Resistivity question??

danielc4514 · Sep 3, 2015

Hi all,
i have a practice question that asks what the resistance will be at 20oC
length of copper 55m
having an area of 2.5mm2
has a specific resistance of 1.724x10-8ohm metre.

my calculation is as follows
(1.724^-8 x 55) / 2.5^-6 = 0.379 ohm

correct me if this is wrong, now how would i calculate this to a temperature?

Thanks

Rpa07 · Sep 3, 2015

My lecturer recently told me that you can work out temp by the additional math 1 + 0.004 per degree C up or down so your cable is presumed start at 30oC and you want 20 so 10 difference. 10 x 0.004 + 1 = 1.04 then you divide your 0.379 because you want to reduce the cable temp from 30 to 20 degrees and to check the impedance will be greater as the temp increases. It's another rating factor and a good one to remember.

Richard Burns · Sep 3, 2015

Resistivity is normally given as a value at room temperature / 20°C and your value for the resistivity (not resistance) of copper is the value at that temperature so there is no need to make any correction.

You have correctly used the formula R=ρl/A
to calculate the answer, I have not done the maths to check but the method is correct.

rpa07 has shown the approximation for the correction of a resistance at a given temperature to another temperature.