. to try and find a match.. ?:|

it can be seen that 25mm CSA has a volt drop of 1.75mV/A/m

so I would suggest it is 25mm copper cable!

calc it back again using the 25mm volt drop figure..

(1.75 x 36 x 42)/1000 = 2.646volts

as said at start that does leave us a 0.044v error!

but its the best I can do! :|

 

[cm304eex]

thanks for that the only problem is the book says that it should be 10mm csa.

The book could be wrong

 

[SPECIAL LOCATION]

thanks for that the only problem is the book says that it should be 10mm csa.The book could be wrong

AH!!!

I did say the numbers didn't crunch up exactly IMHO...

Now say for example if it was...

6.69A (NOT 2.69A)

42m

36A

we would then calculate out to....

(6.69/42/36)*1000 = 4.42mv/a/m

which is within a gnats whisker of 10.0mm CSA cable 4.4mV/a/m

so maybe the volt drop figure has been written down wrong!? :|

 

[redman]

This is how I`d work it out,

2.69 V / 36 A = 0.0747 ohms

Resistivity of copper = 1.78 x 10-8 ohmsm

1.78 x 10-8 x 42

/

0.0747 x 10-6 = 1000 x 10-2

or 1000 / 100 = 10mm

Hope this makes sense, just been doing this myself for the 2330.

 

[cm304eex]

thanks redman and septic think I have got it now thanks for all your hel pApplaud Smiley

 

[SPECIAL LOCATION]

This is how I`d work it out,2.69 V / 36 A = 0.0747 ohms

Resistivity of copper = 1.78 x 10-8 ohmsm

1.78 x 10-8 x 42

/

0.0747 x 10-6 = 1000 x 10-2

or 1000 / 100 = 10mm

Hope this makes sense, just been doing this myself for the 2330.

Ah!!!!! "Resistivity of copper = 1.78 x 10-8 ohmsm"

there's one of those phrases you never seem to use out in the real world when bashin. fixing and testing the old T&E!

Far more technical than my more basic "look at volt drop calculation bit"...

I suspect you may well be right there Redman.... ; )Applaud Smiley

and I am most probably doing a wee-wee up the wrong tree.....

or summit like that!!!Blushing:_|

I shall go back to my beer....

and try to remember the last time I used the phrase "Resistivity of copper" in the 'real world'????????????????

Guiness Drink

Guiness DrinkGuiness DrinkGuiness Drink

Guiness DrinkGuiness DrinkGuiness DrinkGuiness Drink

nope!

can't think of any!!!

which is most likely why me brain didn't even consider it an option!:OBlushing

ROTFWLROTFWLROTFWLROTFWL

:|

 

[redman]

" Ah!!!!! "Resistivity of copper = 1.78 x 10-8 m"

there's one of those phrases you never seem to use out in the real world when bashin. fixing and testing the old T&E! "

I was thinking that myself.

I`ll probably never use half of these calculations once I`ve passed my exams.