DC resistance

[sabastian151]

hello,

i'm having a hard time figuring out the actual voltage drop of a DC circuit and i think it's due to a bad calculation with the resistance.
amps=70kA
VDC=100
L=450 ft

Conductor is 7.50" o.d x 5.5" i.d. water cooled bus.
sufficient water flow through out circuit on multiple inlet connections.

cross section is 20.42 sq. in.

can someone help me with this?
I can figure out the voltage drop once i get the right resistance of copper for this size bus.

thanks.

 

[magoo2]

R = V/I

You have both V and I.

The other details don't really matter.

 

[IRstuff]

The drop should be I*[ρ]*len/area

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[VEBill]

Here's an online calculator that can (perhaps) be pressed into service:

http://circuitcalculator.com/wordpress/2006/01/24/trace-resistance-calculator

Entered 20.42 by 1 inches and 450 feet, and the result was 0.000177 ohms.

You certainly shouldn't trust this result without quadruple checking it in different ways. I suggest this primarily as yet another easy datapoint to add to the mix.

PS: What was the "VDC=100" that you mentioned?

 

[sabastian151]

100 v(dc)total output from rect/trans

i need to figure out the total resistance of the conductor across 450 ft. to calculate the total voltage drop of the circuit.

there are are 2 conductors in parallel carrying 70kA @ 100vdc.

i'm having problems figuring the length into the equation.

i know to use r=v/i=.00142, but where do figure in the resistivity of copper of 1.7x10e-8 as the factor? and the length?

 

[IRstuff]

I gave you the equation, didn't I?

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[davidbeach]

Yeah, IR, it seems that you did.

 

[sabastian151]

The drop should be I*?*len/area

I=70000
?=??? (does this factor include the mat'l? this is where i get confused...i think)
L=450
A=in sq. in? cir mil?

 

[benta]

You are driving yourself crazy by insisting on using antique units (how about throwing in a "furlong" just to make things funnier).

Recalculate using meters for length and square meters for cross-section, then the formula given by IRstuff will work using 1.7x10e-8 for p.

Benta.

 

[desertfox]

Hi sabastian

Rho is what you're after here you go:-

http://hypertextbook.com/facts/2004/BridgetRitter.shtm

desertfox

 

[IRstuff]

The equation works for any consistent unit system, and in Mathcad, it will work even with mixed units:

http://files.engineering.com/getfil...-5450-496c-b325-5671ce4afa69&file=IR_drop.tif

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[VEBill]

I just did the manual calculation using the above and it matched the online 'trace resistance calculator' to four digits (higher apparent accuracy than the significant digits of the input variables).

 

[iop95]

If temperature in cable is about 20C, drop voltage is aprox. 0.146V, or 0.146%.

 

[VEBill]

...0.146V..."

Are you sure?

 

[IRstuff]

Round trip resistance is 0.35 milliohm * 70 kA = 24.8V total drop

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[iop95]

If I undestood corectly, area A = (pi*7.5^2/4 - pi*5.5^2/4) = 20.41sq.in = 131.67 cm2.
Cooper resistivity at 20C Rcu = 2*10^-8 ohm.m
Lenght l = 2*450ft = 297m
R = Rcu*l/A = 2*10^-8 * 297/131.67/10^-4 = 4.51*10^-4 ohm
DV=R*i=4.51*10^-4 * 70*10^3 = 31.57V... in first calculation I used a wrong cooper resistivity value...
And power lose in cable is Pcu = 2.2MW... sure it's not a real system wuth such power lose.

 

[VEBill]

I had found 12.4 volts (the IR drop) one-way - both from the on-line calculator (I linked above) and by way of handraulic calculation. This matches what IRstuff found.

iop95 is using a slightly different value for ?.



Regarding the 450 feet - I wonder why they don't move the transformer a bit closer. It would pay for itself from the recycled copper, power savings, and waste heat. ;-)

 

[electricpete]

sure it's not a real system wuth such power lose.

70kA is a little unusual, unless it is a a fault current.

=====================================
(2B)+(2B)' ?

 

[7anoter4]

I think your conductor is a steel of 0.1 ohm.mm^2/m resistivity and 0.005 1/degreeC temperature coefficient.The conductor temperature has to be 90 degrees C.

 

[sabastian151]

70kA is fully load current 24/7.
Customer shows 3.5v drop.

i used so many different k factors and never got close to 3.5.

maybe i should clarify just in case it's not clear.
2 water cooled conductors running in parallel for a total of 450' (450'/conductor)